US3543232A - Traffic signal control system - Google Patents

Description

Nov. 24, SHUNSUKE IWAMOTO ETAL 3,43,

TRAFFIC SIGNAL CONTROL SYSTEM I Filed April 1, 1968 Q mm Qwwwm .& \m a? m I MH \Q Q Q .v N. m \Q. 1 M K w M \m. R

,1 ATTORNEYS United States Patent US. Cl. 340-37 6 Claims ABSTRACT OF THE DISCLGSURE This invention provides a traffic signal control system which controls the period of time of green signal indication being displayed to one of the intersecting streets by the sum of the periods of time all the vehicles must wait that have been stopped by the red signal on the other street.

Trafiic control at a street intersection is usually performed by controlling traffic signal lamps. The sequential turning on and off of the signal lamps may be controlled in accordance with a predetermined program or in response to the trafiic condition on the streets. The traffic signal control which is performed in response to the traflic condition is commonly called the trafiic-actuated control, and there are many types of traffic-actuated control. In one of them, the duration of the green signal indication being displayed to one of the intersecting streets is controlled by (l) the period of time the first vehicle that has been stopped by the red signal on the other street must wait (this period of time will be referred to as time waiting, which is abbreviated to TW) and (2) the number of vehicles that will have been stopped on the other street during the red signal (this will be referred to as the number of vehicles waiting, which is abbreviated to NVW).

According to this system, when the TW and/or the NVW on the street to which a red signal is being displayed (this street will be referred to as the red signal street) has exceeded a predetermined value, the green signal being displayed to the other street (which will be referred to as the green signal street) is changed to red through an intermediate yellow indication. In other words, when a vehicle has been stopped by the red signal for a predetermined period of time, the signal is changed to green for the vehicle to pass through the intersection, or when a predetermined number of vehicles have been stopped on the red signal street, the red signal is changed to green. If the number of vehicles that are waiting (that is, the NVW) on a red signal street is less than the predetermined number, the red signal will not be changed until the maximum period of time of green signal indication has elapsed.

Thus, according to this prior art system, so long as the NVW remains less than a predetermined number, the trallic signal is controlled by the TW only, irrespective of the NVW, so that a sufiiciently smooth traffic control can hardly be expected.

Accordingly, the primary object of the invention is to provide a trafiic signal control system which utilizes, in addition to the above TW and NVW, a different traflic characteristic on the streets. This characteristic is the sum of the periods of time all the vehicles that have been stopped between the stop line and the vehicle detector (that is, in the detection area) on the red signal street at both sides of the intersection must wait for the signal to be changed to green. This will be abbreviated to STVW.

Another object of the invention is to provide a trafiic signal control system which controls traflic signal at a street intersection by both the STVW and the TW.

If only one vehicle has been stopped during the red signal, the STVW becomes equal to the TW. The STVW increases as the TW and NVW increase. If traffic signal is controlled in such a manner that when the STVW on the red signal street has exceeded a predetermined value, the signal is changed from red to green, the red signal lasts longer when the NVW is smaller, but it is shorter when the NVW is larger. This means that in accordance with the present invention, the green signal interval can be more fully utilized for a smoother control of traflic at the intersection than in prior art systems.

The system of the invention includes a circuit for measuring the STVW which comprises a condenser, a circuit for charging said condenser with pulses the width of which increases in proportion to the increase in the NVW, and a circuit operable in response to a predeterminel level of the charged voltage of said condenser. Suppose that when the value of the NVW is 1, that is, only one vehicle is waiting, the condenser is being charged with a series of pulses of a predetermined cycle. When the NVW is n, the charged voltage of the condenser can attain the same level in a shorter time by increasing the pulse width by n times. The STVW can therefore be expressed by the charged voltage level of the condenser. When the charged voltage has reached a predetermined level, the STVW has reached a predetermined value. Instead of being charged from ground potential, the condenser may be discharged from a predetermined level to which it has been charged beforehand.

An additional object of the invention is to provide a circuit for measuring the STVW by varying the rate of charge or discharge of the condenser by the NVW.

In trafiic signal control, the green signal may be extended by adding thereto a unit period of time (which will be referred to as a unit extension) for every one additional vehicle approaching the intersetcion on the green signal street to have actuated a vehicle detector thereon. If, within the unit extension, no succeeding vehicle has actuated the vehicle detector on the green signal street, the right of way is transferred to the other street.

