TRAFFIC SIGNAL CONTROL SYSTEM
United States Patent 3631386
A system for controlling the duration of an extended portion of the green or proceed phase displayed to traffic at an intersection of a major street and at least one minor street comprises first and second oscillators which normally provide pulse outputs having a predetermined frequency fo which are supplied to first and second counters, respectively. These counters provide a control signal to a third counter when separate, predetermined counts therein are reached for terminating the extended portion of the green phase. The first counter is reset by the passage of each vehicle by a detector on a major or minor street to which the green phase is displayed so that its output represents a plurality of unit extensions thereof. On the other hand, the second counter is reset at each change of phase and its output represents a limit to the total duration of the unit extensions. A congestion sensor furnishes an output signal corresponding to traffic occupancy on the major street. Amplifier means supply a control signal to the first and second oscillators in responses to this output signal such that their output frequency is changed from f0 to effect a corresponding change in the duration of both the unit extensions and the extension limit. If the right-of-way is given to the major street, the output frequency is lowered so that each unit extension and the extension limit are lengthened. If right-of-way is given to the minor street, the output frequency is increased such that each unit extension and the extension limit are shortened. An embodiment for immediately shifting right-of-way from the minor to the major street when congestion increases beyond a specified point is shown, as are embodiments permitting control of the green phase displayed to both streets in accordance with the individual traffic congestions thereof.
US Patent References:
Traffic control system of the actuated type with improved time control
Du Vivier - February 1966 - 3234505
/3258744.html
Auer, Jr. - June 1966 - 3258744
/3258745.html
Auer, Jr. - June 1966 - 3258745
Traffic control system
Cress, Jr. et al. - April 1968 - 3376546
TIMED DECISION LOAD CONTROL SYSTEM
Clark et al. - August 1970 - 3522581
Traffic control system of the actuated type with improved time control
Du Vivier - February 1966 - 3234505
/3258744.html
Auer, Jr. - June 1966 - 3258744
/3258745.html
Auer, Jr. - June 1966 - 3258745
Traffic control system
Cress, Jr. et al. - April 1968 - 3376546
TIMED DECISION LOAD CONTROL SYSTEM
Clark et al. - August 1970 - 3522581
Inventors:
Saita, Tutomu (Kyoto, JA)
Yoshida, Yasumasa (Kyoto, JA)
Yamazaki, Norio (Shizuoka, JA)
Yoshida, Yasumasa (Kyoto, JA)
Yamazaki, Norio (Shizuoka, JA)
Application Number:
04/799343
Publication Date:
12/28/1971
Filing Date:
02/14/1969
View Patent Images:
Export Citation:
Assignee:
Omron Tateisi Electronics Co. (Kyoto, JA)
Primary Class:
International Classes:
G08G1/08; G08G1/07; G08G1/08
Field of Search:
340/37,36,31A,31R,38R
Primary Examiner:
Claffy, Kathleen H.
Assistant Examiner:
Myers, Randall P.
Claims:
What we claim is
1. In a system for controlling a traffic signal at an intersection of a major street and at least one minor street which includes means controlling the phase of said traffic signal in response to a control signal, and control means providing said control signal a fixed time after the last change in phase of said traffic signal, an improvement, for controlling said traffic signal in accordance with the traffic congestion on said streets, comprising:
2. The improvement as recited in claim 1, further comprising means providing a green phase signal only when the phase of said traffic signal gives the right-of-way to said major street, and wherein said extension changing means includes first means responsive to said green phase signal for controlling said extension-setting means to lengthen the duration of each of said unit extensions in proportion to said output signal.
3. The improvement as recited in claim 2, wherein said extension-changing means includes second means for controlling said extension-setting means to shorten the duration of each of said unit extensions in proportion to said output signal, and means inhibiting said second means in response to said green phase signal.
4. The improvement as recited in claim 3, further including level detector means coupled to said sensor means and said means providing a green phase signal and being responsive to the absence of said green phase signal for enabling the production of said control signal when said output signal exceeds a predetermined minimum value so that right-of-way is transferred to said major street to relieve traffic congestion thereon.
5. The improvement as recited in claim 1, further comprising an extension-limit-setting means connected to the control means for providing an extension-limit to the delaying of said control signal by said extension setting means, and an extension-limit-changing means responsive to said output signal for controlling said extension-limit-setting means to vary the duration of said extension-limit in accordance therewith.
6. The improvement as recited in claim 5, further comprising means providing a green phase signal only when the phase of said traffic signal gives right-of-way to said major street, and wherein both said extension-changing means and said extension-limit-changing means include first means responsive to said green phase signal for controlling said extension-setting means and said extension-limit-setting means to lengthen the duration of each of said unit extensions, and said extension-limit respectively, in proportion to said output signal.
7. The improvement as recited in claim 6, wherein both said extension-changing means and said extension-limit-changing means further include second means for controlling said extension-setting means and said extension-limit-setting means to shorten the duration of each of said unit extensions, and said extension-limit, respectively, in proportion to said output signal, and means responsive to said green phase signal for inhibiting the operation of said second means.
8. The improvement as recited in claim 5,
9. an oscillator normally providing a pulse output having a predetermined frequency,
10. a counter coupled to said pulse output for enabling the production of said control signal after a predetermined count thereof has been reached, said counter having a reset terminal, and
11. means coupling said reset terminal of said counter to said detector means,
12. a second oscillator normally providing a pulse output having a second predetermined frequency,
13. a second counter coupled to said second pulse output for enabling the production of said control signal after a predetermined count thereof has been reached, said counter having a reset terminal, and
14. means coupling said reset terminal of said second counter to the control means, and
15. An improvement as recited in claim 1, wherein said extension-setting means comprises
16. The improvement as recited in claim 9, wherein said extension-changing means comprises means decreasing said predetermined frequency in inverse proportion to said output signal from said sensor means.
17. In a system for controlling a traffic signal at an intersection of a plurality of streets, which includes means controlling the phase of said traffic signal in response to a control signal thereto, and means providing said control signal a fixed time after the last change in phase of said traffic signal, an improvement for controlling said traffic signal in accordance with the traffic congestion on said plurality of streets, comprising
18. The improvement as recited in claim 11, further comprising means providing a green phase signal only when the phase of said traffic signal gives right-of-way to said one of said plurality of streets, and wherein said extension changing means and said extension-limit-changing means are responsive to said green phase signal for controlling said extension-setting means and said extension-limit-setting means to lengthen the duration of said unit extension and said extension-limit, respectively, in proportion to said output signal.
