Patent GB-1220480-A (GB)
Title: Improvements in or relating to traffic signal control system
IPC: G08G1/08
Filed: 1968-04-01
Abstract: 1,220,480. Road signals for controlling traffic. TATEISI ELECTRONICS CO. 1 April, 1968 [8 April, 1967; 2 May, 1967], No. 15555/68. Heading G4Q. In a system for controlling traffic signals at a street intersection having means for detecting vehicles approaching the intersection the sum of the waiting periods of each of the vehicles that have been detected is measured and used to control the time of duration of the green signal being displayed to the green signal street. At least two vehicle detectors are used for each street so that the outputs from the detectors on the major street are connected in parallel to input 1D, similarly the detectors on the minor street are connected to 2D. If each street has four lanes then four vehicle detectors are employed. Assuming the major street signal is red and the minor street signal is green, the input from the major street red signal to 1R and on detecting a first vehicle cause the AND element 21 to pass a signal to set bi-stable 31 and via differentiator 49 triggers monostable circuit 51. The output from bi-stable 31 enables pulses from a pulse generator t to pass via AND element 33 and trigger monostable circuits 35, 57. The output pulses from monostable circuits 35, 57 are passed via AND elements 36 or 37 and 58 or 59, depending on whether the major or minor street signal is red, to charge capacitors 72, 73. The charge on capacitor 72 represents the waiting time of the first vehicle stopped at the red signal and the charge on the capacitor 73 represents the sum of the waiting periods of each of the vehicles detected at the stop signal. The voltages developed across these capacitors 72, 73 are limited by Zener diodes 74, 75. Each vehicle detected on the major road at the stop signal causes an output pulse from monostable circuit 51 so that the charge developed across a capacitor 54 represents the number of vehicles waiting at the stop signal. The output from this capacitor 54 is passed via a holding circuit 55 (Fig. 3, not shown), including two transistors (109, 112) coupled via a Zener diode (111) so that as the voltage across the capacitor 54 increases the output voltage of the holding circuit 55 decreases. This decreasing voltage is fed via AND element 56 to an input of a two transistor monostable circuit 57 (Fig. 2, not shown), so as to increase the width of the output pulse of the monostable circuit 57. This gives the required charging of the capacitor 73. The outputs from the capacitors 72, 73 are fed via holding circuits 76, 77 to diodes 81, 82 which choose the higher of these voltages for applying to a hold circuit 86. The higher of the two voltages across the capacitors 72, 73 is then used to increase the rate of charge of capacitor 104 and when the voltage across this capacitor 104 reaches a predetermined value Schmitt circuit 106 operates to change the unit extension of the green time. If the next vehicle on the green street is not detected before this unit extension ends the signals change so that for example the major street signal changes to green and the minor street signal to red. The capacitor 104 is however prevented from charging during the yellow period by signal 1Y, 2Y or during the initial green period by signals 1G<SP>1</SP>, 2G<SP>1</SP>. Also assuming that the major street signal is red and the charge on capacitor 104 has not reached the predetermined value then every time a vehicle is detected on the green minor street signals 2G and 2D associated with the green light and detector of the minor street enable AND element 89 to discharge the capacitor 104 which adjusts the unit extension so that the green time is extended. The traffic signals may be changed at the end of a maximum green period. The caapcitors 54, 72, 73 and 104 are discharged by gate circuits 114, 84, 85 and 101 when the yellow signal 1Y, 2Y occurs when the traffic signals change. Control of the green period of the major street is also effected by vehicles detected on the minor street when at red by the detector signals at 2D similar to the above. The major street may be given priority over the minor street by adjusting the resistors 45, 67 to be lower than resistors 46, 68 so that the time taken to charge the capacitors 72, 73 is less for the major road. The yellow signal may be preceded by winking the green signal lamp two times. Instead of the resistors and capacitors as timers, digital timers such as pulse counters may be used. Traffic density and the speed of vehicles may also be used to control the traffic. A different detector may be placed near the stop line for measuring the waiting time of the first vehicle to be detected. The gate circuits 84, 85, 114 may each include a transistor (Fig. 4, not shown).