To effect the above-mentioned extension of the green signal, pulse counters, charge-discharge circuits or the like, which are commonly called timers, may be employed. Such a timer may be connected to the vehicle detector, so that upon actuation of the detector, the timer begins to operate to provide a predetermined period of delay time. When the delay time has elapsed, the traffic signal is changed so that the right of way is transferred from one of the streets to the other. The delay time is variable and may be controlled by various traflic characteristics. In prior art systems, the TW and the NVW have been relied on to control the delay time. However, these traffic characteristics are not suitable for controlling the unit extension. First, the control of the unit extension to be added to the green signal by means of the NVW on the red signal street cannot be based on a proper traffic demand on the red signal street. Since the NVW does not contain any time factor, it cannot properly control the period of time of duration of green signal indication. Secondly, the TW concerns itself only with the first of the many vehicles that have been stopped by the red signal. Therefore, this factor is not suflicient for the purpose of controlling the unit extension, either.

Accordingly, a further object of the invention is to provide a traffic signal control system which controls the unit extension by means of the STVW.

In order to measure the STVW, a vehicle detector may be placed on each intersecting street a predetermined distance from the stop line in front of the intersection, so that time is measured from when the detector is actuated by a vehicle to when the vehicle stops at the stop line or behind the previous vehicle, and the periods of time thus measured for all the vehicles that have been stopped between the stop line and the detector are summed up to obtain a value of the STVW.

A preferred embodiment of the invention will be ex plained in detail with reference to the accompanying drawings, wherein;

FIG. 1 is a circuit diagram, partly in block, of the system of the invention;

FIG. 2 is a circuit diagram of the mono-stable multivibrators employed in the system of FIG. 1;

FIG. 3 is a circuit diagram of the circuit, employed in the system of FIG. 1, which is designed to render the output therefrom smaller as the input thereto becomes greater; and

FIG. 4 is a circuit diagram of the gate circuits employed in the system of FIG. 1.

The portion of the system which directly controls the sequential turning on and off of the signal lamps is so well known that it is not shown in FIG. 1. This portion of the system, which will be referred to as the signal lamp control, includes timers and controls the periods of time of duration of the green, yellow and red signal indications to be displayed to each of the streets crossing at an intersection.

In the following description, when traftic on one of the streets is more favoured than that on the other, the former street will be referred to as the major street and the latter, as the minor street. Also, when an electrical signal is produced by a circuit component or exists on a line, the condition will be referred to as a signal 1 having been produced by the circuit component on the line or the signal on the line having become 1; while when no signal is produced by a circuit component or exists on a line, or when a signal 1 on a line has disappeared, the condition will be referred to as a signal 0 having been produced by the circuit component or existing on the line or the signal 1 on the line having become 0. The signal 1 on a line may for example be the condition under which a voltage of +12 volts is being applied to the line, and the signal 0, the condition under which the line is grounded.

During the green signal indication the signal lamp control produces two electrical outputs, one of which is a signal G which continues to be 1 during the time the signal indication remains green; while the other is a signal G which is 1 during a predetermined minimum period of time of green signal indication, that is, the minimum initial portion of the green signal indication. The signal lamp control also produces electrical signals Y and R corresponding to the yellow and red signal indications, respectively. These electrical signals will be designated by their respective reference symbols prefixed with the numerals 1 and 2 when they are related to the major and minor streets, respectively. Thus, the signals 16 1, 1Y 1 and 1R 1 produced by the signal lamp control correspond to the green, yellow and red signal indications being displayed to the major street; while the signals 2G 1, 2Y 1 and 2R 1 correspond to the green, yellow and red signal indications being displayed to the minor streets. The signal 16' 1 and 2G 1 are produced by the signal lamp control during the minimum initial portions of the green indication being displayed to the major and minor streets, respectively.

These electrical signals are applied as inputs to the circuit of FIG. 1 at the terminals indicated by the same reference symbols as those of the signals. Three other inputs t, 1D and 2D are also applied to the circuit. The input t is the pulses of a predetermined cycle produced by a pulse generator, not shown. The signals 1D and 2D are the outputs from the vehicle detectors installed in the major and minor streets, respectively. The signals 1D and 2D become 1 every time the vehicle detectors detect one vehicle.