19. The improvement as recited in claim 12, further comprising a second sensor means furnishing a second output signal corresponding to the degree of traffic-occupancy on a second of said plurality of streets, and a level detector means coupled to said second output signal and responsive to said green phase signal for enabling the production of said control signal when said second output signal exceeds a predetermined minimum level so that right-of-way is transferred to said second one of said plurality of streets to relieve the traffic congestion thereon.
20. The improvement as recited in claim 18, further comprising:
1. In a system for controlling a traffic signal at an intersection of a major street and at least one minor street which includes means controlling the phase of said traffic signal in response to a control signal, and control means providing said control signal a fixed time after the last change in phase of said traffic signal, an improvement, for controlling said traffic signal in accordance with the traffic congestion on said streets, comprising:
2. The improvement as recited in claim 1, further comprising means providing a green phase signal only when the phase of said traffic signal gives the right-of-way to said major street, and wherein said extension changing means includes first means responsive to said green phase signal for controlling said extension-setting means to lengthen the duration of each of said unit extensions in proportion to said output signal.
3. The improvement as recited in claim 2, wherein said extension-changing means includes second means for controlling said extension-setting means to shorten the duration of each of said unit extensions in proportion to said output signal, and means inhibiting said second means in response to said green phase signal.
4. The improvement as recited in claim 3, further including level detector means coupled to said sensor means and said means providing a green phase signal and being responsive to the absence of said green phase signal for enabling the production of said control signal when said output signal exceeds a predetermined minimum value so that right-of-way is transferred to said major street to relieve traffic congestion thereon.
5. The improvement as recited in claim 1, further comprising an extension-limit-setting means connected to the control means for providing an extension-limit to the delaying of said control signal by said extension setting means, and an extension-limit-changing means responsive to said output signal for controlling said extension-limit-setting means to vary the duration of said extension-limit in accordance therewith.
6. The improvement as recited in claim 5, further comprising means providing a green phase signal only when the phase of said traffic signal gives right-of-way to said major street, and wherein both said extension-changing means and said extension-limit-changing means include first means responsive to said green phase signal for controlling said extension-setting means and said extension-limit-setting means to lengthen the duration of each of said unit extensions, and said extension-limit respectively, in proportion to said output signal.
7. The improvement as recited in claim 6, wherein both said extension-changing means and said extension-limit-changing means further include second means for controlling said extension-setting means and said extension-limit-setting means to shorten the duration of each of said unit extensions, and said extension-limit, respectively, in proportion to said output signal, and means responsive to said green phase signal for inhibiting the operation of said second means.
8. The improvement as recited in claim 5,
9. an oscillator normally providing a pulse output having a predetermined frequency,
10. a counter coupled to said pulse output for enabling the production of said control signal after a predetermined count thereof has been reached, said counter having a reset terminal, and
11. means coupling said reset terminal of said counter to said detector means,
12. a second oscillator normally providing a pulse output having a second predetermined frequency,
13. a second counter coupled to said second pulse output for enabling the production of said control signal after a predetermined count thereof has been reached, said counter having a reset terminal, and
14. means coupling said reset terminal of said second counter to the control means, and
15. An improvement as recited in claim 1, wherein said extension-setting means comprises
16. The improvement as recited in claim 9, wherein said extension-changing means comprises means decreasing said predetermined frequency in inverse proportion to said output signal from said sensor means.
17. In a system for controlling a traffic signal at an intersection of a plurality of streets, which includes means controlling the phase of said traffic signal in response to a control signal thereto, and means providing said control signal a fixed time after the last change in phase of said traffic signal, an improvement for controlling said traffic signal in accordance with the traffic congestion on said plurality of streets, comprising
18. The improvement as recited in claim 11, further comprising means providing a green phase signal only when the phase of said traffic signal gives right-of-way to said one of said plurality of streets, and wherein said extension changing means and said extension-limit-changing means are responsive to said green phase signal for controlling said extension-setting means and said extension-limit-setting means to lengthen the duration of said unit extension and said extension-limit, respectively, in proportion to said output signal.
19. The improvement as recited in claim 12, further comprising a second sensor means furnishing a second output signal corresponding to the degree of traffic-occupancy on a second of said plurality of streets, and a level detector means coupled to said second output signal and responsive to said green phase signal for enabling the production of said control signal when said second output signal exceeds a predetermined minimum level so that right-of-way is transferred to said second one of said plurality of streets to relieve the traffic congestion thereon.
20. The improvement as recited in claim 18, further comprising:
Description:
This invention relates to an automatic traffic signal control system, and more particularly to a system for controlling the traffic signal at a street intersection in accordance with the traffic condition on the streets adjacent the intersection.
Among various types of traffic signal control systems, the traffic-actuated type of control system is well known. In this type of system, the duration or length of the proceed or green signal usually comprises a minimum initial portion and an extension added to the minimum initial portion. The minimum initial portion is the minimum period of time of duration of the green signal indication to be displayed to each street, no matter whether or not there is traffic on that street; and the extension is a unit period of time of green signal indication to be added to the minimum initial portion to extend the total green time so as to pass the vehicles that could not pass the intersection during the minimum initial portion of the green signal. As many unit extensions as the traffic condition requires may be added, but there is a limit to such extension of the green signal. The limit will be referred to as the extension limit.
When there has occurred a traffic delay or congestion on a street, it has hitherto been customary to extend the period of time of the green signal indication to the maximum limit regardless of the degree of the traffic delay or congestion on that street or on the street crossing that street. With such prior art arrangements, once a traffic congestion has occurred, the traffic-actuated control system is changed into a fixed-cycle control system, and it is when traffic is being delayed or congested that the traffic-actuated control is expected to perform its proper functions.
Accordingly, it is one object of the invention to provide a system which is capable of controlling the traffic signal at a street intersection in accordance with the traffic condition adjacent the intersection so as to relieve the streets adjacent the intersection of any traffic delay or congestion.
Another object of the invention is to provide a traffic signal control system which controls the unit extension and the extension limit in accordance with the traffic condition around a street intersection, thereby relieving the streets around the intersection of any traffic delay or congestion.
Another object of the invention is to provide such a traffic signal control system as aforesaid, wherein as the degree of traffic congestion increases on one of the intersecting streets, the unit extension and/or the extension limit of the green time being displayed to the street on which the congestion occurs is lengthened, thereby relieving the street of the congestion.