There are at least two vehicle detectors on each of the crossing streets at the opposite sides of the intersection so as to detect vehicles proceeding in the opposite directions. Therefore the outputs from the two detectors are parallelly applied to each of the terminals 1D and 2D. If each street comprises four lanes, two for outbound trafiic and the other two for inbound trafiic, four detectors are required for each street. In this case, the outputs from the four detectors may be applied to the terminal 1D or 2D through an OR element.

The circuit for measuring the TW will now be described. The input signals IR and 1D are applied to an AND element 21 comprising a pair of diodes 23 and 25 and a resistor 27 and the input signals 2R and 2D, to an AND element 22 comprising a pair of diodes 24 and 26 and a resistor 28. The resistors 27 and 28 are connected at one side to a positive potential source shown as a square with a plus sign enclosed therein. As is well known, the diode AND circuits 21 and 22 are such that when the two inputs are both 1 at the same time, the output signal therefrom becomes 1. Thus, with a red signal being displayed to the major street and consequently the input 1R being 1, whenever the detectors on the major street detect one vehicle, the signal 1D becomes 1 so that the output from the AND element 21 becomes 1. At this time, the input signal 2R remains 0, so that the output from the AND element 22 remains 0 regardless of the operation of the detectors on the minor street. On the contrary, with a red signal being displayed to the minor street and consequently the input signal 2R being 1, whenever the detectors on the minor street detect one vehicle, the signal 2D becomes 1 so that the output from the AND element 22 becomes 1. At this time the input signal IR is 0, so that the output from the AND element 21 remains 0 regardless of the actuation of the detectors on the major street.

The output signals 1" from the AND elements 21 and 22 are applied as an input to an OR element 29, the output signal 1 from which is applied as a set input signal to a flip-flop 31 on the one hand and a differentiator 49 on the other. Since it is when the detectors on the red signal street are actuated that the output signal from the OR element 29 becomes 1, the setting of the flip-flop 31 means that one of the detectors on the red signal street is actuated for the first time. In other words, the flip-flop 31 is set when a first vehicle has been detected on the red signal street.

The signals 1Y and ZY are applied as an input to an OR element 32, the output from which is applied as a reset input to the flip-flop 31. It is recalled that the signal 1Y is 1 while the trafiic signal being displayed to the major street is yellow, and that the signal 2Y is 1 with a yellow signal being displayed to the minor street. Thus, when the signal being displayed to the major or minor street is changed to red, the flip-flop 31 is in the reset condition ready to be set.

The set output from the flip-flop 31 is applied as one input to an AND element 33, tot he other input to which is applied the signal 2. Therefore, from the time the flip-flop 31 has been set, that is, when the first vehicle has been detected on the red signal street, the input pulses t are applied through the AND element 33 to a diiferentiator 34, which produces trigger pulses corresponding to the input pulses.

Each trigger pulse is applied to a pair of mono-stable multivibrators 35 and 57. As shown in FIG. 2, each monostable multivibrator includes a resistor 108 and a condenser 107 and is triggered by each trigger pulse to produce an output pulse the width of which is determined by the resistor 108 and condenser 107. The mono-stable multivibrators are provided so that condensers 72 and '73 to be described later in detail will be charged with voltage in the form of pulses to enable them to measure a longer time than otherwise.

The output from the mono-stable multivibrator 35 is applied as one input to a pair of AND elements 36 and 37 which receives as the other input the signals IR and 2R,

respectively. The AND element 36 comprises a pair of diodes 38 and 41 connected in the two input sides, respectively, and a resistor 43 connected to a positive potential source. Similarly, the AND element 37 comprises a pair of diodes 39 and 42 connected in the two input sides thereof and a resistor 44 connected to the positive potential source. When the signal 1R becomes 1 with the traffic signal being displayed to the major street being red, the AND element 36 produces an output signal 1 every time the output signal from the multivibrator 35 becomes 1. On the other hand, when the signal 2R becomes 1 with the traffic signal being displayed to the minor street being red, the AND element 37 produces an output "1 every time the output signal from the mono-stable multivibrator 35 becomes 1.

The output from the AND element 36 are applied to the condenser 72 through a variable resistor 45 and a diode 47. Similarly, the output from the other AND element 37 are applied to the same condenser 72 through a variable resistor 46 and a diode 48. The variable resistors 45 and 46 are for the purpose of varying the time it takes the condenser 72 to be charged to a predetermined voltage level. This level of the terminal voltage of the condenser 72 is determined by a Zener diode 74.

As the condenser 72 is charged, the unit extension is shortened until the condenser 72 is fully charged to the predetermined level, wehereupon the unit extension is shortened to a minimum. This will be referred to again in further detail later. The reason why the unit extension to be added to the green signal being displayed to one of the streets is thus shortened is because the TW on the other street the signal indication to which is red increases wih resulting increase in the demand of trafiic on the red signal street for passage through the intersection. Traflic on the major street is favoured more than that on the minor street. Therefore, the TW on the major street, that is, the period of time it takes the condenser '72 to be charged to the predetermined level is rendered shorter than that on the minor street. This is eifected by rendering the resistance value of the variable resistor 45 lower than that of the variable resistor 46.

When the trafiic signal being displayed to the major or minor street has been changed to red, that is, when the signal 1R 1 or 2R 1 has been applied to the circuit, it is required that the condenser 72 have already been discharged to zero voltage so as to be charged again. For this purpose a discharge circuit is provided comprising an OR element 83 and a gate circuit 84 connected between the one side of the condenser 72 and the diodes 47 and 48. The OR element 83 receives as an input the signal 1Y (or 2Y) and produces an output to be applied to the gate circuit 84.

As shown in FIG. 4, when the input signal to the gate circuit 84 is 1, its output is grounded. Since the red signal is preceded by a yellow indication, when the signal 1Y becomes 1, the gate circuit 84 has its output grounded, so that the opposite sides of the condenser 72 are grounded so as to be discharged.

The circuit for measuring the STVW will now be described. It is recalled that the output pulses produced by the OR element 29, one pulse upon detection of every one vehicle, are applied also to the difierentiator 49. The output from this diiierentiator 49 triggers a monostable multivibrator 51, the output from which is in turn applied to a condenser 54 through a resistor 52 and a diode 53, so that the charged voltage level of the condenser 54 increases with the output pulses from the OR element 29. Since the number of output pulses from the OR element 29 correspond to the number of pulses produced by the detectors on the red signal street and consequently the NVW on the red signal street, the terminal voltage of the condenser 54 increases with the NVW.

Just as in the case of the previously mentioned condenser 72, it is required that the condensed 54 should have. already been discharged when the trafiic signal is changed to red. For this purpose a discharge circuit is provided, comprising an OR element 113 and a gate circuit 114- of the same construction as the gate circuit 84. The red signal indication to the major or minor street is preceded by a yellow indication. When the signal has been changed to yellow, the input signal 1Y or 2Y to be applied to the OR element 113 becomes 1, so that the condenser 54 is discharged through the gate circuit 114 in the same manner as previously mentioned.

The terminal voltage of the condenser 54 is applied as an input to a hold circuit 55, the output from which is applied as an input to a circuit 56. As shown in FIG. 3, the circuit 56 includes a pair of transistors 109 and 112 and a Zener diode 111, and is arranged so that as the input to the circuit 56 increases, its output decreases. The output from the circuit 56 is applied as a bias input to a mono-stable multivibrator 57. As indicated by a dotted line in FIG. 2, the bias is applied intermediate the condenser 107 and the resistor 108 which determines the time constant of the condenser 107. When the bias is impressed on the condenser 107, the width of the output pulse from the mono-stable multivibrator 57 decreases with increase in the input voltage.

As previously mentioned, as the NVW increases, the terminal voltage of the condenser 54 increases and the output from the circuit 56 decreases, so that the width of the output pulses from the mltivibrator 57 increases. It is recalled that the flip-flop 31 is set when a vehicle detector on the red signal street has detected a first vehicle under the red signal indication, and that while the flip-flop is in the set condition, the differentiator 34 applies to the mono-stable multivibrator 57 a series of trigger pulses in the same cycle as that of the input pulses t applied to the AND element 33. The output pulses from the multivibrator 57 are applied to a condenser 73 to charge it, as will be described below in detail.