Still another object of the invention is to provide a traffic signal control system, wherein the degree of traffic congestion on that one of the intersecting streets to which the right-of-way is given is compared with the degree of traffic congestion on the other of the streets to which the right-of-way is not given, and wherein on the basis of the result of the comparison the unit extension and/or the extension limit is controlled so as to give the right-of-way to each street in accordance with the traffic congestion on each street.
A further object of the invention is to prevent occurrence of traffic congestion on the intersecting streets by controlling the traffic signal in accordance with the traffic condition on the streets. To this end, the invention provides a system wherein when the traffic congestion on the street to which the right-of-way is not given has reached a predetermined degree, the right-of-way is transfered to that street so as to relieve the street of the traffic congestion.
As previously mentioned, in accordance with the invention, the unit extension and/or the extension limit is controlled in accordance with the information about the traffic condition on the intersecting streets. Such information may be obtained in terms of what is referred to in the art as "occupancy," the vehicle speed, the number of cars waiting at a street intersection for the signal to change, etc. The occupancy is available most easily among them, and includes time-occupancy and space-occupancy. The time-occupancy and the space-occupancy have similar characteristics so that one of them may be relied on as an information about the traffic stagnation. The time-occupancy can be measured by the use of a vehicle detector of the presence type. The detector is placed at a suitable place on a street. When the detector detects a vehicle, it produces an output, which continues as long as the vehicle is being detected. The time-occupancy can be obtained by integrating all the periods of time of existence of the output signal from the detector. The space-occupancy can be measured by the use of a vehicle detector having as a sensor or pickup a large loop of wire placed on a street so as to cover a predetermined relatively large area of the surface of the street. The output from the detector varies with the number of vehicles existing within the enclosed area of the sensor loop. It will be seen that the output from the detector shows the magnitude of the area that is occupied by vehicles within the loop, that is, the space-occupancy.
If the degree of traffic congestion on that one of the crossing streets to which the right-of-way is given increases, the unit extension and/or the extension limit of the green signal being displayed to that one street may be lengthened, so that the degree of congestion will be decreased. If a traffic congestion has occurred on that one of the streets to which the right of way is not given, the unit extension and/or the extension limit of the green signal being displayed to the other street may be shortened to transfer the right-of-way to that one street as soon as possible.
The invention will be better understood from the following detailed description of some preferred embodiments thereof with reference to the accompanying drawings, wherein like reference numerals denote corresponding parts, and wherein:
FIG. 1 is a block diagram of one embodiment of the invention, with a block enclosed by a dash-and-dot line being added to show a modified embodiment thereof;
FIG. 2 is a detailed circuit diagram of FIG. 1;
FIG. 3 is a block diagram of another embodiment of the invention;
FIG. 4 is a graph showing the relation between the degree of traffic congestion and the output frequencies of the oscillators used in the circuit of FIGS. 1-3;
FIG. 5 is a block diagram of another embodiment of the invention; and
FIG. 6-8 are graphs showing the operation of the system of FIG. 5.
Now referring to the drawings, first to FIG. 1, the block 10 designates a device for converting the information about the traffic congestion on the major street into a corresponding voltage signal. As shown in FIG. 2, the device 10 comprises a vehicle detector 11 installed on the major street and a space-occupancy detector 12 which is adapted to receive the output from the vehicle detector 11 to produce a direct current voltage signal corresponding to the space-occupancy on the major street. Such a space-occupancy detector is well known.
The output from the circuit 10 is applied to an input adjuster 20 which includes a potentiometer 21 for adjusting the input voltage level received from the device 10 and a subtracting circuit 22 for subtracting a predetermined value from the input voltage level. The output from the subtracting circuit 22 is applied to an oscillating frequency-changing circuit 30 through a buffer circuit 23 such as an emitter follower. The circuit 30 comprises four amplifiers 31-34, to which the output from the input adjuster 20 is applied through variable resistors 35-38, respectively, The amplifiers 31 and 33 has such a characteristic that their outputs decrease in proportion to the increase in the inputs to them; while the other amplifiers 32 and 34 have such a characteristic that their outputs increase in proportion to the increase in the inputs thereto. The amplifiers 31-34 are provided with potentiometers 39-42 and gate terminals 43-46, respectively, The potentiometers are provided for the purpose of adjusting the output voltages of the amplifiers, and only those of the amplifiers which receive a gate signal through their respective gate terminals perform their amplifying operation. The gate terminals 43 and 45 are directly connected to a terminal 49, to which the gate terminals 44 and 46 are connected indirectly through NOT-elements 47 and 48, respectively. A signal 1G is applied to the terminal 49 by a signal controller 73' while the traffic signal to the major street is green. Therefore, while the signal 1G is being applied to the terminal 49, the amplifiers 31 and 33 work; and while there is no signal 1G being applied to the terminal 49, the amplifiers 32 and 34 operate.
The outputs from the amplifiers 31 and 32 are applied to an OR-element OR1, the output from which is applied to an oscillator 51, while the outputs from the amplifiers 33 and 34 are applied to an OR-element OR2, the output from which is applied to another oscillator 52.
The oscillating frequencies of the oscillators 51 and 52 vary in proportion to the magnitude of the input applied thereto in the following manner. When the traffic signal being displayed to the major street is green, the oscillators 51 and 52 receive their respective input voltages from the amplifiers 31 and 33. Under the condition, as the degree of traffic congestion on the major street increases, the outputs from the amplifiers decreases, so that the oscillating frequencies of the oscillators 51 and 52 vary as shown by the curve B in FIG. 4.
When the traffic signal being displayed to the minor street (that is, the street crossing the major street) is green and there is no signal 1G at the terminal 49, the oscillators 51 and 52 receive their respective input voltages from the amplifiers 32 and 34. Under the condition, when the degree of traffic congestion on the major street has exceeded a point P1 in FIG. 4, the output voltage levels of the amplifiers 32 and 34 begin to rise, so that the oscillating frequencies of the oscillators 51 and 52 change as shown by the curve A in FIG. 4. When the traffic congestion has exceeded a predetermined degree P2, both curves A and B become flat at frequencies f2 and f1, respectively. The values of the frequencies f1 and f2 can be adjusted by means of the potentiometers 39-42.
The output pulses from the oscillators 51 and 52 are applied to a block 60 including a unit extension-setting circuit and an extension limit-setting circuit. The output pulses from the oscillators 51 and 52 are first applied as one input to OR-elements OR3 and OR4, respectively. A predetermined voltage is applied to a terminal V and thence through a switch 61 to an OR-element OR5 as well as to the OR-elements OR3 and OR4. The switch 61 is opened when it is desired to control the traffic signal by the system of the invention on the basis of the information about the traffic congestion on the crossing streets. A signal appears on a line 62 from signal controller 73' upon termination of the extension limit or when there is no more vehicles to be detected during the extension and disappears upon termination of the minimum initial portion. In other words, there is no signal on the line 62 during the extended period of time of the green signal. The signal on the line 62 is also applied to the OR-elements OR3-OR5.
The outputs from the OR-elements OR3-OR5 are applied to NOT-elements 63-65, respectively. Therefore, the NOT-elements 65 produces an output when the switch 61 is opened and at the same time there is no signal on the line 62. This output from the NOT-element 65 is applied as an inhibit input to INHIBIT-elements 66 and 67. A voltage of a predetermined frequency, say, 60 c./s. is applied to a terminal 68 and thence as an input to the INHIBIT-elements 66 and 67 through a resistor 69 and a rectifier 70. The output from the INHIBIT-element 66 and the output from the NOT-element 63 are applied to a unit extension-setting counter 71 through an OR-element OR6; while the output from the INHIBIT element 67 and the output from the NOT-element 64 are applied to an extension limit-setting counter 72 through an OR-element OR7. The outputs produced by the counters 71 and 72 when they have counted up are applied to a "phase"-shifting counter 73 through an OR element OR8. A suitable pulse generator, not shown, applies phase-shifting pulses to the counter 73 through the OR-element OR8. As the counter 73 counts the input pulses, it steps forward shifting its output from one of its output terminals onto the next. These output signals from the counter 73 may be utilized as phase-shifting signals to change one phase to the next by application to signal controller 73' which in turn directly controls the traffic signal at the intersection The counter 71 is reset by a reset signal applied through a reset line R1 upon detection of every one vehicle by a vehicle detector 71' also located on the major street; and the counter 72 is reset by a reset signal applied thereto through a reset line R2 by signal controller 73' upon change of one phase to the next.
In operation, the output voltage from the circuit 10, that is, the voltage corresponding to the degree of congestion on the major street, is applied to the subtracting circuit 22, which subtracts from the input voltage the voltage corresponding to the normal volume of traffic on the major street. When there is no traffic congestion on the major street, the oscillators 51 and 52 oscillate at a normal frequency f0 (see FIG. 4), thereby setting a lower limit to the length of the unit extension and also that of the extension limit. The input voltages to the amplifiers 3114 35 are regulated by the variable resistors 35-38, respectively. These resistors determine the ratio of change of the oscillating frequencies of the oscillators 51 and 52 within a range exceeding the minimum degree of congestion as shown at P1 in FIG. 4. This determination is made in view of the relation between the degree of congestion and the actual difficulty with which vehicles pass the intersection. The inputs to the oscillators 51 and 52 are regulated by the potentiometers 39-42, so that the frequencies f1 and f2 are set for the degree P2 of traffic congestion. In other words, upper limits are set to the variable lengths of the unit extension and the extension limit which may increase as traffic congestion aggravates.
Suppose that the degree of traffic congestion on the major street has exceeded the point P1 in FIG. 4. As the degree of congestion increases, the oscillating frequencies of the oscillators 51 and 52 decrease as shown by the curve B in FIG. 4. When the switch 61 is open and there is no signal on the line 62, the output from the oscillator 51 is applied to the counter 71 through the OR-element OR3, the NOT-element 63 and the OR-element OR6. As the output oscillating frequency of the oscillator 51 decreases, the time required for the counter 71 to have finished counting all the predetermined number of input pulses becomes longer than when the degree of congestion is below P1. This means that the unit extension has now become longer than before. When the unit extension has thus been lengthened, the right of way is given to the major street for a longer period of time than otherwise, thereby helping causing the traffic congestion to disappear.
If, before the counter 71 has counted up, a succeeding vehicle is detected and a reset signal is applied to the reset terminal R1 thereof, the counter 71 is reset to resume counting the input pulses from the beginning. Therefore, it is only when the interval between two successive reset inputs to be applied to the reset terminal R1 (that is between two succeeding vehicles approaching the intersection) has exceeded the period of time required for the counter 71 to have counted up that the counter produces an output signal to change phases.
On the other hand, the period of time for the other counter 72 to have counted up also becomes longer, so that the extension limit is also lengthened, thereby helping cause the congestion on the major street to disappear.
When the unit extension has been lengthened, the extension limit must also be lengthened. Therefore, in the illustrated embodiment the two counters 71 and 72 are provided. However, there may be a case in which the extension limit must be lengthened while the unit extension need not be. In such cases, only one of the two counters may be employed.
When the right of way is not given to the major street, that is, when no signal 1G is being applied to the terminal 49, the oscillators 51 and 52 receive the outputs from the amplifiers 32 and 34, respectively. In this case, as the congestion on the major street increases, the outputs from the amplifiers increase until the congestion has exceeded the point P1 in FIG. 4, whereupon the oscillating frequencies of the oscillators 51 and 52 begins to increase as shown by the curve A in FIG. 4, so that the time it takes for the counters 71 and 72 to have counted up becomes shorter and shorter. This means that both the unit extension and the extension limit of the green signal being displayed to the minor street is shortened. In other words, the time during which the right of way is given to the minor street is shortened so that the right of way is transferred to the major street sooner than otherwise, thereby helping congestion thereon to disappear sooner.
In the arrangement of the above embodiment, the traffic congestion on the major street is detected so as to remove the congestion therefrom either when the right-of-way is given to the major street or when it is not, regardless of the traffic condition on the minor street. However, there are cases in which while the right-of-way is being given to one of the crossing streets, the traffic congestion on the other street must be taken into consideration. That is, if the traffic congestion increases on the other street, the right-of-way is transferred from the one street to which it has until then been given to that other street so as to remove the congestion therefrom. To this end, the level detector 81 is added to the system of FIG. 1. The detector 81 receives an an input the output from the circuit 20. The output from the NOT-element 47 is applied as an actuating signal to the detector 81. Therefore, while the signal from the NOT-element 47 is being applied to the level detector 81, that is, when the right of way is being given to the minor street, and not to the major street, the level detector detects the level of the output signal from the circuit 20, that is, the degree of traffic congestion on the major street. When the traffic congestion has reached a predetermined degree, the level detector 81 produces an output to be directly applied to the OR-element OR8, so that the counter 73 has its output shifted, thereby transferring the right-of-way from the minor street to the major street.