The output from the multivibrator 57 is applied as one input to AND elements 58 and 59, the other inputs to which are the signals IR and 2R, respectively. The AND element 58 comprises a pair of diodes 61 and 63 and a resistor 65 connected at one end to a positive potential source; and the AND element 59 comprises a pair of diodes 62 and 64 and a resistor 66 connected on one end to the positive potential source. As previously mentioned, the signal IR is 1" while the traffic signal being displayed to the major street is red. Under the condition, every time the mono-stable multivibrator 57 produces an output pulse, the AND element 58 produces an output pulse of the same width as the pulse produced by the mono-stable multivibrator 57. On the other hand, the signal 2R is 1 while the trafiic signal being displayed to the minor street is red. Under the condition, every time the mono-stable multivibrator 57 produces an output pulse, the AND element 59 produces an output pulse of the same width as the pulse produced by the multivibrator 57.

The output pulses from the AND element 58 are ap plied through a variable resistor 67 and a diode 69 to the condenser 73. Similarly, the output pulses from the other AND element 59 are applied through a variable resistor 68 and a diode 71 to the same condenser 73. The variable resistors 67 and 68 are provided to vary the time it takes the charged voltage of the condenser 73 to reach a predetermined level. The voltage level is determined by a Zener diode 75 connected in parallel with the condenser 73.

With a series of input pulses being applied to the condenser 73, its terminal voltage increases as time passes. As previously mentioned, the width of the pulses from the mono-stable multivibrator 57 increases as the NVW increases. This means that the terminal voltage of the condenser 73 increases as the STVW increases. If the arrangement is such that as the NVW increases by one, the width of the output pulses from the monostable multivibrator 57 increases by a predetermined length and, consequently, the terminal voltage of the condenser 73 increases in proportion to the pulse width, the terminal voltage of the condenser 73 expresses or measures the STVW. To enable this measurement, the circuit constants are properly selected.

As the terminal voltage of the condenser 73 is increased to a predetermined level, the unit extension is decreased to a predetermined minimum length, as previously mentioned with reference to the condenser 72. The reason why the unit extension is thus decreased is because the STVW on the red signal street is increased with resulting increase in the demand for the right of way on that street. With the same STVW, trafiic on the major street is favoured more than that on the minor street. Therefore, the arrangement is such that the condenser 73 is charged to the predetermined level with a less amount of STVW on the major street than on the minor street. This can be accomplished by setting the variable resistor 67 to a lower resistance value than that of the variable resistor 68.

When the trafiic signal has been changed to red, the condenser 73 must have its previously charged potential discharged so as to be ready to be charged again. To this end, a discharge circuit is provided, which comprises the OR element 83 and a gate circuit 85 such as shown in FIG. 4. The input signal 1Y or 2Y becomes 1 immediately before the traffic signal is changed to red. This causes the output from the gate circuit 85 to be grounded, so that the condenser 73 is discharged before the signal 1R or 2R becomes 1.

When a demand for the right of way caused by the TW coincides with a demand for the right of way caused by STVW, selection must be made between the two. A circuit arrangement for this purpose will now be described below. A Zener diode 74 is connected in parallel with the condenser 72 for measuring the TW, and a Zener diode 75 is connected in parallel with the condenser 73 for measuring the STVW. The terminal voltage of the Zener diode 74 is applied as an input to a hold circuit 76, while the terminal voltage of the Zener diode 75 is applied as an input to a hold circuit '77. Since the terminal voltage of the condenser 72 increase as the TW increases, the input to the hold circuit 76 accordingly increases. However, even when the terminal voltage of the condenser 72 exceeds that of the Zener diode 74, the input to the hold circuit 76 does not exceed the terminal voltage of the Zener diode 74. Similarly, the terminal voltage of the condenser 73 increases as the STVW increases, so that the input to the hold circuit 77 accordingly increases. However, because of the Zener diode 75, the input to the hold circuit 76 does not exceed the terminal voltage of the Zener diode 75 even when the terminal voltage of the condenser 73 has exceeded that of the Zener diode 75. The outputs from the hold circuits 76 and 77 exponentially increase with their respective inputs.

The output voltages from the hold circuits 76 and 77 are divided by variable resistors 78 and 79, respectively. The variable resistors are for the purpose of applying a bias to a condenser 104. The condenser 104 has a predetermined time constant and is charged by a positive potential source from the bias level up to a predetermined higher level. Therefore, the higher is the bias voltage level taken out from the variable resistor 78 or 79, the less time it takes the condenser 104 to be charged to the predetermined level. This will be referred to again in detail later.

The voltages taken out from the resistors 78 and 79 are applied through diodes 81 and 82, respectively, to a hold circuit 86. Therefore, the hold circuit 86 chooses as its input the higher of the two voltages.

The unit extension is continuously added to the green signal upon every actuation of the vehicle detectors on the green signal street. However, the unit extension is 8 shortened as the TW or the STVW on the red signal street increases. A circuit arrangement for effecting this result will now be described.

The condenser 104 is biased by the output from the hold circuit 86 through a diode 87 and at the same time is charged by a positive potential source through a resistor 102 and a diode 103. The resistor 102 determines the time constant of the condenser 104. A gate circuit 101 has its output connected to the condenser 104 and its input connected to AND elements 88 and 89 through diodes 95 and 96, respectively, and also to an OR element 99. The OR element 99 receives as its input the signals 1Y, 2Y, 1G and 26. As previously mentioned, the signals 1Y and 2Y are 1 while the traffic signal indication to the major and minor streets, respectively, is yellow. Therefore, when the signal 1Y or 2Y becomes 1, the condenser 104 is discharged. The signals 1G and 2G are 1 during the minimum initial portion of the green signal being displayed to the major and minor streets, respectively. The lengths of the minimum initial portions and consequently the periods of time of duration of the signals 16 and 26' are fixed because they are set in the signal lamp control device separate from the circuit shown in FIG. 1, independently of the operation of the vehicle detectors. When the signals 1G or 2G become 1, the condenser 104- is discharged through the gate circuit 101. Presently, after the fixed period of time, the signal 16 or 26 becomes 0 so that the condenser begins to be charged again until the signal 1Y or 2Y again becomes 1. Thus, the condenser 104 is alternately charged and discharged, so that the green signal is extended by the unit extension added thereto, as will be described below.

The AND element 88 comprises a pair of diodes 91 and 93 and a resistor 97 connected to a positive potential source and the AND element 88, a pair of diodes 92 and 94 and a resistor 98 connected to the positive potential source. The signals 16 and ID are the inputs to the AND element 88 while the signals 2G and 2D are the inputs to the AND element 89.

During the green signal indication being displayed to the major street the signal 1G is 1. Under the condition, every time the vehicle detectors on the major street are actuatde the signal 1D becomes 1, so that the output from the gate circuit 101 is grounded with resulting discharge of the condenser 104. Similarly, when the signal being displayed to the minor street is green, the signal 26 is 1, and under the condition every time the vehicle detectors on the minor street are actuated to render the signal 2D 1, the output signal from the AND element 89 becomes 1, so that the output from the gate circuit 101 is grounded with resulting discharge of the condenser 104.

The instant the condenser has been discharged, it is biased again by the output from the hold circuit 86 applied thereto through the diode 87 and then charged again with the positive potential coming through the diode 103.

The terminal voltage of the condenser 104 is applied as an input to a hold circuit 105, the output from which is applied to a Schmitt circuit 106. As is well known, when the input to the Schmitt circuit exceeds a predetermined level, it produces an output B. The arrangement is such that it is only when the vehicle detectors on the green signal street are not actuated that the condenser 1041 continues being charged, until its terminal voltage becomes high enough to cause the signal B to become 1. In other words, the signal B becomes 1 when there is no vehicle on the green signal street.

The signal B is utilized to change the traffie signal indication displayed to the major and minor streets. Suppose, for example, that the signal being displayed to the major street is green now. When the signal B becomes 1, the signal indication to the major street is changed to red through an appropriate interval of yellow and at the same time the signal indication to the minor street is changed from red to green. The yellow signal may be preceded by winking of the green signal lamp a couple of times. When there is a maximum period of time of duration of the green signal, the signal is changed upon lapse of the maximum period of time even when the signal B is yet 0.

The operation of the system will now be described. Suppose that the traflic signal indication to the major street is green now. When the output signal B from the system becomes 1 or when the maximum period of time of duration of the green signal indication has elapsed, the signal indication is changed from green to yellow, so that the input signal 1Y becomes 1. This signal is applied to the OR elements 32, 83, 113 and 99. The output from the OR element 32 resets the flip-flop 31; the output from the OR element 83 causes the condensers 72 and 73 to be discharged through the gate circuits 84 and 85, respectively; the output from the OR element 113 causes the condenser 54 to be discharged through the gate circuit 114; and the output from the OR element 99 causes the condenser 104 to be discharged through the gate circuit 101. Thus, the system has been reset to the original condition.