In order to remove the congestion on the minor street while the right-of-way is being given to the major street, a congestion detector similar 10' to that 10 in FIG. 1 can also be provided on the minor street, as shown in FIG. 3, so that the output from the congestion detector 10' is applied to a level detector 81' which operates when the right-of-way is given to the major street, and when the input to the level detector 10' has exceeded a predetermined level, the output produced by the level detector may be applied to the OR-element OR8. In short, when the degree of traffic congestion on the street to which the right-of-way is not given has exceeded a predetermined level, the right-of-way is transferred to that street so as to remove the congestion therefrom.
Now turning to FIG. 5, there is shown an arrangement that the degree of traffic congestion on both the major and the minor streets are compared, so that the congestion on that street the degree of which is greater is first removed. In FIG. 5 the same reference numerals as in FIGS. 1 and 2 denote corresponding parts. The numeral 100 denotes a circuit similar to the circuit 10 in FIG. 1, which detects the degree of traffic congestion on the minor street to produce a corresponding output voltage signal. The output is applied as one input to a voltage comparator 110, to which the output from the circuit 10 is applied as another input. The comparator 110 compares the two inputs and produces an output signal corresponding to the difference or ratio between the two input signals. The output signal changes as time passes. Let it be assumed that the output signal changes as shown by the curve in FIG. 8. Between tand t2, and t3 and t4 the degree of traffic congestion on the major street is greater than that on the minor street, and between t2 and t3 the degree of traffic congestion on the minor street is greater than that on the major street. In the former case (between 11 and t2, and t3 and t 4), the output (the difference voltage) from the comparator 110 appears on a line 112, but no signal is produced on a line 111. In the latter case (between t2 and t3) the difference voltage appears on the line 111 but not signal is produced on the line 112. At t1, t2, t3 and t4 when the difference voltage is zero, no signal appears on either the line 111 or 112. The signals on the lines 111 and 112 are applied to subtracting circuits 113 and 114, respectively.
The substracting circuit 113 subtracts the voltage on the line 111 from the output voltage from the circuit 10 and produces an output corresponding to the result of subtraction; while the subtracting circuit 114 subtracts the voltage on the line 112 from the output voltage from the circuit 110 and produces an output corresponding to the result of the subtraction.
The output from the subtracting circuit 113 is applied to amplifiers 131 and 132, and the output from the subtracting circuit 114 is applied to amplifiers 133 and 143. The outputs from the amplifiers 131-134 changes in inverse proportion to the input thereto just as in the case of the amplifiers 31 and 33 in FIG. 1. The amplifiers 131 and 132 operate while the signal 1G is being applied thereto by signal controller 73' , with the right of way being given to the major street; and the amplifiers 133 and 134 operate while the signal 2G is being applied thereto by signal controller 73', with the right-of-way being given to the minor street. The outputs from the amplifiers 131 and 133 are applied to the oscillator 51 and the outputs from the amplifiers 132 and 134, to the oscillator 52.
The oscillating frequencies of the oscillators 51 and 52 vary with the degree of traffic congestion as shown by the curve C in FIG. 6, just as in the case of the curve B in FIG. 4. The other circuit connections of FIG. 5 are the same as in FIG. 1, with the exception that the reset terminal of counter 71 is connected through reset line R1 to vehicle detector 71a, 71b for the major and minor streets of the intersection.
In operation, the comparator 110 compares the voltages corresponding to the degrees of traffic congestion on the major and minor streets. If the degree of congestion on the major street has been found greater than that on the minor street, the difference voltage is applied to the subtracting circuit 114. As previously mentioned, the output from the subtracting circuit 114 is the output voltage from the detector 100 minus the output voltage on the line 112. On the other hand, the output from the other subtracting circuit 113 is the output voltage from the detector 10.
Suppose that the above conditions have come into existence while the right-of-way is being applied to the major street. Then, the amplifiers 131 and 132 alone operate to produce an output since the signal 1G is applied to the amplifiers 131 and 132. In other words, the voltage corresponding to the output from the detector 10 is applied to the oscillators 51 and 52, the output frequencies of which will then decrease, so that both the unit extension and the extension limit of the green signal being displayed to the major street are lengthened as previously explained with reference to FIGS. 1 and 2.
Suppose that the previously mentioned conditions have occurred while the right-of-way is being applied to the minor street. Then, the amplifiers 133 and 134 alone operate to produce an output since the signal 2G is now being applied to the amplifiers 133 and 134. In other words, the output voltage from the detector 100 minus the voltage on the line 112 is applied to the oscillators 51 and 52, the output frequencies of which will then decrease as shown by the curve D in FIG. 7. It is seen that in the curve D the decreasing change in the output frequency takes place more gradually with respect to the degree of congestion than in the curve C which results from the output from the detector 100 alone. It may be mentioned that the curves C and D presupposes that the difference voltage on the line 112 remains unchanged. The decrease in the oscillating frequencies of the oscillators 51 and 52 results in the increase in the length of the unit extension and the extension limit of the green signal being displayed to the minor street. In this case, however, the degree of stagnation on the major street which has been found greater than that on the minor street is taken into consideration. That is, the increase in the lengths of the unit extension and the extension limit are less than those which would result if the traffic congestion on the major street had not been taken into consideration or if there were no traffic congestion on the major street. In other words, the unit extension and the extension limit of the green signal being displayed to the minor street will be lengthened, but less than if no consideration is given to the traffic condition on the major street, so that the right-of-way is transferred to the major street sooner to remove the traffic congestion therefrom.
Among various types of traffic signal control systems, the traffic-actuated type of control system is well known. In this type of system, the duration or length of the proceed or green signal usually comprises a minimum initial portion and an extension added to the minimum initial portion. The minimum initial portion is the minimum period of time of duration of the green signal indication to be displayed to each street, no matter whether or not there is traffic on that street; and the extension is a unit period of time of green signal indication to be added to the minimum initial portion to extend the total green time so as to pass the vehicles that could not pass the intersection during the minimum initial portion of the green signal. As many unit extensions as the traffic condition requires may be added, but there is a limit to such extension of the green signal. The limit will be referred to as the extension limit.
When there has occurred a traffic delay or congestion on a street, it has hitherto been customary to extend the period of time of the green signal indication to the maximum limit regardless of the degree of the traffic delay or congestion on that street or on the street crossing that street. With such prior art arrangements, once a traffic congestion has occurred, the traffic-actuated control system is changed into a fixed-cycle control system, and it is when traffic is being delayed or congested that the traffic-actuated control is expected to perform its proper functions.