After the predetermined period of time set for the yellow signal, the signal indication is changed to red to the major street and green to the minor street. As a result, the signals 1R, 2G and 2G become 1. The signal 26' l is applied through the OR element 99 to ground the output from the gate circuit 101 so that the condenser 104 has its opposite sides grounded. This means that during the minimum initial portion of the green signal indication while the signal 2G remains l, the condenser 104 can be neither charged nor discharged. At this time, however, the signal 1R becomes 1 so that the AND element 21 produces an output pulse every time the other input signal 1D becomes 1 upon detection of every one vehicle on the major street.

When the first vehicle is detected on the major street to which the signal indication is red, the signal 1D becomes 1, so that the output signal from the AND element 21 becomes 1 and set the flip-flop 31. While the set output from the flip-flop 31 continues to be 1, the input pulses t are applied to the ditferentiator 34 through the AND element 33 so as to provide a series of trigger pulses to be applied to the mono-stable multivibrators 53 and 57.

When the signal IR is 1, the signal 2R is 0, so that the condenser 72 is charged with the output pulses from the AND element 36 through the resistor 45. As time passes, the terminal voltage of the condenser 72 increases, with resulting increase in the bias voltage to be applied to the condenser 104. It is recalled that the condenser 72 is now measuring the TW on the major street. Simultaneously with this measurement of the TW, the STVW is also measured in the following manner. With the signal 1R being 1, upon detection of every one vehicle on the major street the OR element 29 produces an output signal 1, which triggers the mono-stable multivibrator 51. The output from the multivibrator 51 is applied through the resistor 52 to the condenser 54 to charge the same. As the number of vehicles that are detected, that is the NVW, on the red signal street increases, the output voltage from the AND element 56 decreases, thereby increasing the width of the pulses produced by the mono-stable multivibrator 57. Since the signal IR is l and consequently the signal 2R is 0, the condenser 73 is charged with the pulses applied thereto through the AND element 58 and the resistor 67 and the diode 69. Thus, the terminal voltage of te condenser 73 increases as time passes and the NVW on the major street increases.

When the minimum initial portion of the green signal being displayed to the minor street has passed, the signal 2G becomes 0, so that the condenser 104 is biased by the voltage caused by the TW or the STVW to be 10 applied thereto through the diode 87, and at the same time charged with the positive potential applied through the diode 103.

Since the signal 1G is 0 and the signal 2G is 1, every time a vehicle detector on the minor street is actuated to render the signal 2D 1, the output from the AND element 89 becomes 1 so that the condenser 104 is discharged. So long as the condenser 104 is discharged by the actuations of the vehicle detectors on the minor street before its terminal voltage has reached a level high enough to actuate the Schmitt circuit 106, the output signal B therefrom remains 0.

As the TW or the STVW increases, the input to the hold circuit 86 is changed from one of the voltages applied through the diodes 81 and 82 to the other, depending upon by which of the TW and the STVW the trafiic demand on the major street is caused to increase. In either case, the output from the hold circuit 86 to be applied as a bias to the condenser 104 increases, thereby decreasing the time required for the condenser 104 to be charged to a predetermined voltage level. In other words, as the TW or STVW increases on the major street, the unit extension is shortened so that the right of way is transferred to the major street unless a succeeding vehicle approaches the intersection on the minor street with a smaller headway than before.

When the terminal voltages of the condensers 72 and 73 have reached the Zener voltages of the Zener diodes 74 and 75, respectively, the bias voltage to be applied to the condenser 104 as determined by the resistors 78 and 79 has reached a predetermined level. As a result, the unit extension is shortened to a predetermined minimum length. In this case, in order that the unit extension is reduced to the minimum length by the STVW the resistors 78 and 79 are adjusted so that the bias voltage obtained through the resistor 79 are adjusted so that the bias voltage obtained through the resistor 78.