Accordingly, it is one object of the invention to provide a system which is capable of controlling the traffic signal at a street intersection in accordance with the traffic condition adjacent the intersection so as to relieve the streets adjacent the intersection of any traffic delay or congestion.
Another object of the invention is to provide a traffic signal control system which controls the unit extension and the extension limit in accordance with the traffic condition around a street intersection, thereby relieving the streets around the intersection of any traffic delay or congestion.
Another object of the invention is to provide such a traffic signal control system as aforesaid, wherein as the degree of traffic congestion increases on one of the intersecting streets, the unit extension and/or the extension limit of the green time being displayed to the street on which the congestion occurs is lengthened, thereby relieving the street of the congestion.
Still another object of the invention is to provide a traffic signal control system, wherein the degree of traffic congestion on that one of the intersecting streets to which the right-of-way is given is compared with the degree of traffic congestion on the other of the streets to which the right-of-way is not given, and wherein on the basis of the result of the comparison the unit extension and/or the extension limit is controlled so as to give the right-of-way to each street in accordance with the traffic congestion on each street.
A further object of the invention is to prevent occurrence of traffic congestion on the intersecting streets by controlling the traffic signal in accordance with the traffic condition on the streets. To this end, the invention provides a system wherein when the traffic congestion on the street to which the right-of-way is not given has reached a predetermined degree, the right-of-way is transfered to that street so as to relieve the street of the traffic congestion.
As previously mentioned, in accordance with the invention, the unit extension and/or the extension limit is controlled in accordance with the information about the traffic condition on the intersecting streets. Such information may be obtained in terms of what is referred to in the art as "occupancy," the vehicle speed, the number of cars waiting at a street intersection for the signal to change, etc. The occupancy is available most easily among them, and includes time-occupancy and space-occupancy. The time-occupancy and the space-occupancy have similar characteristics so that one of them may be relied on as an information about the traffic stagnation. The time-occupancy can be measured by the use of a vehicle detector of the presence type. The detector is placed at a suitable place on a street. When the detector detects a vehicle, it produces an output, which continues as long as the vehicle is being detected. The time-occupancy can be obtained by integrating all the periods of time of existence of the output signal from the detector. The space-occupancy can be measured by the use of a vehicle detector having as a sensor or pickup a large loop of wire placed on a street so as to cover a predetermined relatively large area of the surface of the street. The output from the detector varies with the number of vehicles existing within the enclosed area of the sensor loop. It will be seen that the output from the detector shows the magnitude of the area that is occupied by vehicles within the loop, that is, the space-occupancy.
If the degree of traffic congestion on that one of the crossing streets to which the right-of-way is given increases, the unit extension and/or the extension limit of the green signal being displayed to that one street may be lengthened, so that the degree of congestion will be decreased. If a traffic congestion has occurred on that one of the streets to which the right of way is not given, the unit extension and/or the extension limit of the green signal being displayed to the other street may be shortened to transfer the right-of-way to that one street as soon as possible.
The invention will be better understood from the following detailed description of some preferred embodiments thereof with reference to the accompanying drawings, wherein like reference numerals denote corresponding parts, and wherein:
FIG. 1 is a block diagram of one embodiment of the invention, with a block enclosed by a dash-and-dot line being added to show a modified embodiment thereof;
FIG. 2 is a detailed circuit diagram of FIG. 1;
FIG. 3 is a block diagram of another embodiment of the invention;
FIG. 4 is a graph showing the relation between the degree of traffic congestion and the output frequencies of the oscillators used in the circuit of FIGS. 1-3;
FIG. 5 is a block diagram of another embodiment of the invention; and
FIG. 6-8 are graphs showing the operation of the system of FIG. 5.
Now referring to the drawings, first to FIG. 1, the block 10 designates a device for converting the information about the traffic congestion on the major street into a corresponding voltage signal. As shown in FIG. 2, the device 10 comprises a vehicle detector 11 installed on the major street and a space-occupancy detector 12 which is adapted to receive the output from the vehicle detector 11 to produce a direct current voltage signal corresponding to the space-occupancy on the major street. Such a space-occupancy detector is well known.
The output from the circuit 10 is applied to an input adjuster 20 which includes a potentiometer 21 for adjusting the input voltage level received from the device 10 and a subtracting circuit 22 for subtracting a predetermined value from the input voltage level. The output from the subtracting circuit 22 is applied to an oscillating frequency-changing circuit 30 through a buffer circuit 23 such as an emitter follower. The circuit 30 comprises four amplifiers 31-34, to which the output from the input adjuster 20 is applied through variable resistors 35-38, respectively, The amplifiers 31 and 33 has such a characteristic that their outputs decrease in proportion to the increase in the inputs to them; while the other amplifiers 32 and 34 have such a characteristic that their outputs increase in proportion to the increase in the inputs thereto. The amplifiers 31-34 are provided with potentiometers 39-42 and gate terminals 43-46, respectively, The potentiometers are provided for the purpose of adjusting the output voltages of the amplifiers, and only those of the amplifiers which receive a gate signal through their respective gate terminals perform their amplifying operation. The gate terminals 43 and 45 are directly connected to a terminal 49, to which the gate terminals 44 and 46 are connected indirectly through NOT-elements 47 and 48, respectively. A signal 1G is applied to the terminal 49 by a signal controller 73' while the traffic signal to the major street is green. Therefore, while the signal 1G is being applied to the terminal 49, the amplifiers 31 and 33 work; and while there is no signal 1G being applied to the terminal 49, the amplifiers 32 and 34 operate.
The outputs from the amplifiers 31 and 32 are applied to an OR-element OR1, the output from which is applied to an oscillator 51, while the outputs from the amplifiers 33 and 34 are applied to an OR-element OR2, the output from which is applied to another oscillator 52.
The oscillating frequencies of the oscillators 51 and 52 vary in proportion to the magnitude of the input applied thereto in the following manner. When the traffic signal being displayed to the major street is green, the oscillators 51 and 52 receive their respective input voltages from the amplifiers 31 and 33. Under the condition, as the degree of traffic congestion on the major street increases, the outputs from the amplifiers decreases, so that the oscillating frequencies of the oscillators 51 and 52 vary as shown by the curve B in FIG. 4.
When the traffic signal being displayed to the minor street (that is, the street crossing the major street) is green and there is no signal 1G at the terminal 49, the oscillators 51 and 52 receive their respective input voltages from the amplifiers 32 and 34. Under the condition, when the degree of traffic congestion on the major street has exceeded a point P1 in FIG. 4, the output voltage levels of the amplifiers 32 and 34 begin to rise, so that the oscillating frequencies of the oscillators 51 and 52 change as shown by the curve A in FIG. 4. When the traffic congestion has exceeded a predetermined degree P2, both curves A and B become flat at frequencies f2 and f1, respectively. The values of the frequencies f1 and f2 can be adjusted by means of the potentiometers 39-42.
The output pulses from the oscillators 51 and 52 are applied to a block 60 including a unit extension-setting circuit and an extension limit-setting circuit. The output pulses from the oscillators 51 and 52 are first applied as one input to OR-elements OR3 and OR4, respectively. A predetermined voltage is applied to a terminal V and thence through a switch 61 to an OR-element OR5 as well as to the OR-elements OR3 and OR4. The switch 61 is opened when it is desired to control the traffic signal by the system of the invention on the basis of the information about the traffic congestion on the crossing streets. A signal appears on a line 62 from signal controller 73' upon termination of the extension limit or when there is no more vehicles to be detected during the extension and disappears upon termination of the minimum initial portion. In other words, there is no signal on the line 62 during the extended period of time of the green signal. The signal on the line 62 is also applied to the OR-elements OR3-OR5.
The outputs from the OR-elements OR3-OR5 are applied to NOT-elements 63-65, respectively. Therefore, the NOT-elements 65 produces an output when the switch 61 is opened and at the same time there is no signal on the line 62. This output from the NOT-element 65 is applied as an inhibit input to INHIBIT-elements 66 and 67. A voltage of a predetermined frequency, say, 60 c./s. is applied to a terminal 68 and thence as an input to the INHIBIT-elements 66 and 67 through a resistor 69 and a rectifier 70. The output from the INHIBIT-element 66 and the output from the NOT-element 63 are applied to a unit extension-setting counter 71 through an OR-element OR6; while the output from the INHIBIT element 67 and the output from the NOT-element 64 are applied to an extension limit-setting counter 72 through an OR-element OR7. The outputs produced by the counters 71 and 72 when they have counted up are applied to a "phase"-shifting counter 73 through an OR element OR8. A suitable pulse generator, not shown, applies phase-shifting pulses to the counter 73 through the OR-element OR8. As the counter 73 counts the input pulses, it steps forward shifting its output from one of its output terminals onto the next. These output signals from the counter 73 may be utilized as phase-shifting signals to change one phase to the next by application to signal controller 73' which in turn directly controls the traffic signal at the intersection The counter 71 is reset by a reset signal applied through a reset line R1 upon detection of every one vehicle by a vehicle detector 71' also located on the major street; and the counter 72 is reset by a reset signal applied thereto through a reset line R2 by signal controller 73' upon change of one phase to the next.
In operation, the output voltage from the circuit 10, that is, the voltage corresponding to the degree of congestion on the major street, is applied to the subtracting circuit 22, which subtracts from the input voltage the voltage corresponding to the normal volume of traffic on the major street. When there is no traffic congestion on the major street, the oscillators 51 and 52 oscillate at a normal frequency f0 (see FIG. 4), thereby setting a lower limit to the length of the unit extension and also that of the extension limit. The input voltages to the amplifiers 3114 35 are regulated by the variable resistors 35-38, respectively. These resistors determine the ratio of change of the oscillating frequencies of the oscillators 51 and 52 within a range exceeding the minimum degree of congestion as shown at P1 in FIG. 4. This determination is made in view of the relation between the degree of congestion and the actual difficulty with which vehicles pass the intersection. The inputs to the oscillators 51 and 52 are regulated by the potentiometers 39-42, so that the frequencies f1 and f2 are set for the degree P2 of traffic congestion. In other words, upper limits are set to the variable lengths of the unit extension and the extension limit which may increase as traffic congestion aggravates.
Suppose that the degree of traffic congestion on the major street has exceeded the point P1 in FIG. 4. As the degree of congestion increases, the oscillating frequencies of the oscillators 51 and 52 decrease as shown by the curve B in FIG. 4. When the switch 61 is open and there is no signal on the line 62, the output from the oscillator 51 is applied to the counter 71 through the OR-element OR3, the NOT-element 63 and the OR-element OR6. As the output oscillating frequency of the oscillator 51 decreases, the time required for the counter 71 to have finished counting all the predetermined number of input pulses becomes longer than when the degree of congestion is below P1. This means that the unit extension has now become longer than before. When the unit extension has thus been lengthened, the right of way is given to the major street for a longer period of time than otherwise, thereby helping causing the traffic congestion to disappear.
If, before the counter 71 has counted up, a succeeding vehicle is detected and a reset signal is applied to the reset terminal R1 thereof, the counter 71 is reset to resume counting the input pulses from the beginning. Therefore, it is only when the interval between two successive reset inputs to be applied to the reset terminal R1 (that is between two succeeding vehicles approaching the intersection) has exceeded the period of time required for the counter 71 to have counted up that the counter produces an output signal to change phases.
On the other hand, the period of time for the other counter 72 to have counted up also becomes longer, so that the extension limit is also lengthened, thereby helping cause the congestion on the major street to disappear.
When the unit extension has been lengthened, the extension limit must also be lengthened. Therefore, in the illustrated embodiment the two counters 71 and 72 are provided. However, there may be a case in which the extension limit must be lengthened while the unit extension need not be. In such cases, only one of the two counters may be employed.
When the right of way is not given to the major street, that is, when no signal 1G is being applied to the terminal 49, the oscillators 51 and 52 receive the outputs from the amplifiers 32 and 34, respectively. In this case, as the congestion on the major street increases, the outputs from the amplifiers increase until the congestion has exceeded the point P1 in FIG. 4, whereupon the oscillating frequencies of the oscillators 51 and 52 begins to increase as shown by the curve A in FIG. 4, so that the time it takes for the counters 71 and 72 to have counted up becomes shorter and shorter. This means that both the unit extension and the extension limit of the green signal being displayed to the minor street is shortened. In other words, the time during which the right of way is given to the minor street is shortened so that the right of way is transferred to the major street sooner than otherwise, thereby helping congestion thereon to disappear sooner.