If a succeeding vehicle approaches the intersection on the minor street with a headway longer than the unit extension as determined by the bias voltage impressed on the condenser 104, the condenser will have been charged to a voltage level high enough to actuate the Schmitt circuit 106 so that the output signal B becomes 1 and consequently the green signal being displayed to the minor street is changed to yellow.

When the signal to the minor street has thus been changed to yellow, the signal 2Y becomes 1 so that the flip-flop 31 is reset with resulting discharge of all the condensers in the circuit.

Presently, the yellow signal to the minor street is changed to red, and at the same time the signal to the major street is changed from red to green, whereupon the unit extension to be added to the green signal displayed to the major street will be controlled by the TW or the STVW on the minor street in a manner similar to the above mentioned manner, so that no further explanation will be required.

Having illustrated and described a preferred embodiment of the invention, it is understood that the invention is not limited thereto and that there are many changes and modifications thereof without departing from the scope of the invention as defined in the appended claims. For example, instead of the resistors and condensers as timers, timers of a digital type such as pulse counters may also be employed. As traffic characteristics to be relied on for control purpose, traflic density, the speed of vehicles, etc. may also be utilized in addition to the TW and the STVW. A different detector may be placed near the stop line for the exclusive purpose of measuring the TW.

What we claim is:

1. A system for controlling traflic signal at a street intersection, comprising: first detecting means for detecting vehicles approaching said intersection on a red signal street; means for measuring the sum of the periods of time all the vehicles that have been detected by said first 1 l detecting means must Wait before said intersection; second detecting means for detecting said sum having reached a predetermined value; and means operable in response to said second detecting means to control the period of time of duration of the green signal being displayed to a green signal street.

2. A system for controlling trafiic signal at a street intersection, comprising: first detecting means for detecting vehicles approaching sad intersection on a red signal street; first measuring means for measuring the sum of the periods of time all the vehicles that have been detected by said first detecting means must wait before said intersection; second detecting means for detecting said sum having reached a predetermined value; second measuring means for measuring the period of time the first of said vehicles that have been detected by said first detecting means must wait before said intersection; third detecting means for detecting said period of time of waiting of said first vehicle having reached a predetermined value; and means operable in response to one of said second and third detecting means to control the period of time of duration of the green signal being displayed to a green signal street.

3. A system for controlling traific signal at a street intersection, comprising: first detecting means for detecting vehicles approaching said intersection on a red signal street; means for measuring the sum of the periods of time all the vehicles that have been detected by said first detecting means must Wait before said intersection; second detecting means for detecting vehicles approaching said intersection on a green signal street; means for extending the green signal being displayed to said green signal street by continuously adding thereto a predetermined unit extension per vehicle detected by said second detecting means; and means operable in response to said measuring means to vary said unit extension.

4. A system for controlling traffic signal at a street intersection, comprising: first detecting means for detecting vehicles approaching said intersection on a red signal street; first measuring means for measuring the sum of the periods of time all the vehicles that have been detected by said first detecting means must Wait before said intersection; second measuring means for measuring the period of time the first of said vehicles that have been detected must Wait before said intersection; second detecting means for detecting vehicles approaching said intersection on a green signal street; means for extending the green signal being displayed to said green signal street by continuously adding thereto a predetermined unit extension per vehicle detected by said second detecting means; and means operable in response to one of said first and second measuring means to vary said unit extension.

5. A system for controlling trafiic signal at a street intersection, comprising: a condenser; first circuit means for providing a pulse voltage to vary the terminal voltage of said condenser; second circuit means for controlling the pulse width of said pulse voltage by the number of vehicles that are stopped before said intersection on a red signal street to wait for the signal to be changed to green; and third circuit means operable in response to the terminal voltage of said condenser to vary the period of time of duration of the green signal being displayed to a green signal street crossing said red signal street.

6. The system of claim 5, wherein said first circuit means comprises a multivibrator the output pulse Width of which decreases with increase in the control input thereto, and wherein said second circuit means comprises a first circuit the output from which increases with increase in said number of vehicles Waiting and a second circuit which receives as an input the output from said first circuit and produces an output which decreases with increase in said output from said first circuit, the output from said second circuit being utilized as said control input to said multivibrator.

References Qited UNITED STATES PATENTS 3,464,059 8/1969 Brockett 340-37 THOMAS B. HABECKER, Primary Examiner US. Cl. X.R. 340-31

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