In the arrangement of the above embodiment, the traffic congestion on the major street is detected so as to remove the congestion therefrom either when the right-of-way is given to the major street or when it is not, regardless of the traffic condition on the minor street. However, there are cases in which while the right-of-way is being given to one of the crossing streets, the traffic congestion on the other street must be taken into consideration. That is, if the traffic congestion increases on the other street, the right-of-way is transferred from the one street to which it has until then been given to that other street so as to remove the congestion therefrom. To this end, the level detector 81 is added to the system of FIG. 1. The detector 81 receives an an input the output from the circuit 20. The output from the NOT-element 47 is applied as an actuating signal to the detector 81. Therefore, while the signal from the NOT-element 47 is being applied to the level detector 81, that is, when the right of way is being given to the minor street, and not to the major street, the level detector detects the level of the output signal from the circuit 20, that is, the degree of traffic congestion on the major street. When the traffic congestion has reached a predetermined degree, the level detector 81 produces an output to be directly applied to the OR-element OR8, so that the counter 73 has its output shifted, thereby transferring the right-of-way from the minor street to the major street.
In order to remove the congestion on the minor street while the right-of-way is being given to the major street, a congestion detector similar 10' to that 10 in FIG. 1 can also be provided on the minor street, as shown in FIG. 3, so that the output from the congestion detector 10' is applied to a level detector 81' which operates when the right-of-way is given to the major street, and when the input to the level detector 10' has exceeded a predetermined level, the output produced by the level detector may be applied to the OR-element OR8. In short, when the degree of traffic congestion on the street to which the right-of-way is not given has exceeded a predetermined level, the right-of-way is transferred to that street so as to remove the congestion therefrom.
Now turning to FIG. 5, there is shown an arrangement that the degree of traffic congestion on both the major and the minor streets are compared, so that the congestion on that street the degree of which is greater is first removed. In FIG. 5 the same reference numerals as in FIGS. 1 and 2 denote corresponding parts. The numeral 100 denotes a circuit similar to the circuit 10 in FIG. 1, which detects the degree of traffic congestion on the minor street to produce a corresponding output voltage signal. The output is applied as one input to a voltage comparator 110, to which the output from the circuit 10 is applied as another input. The comparator 110 compares the two inputs and produces an output signal corresponding to the difference or ratio between the two input signals. The output signal changes as time passes. Let it be assumed that the output signal changes as shown by the curve in FIG. 8. Between tand t2, and t3 and t4 the degree of traffic congestion on the major street is greater than that on the minor street, and between t2 and t3 the degree of traffic congestion on the minor street is greater than that on the major street. In the former case (between 11 and t2, and t3 and t 4), the output (the difference voltage) from the comparator 110 appears on a line 112, but no signal is produced on a line 111. In the latter case (between t2 and t3) the difference voltage appears on the line 111 but not signal is produced on the line 112. At t1, t2, t3 and t4 when the difference voltage is zero, no signal appears on either the line 111 or 112. The signals on the lines 111 and 112 are applied to subtracting circuits 113 and 114, respectively.
The substracting circuit 113 subtracts the voltage on the line 111 from the output voltage from the circuit 10 and produces an output corresponding to the result of subtraction; while the subtracting circuit 114 subtracts the voltage on the line 112 from the output voltage from the circuit 110 and produces an output corresponding to the result of the subtraction.
The output from the subtracting circuit 113 is applied to amplifiers 131 and 132, and the output from the subtracting circuit 114 is applied to amplifiers 133 and 143. The outputs from the amplifiers 131-134 changes in inverse proportion to the input thereto just as in the case of the amplifiers 31 and 33 in FIG. 1. The amplifiers 131 and 132 operate while the signal 1G is being applied thereto by signal controller 73' , with the right of way being given to the major street; and the amplifiers 133 and 134 operate while the signal 2G is being applied thereto by signal controller 73', with the right-of-way being given to the minor street. The outputs from the amplifiers 131 and 133 are applied to the oscillator 51 and the outputs from the amplifiers 132 and 134, to the oscillator 52.
The oscillating frequencies of the oscillators 51 and 52 vary with the degree of traffic congestion as shown by the curve C in FIG. 6, just as in the case of the curve B in FIG. 4. The other circuit connections of FIG. 5 are the same as in FIG. 1, with the exception that the reset terminal of counter 71 is connected through reset line R1 to vehicle detector 71a, 71b for the major and minor streets of the intersection.
In operation, the comparator 110 compares the voltages corresponding to the degrees of traffic congestion on the major and minor streets. If the degree of congestion on the major street has been found greater than that on the minor street, the difference voltage is applied to the subtracting circuit 114. As previously mentioned, the output from the subtracting circuit 114 is the output voltage from the detector 100 minus the output voltage on the line 112. On the other hand, the output from the other subtracting circuit 113 is the output voltage from the detector 10.
Suppose that the above conditions have come into existence while the right-of-way is being applied to the major street. Then, the amplifiers 131 and 132 alone operate to produce an output since the signal 1G is applied to the amplifiers 131 and 132. In other words, the voltage corresponding to the output from the detector 10 is applied to the oscillators 51 and 52, the output frequencies of which will then decrease, so that both the unit extension and the extension limit of the green signal being displayed to the major street are lengthened as previously explained with reference to FIGS. 1 and 2.
Suppose that the previously mentioned conditions have occurred while the right-of-way is being applied to the minor street. Then, the amplifiers 133 and 134 alone operate to produce an output since the signal 2G is now being applied to the amplifiers 133 and 134. In other words, the output voltage from the detector 100 minus the voltage on the line 112 is applied to the oscillators 51 and 52, the output frequencies of which will then decrease as shown by the curve D in FIG. 7. It is seen that in the curve D the decreasing change in the output frequency takes place more gradually with respect to the degree of congestion than in the curve C which results from the output from the detector 100 alone. It may be mentioned that the curves C and D presupposes that the difference voltage on the line 112 remains unchanged. The decrease in the oscillating frequencies of the oscillators 51 and 52 results in the increase in the length of the unit extension and the extension limit of the green signal being displayed to the minor street. In this case, however, the degree of stagnation on the major street which has been found greater than that on the minor street is taken into consideration. That is, the increase in the lengths of the unit extension and the extension limit are less than those which would result if the traffic congestion on the major street had not been taken into consideration or if there were no traffic congestion on the major street. In other words, the unit extension and the extension limit of the green signal being displayed to the minor street will be lengthened, but less than if no consideration is given to the traffic condition on the major street, so that the right-of-way is transferred to the major street sooner to remove the traffic congestion therefrom.