1,223,443. Road traffic signals. LABORATORY FOR ELECTRONICS Inc. 2 April, 1968 [14 April, 1967], No. 15767/68. Heading G4Q. A traffic control system enabling traffic on a minor road 18 to enter a three-lane expressway 10 by means of an entrance ramp 16 and merging area 20, includes a traffic signal 24 situated approximately mid-way along the ramp to allow vehicles to enter the area 20 one at a time when there is capacity on the expressway to take extra traffic. Vehicle detectors monitor traffic flow on expressway 10, ramp 16 and merging area 20. Thus, detectors D1, D2, D3 upstream of the area 20 each indicate the passage of a vehicle and also its speed and detectors D4, D5, D6 downstream of the area perform similar functions. Detector D7 in the right-hand lane just upstream of the merging area 20 indicates the vehicle speed. Detector D8 in the area 20 indicates the vehicle speed and gives a continuous output when a vehicle stops in its zone of detection. Detector D9 just in front of the signal 24 indicates the presence of a vehicle. Detectors D1 . . . D9 are connected to circuitry, Figs. 2A, 2B, these Figures fitting side-by-side as in Fig. 5. Detectors D2, D5, D7, D8 are connected to computers 30, 32, 34, 36, each of which generates an output indicative of the average speed of the vehicles passing its detector. Each computer 30, 32, 34, 36 is connected to a level monitor circuit 38, 40, 42, 44 providing discrete outputs indicative of the level of the input signal, each output corresponding to a certain range of vehicle speed. Thus, for example, level monitor 38 has three output lines, indicated by line 46, related to three speed ranges, whereby if the speed of vehicles passing detector D2 is within a first range level monitor applies a signal to AND gate 48, if the speed is within a second range a signal is applied to a timer 50 and if the speed is within a third range a signal is applied to AND gate 52. The level monitor 40 similarly has three output lines 54 respectively supplying signals to a selector 56, a timer 58 and AND gate 52. The level monitor 42 similarly has three outputs supplied, respectively, to a line 60, a line 62 and AND gate 48. Level monitor 44 has eight output levels indicative of the speed of vehicles passing the detector D8, each of these outputs being associated with an individual voltage control component in a weighting unit 64 to determine the potential on an output line 65. Computer 34 is also connected to a differentiator 35 coupled through an inverter 37 to a weighting unit 39. When AND gate 48 receives signals on both its inputs corresponding to the detection of low vehicle speed by both detectors D2, D7 it applies a signal to timer 68. Timer 68 times a minimum duration for this condition and then turns on switching unit 70 applying a potential on line 71. When AND gate 52 receives a high vehicle speed signal from detector D2 and a low vehicle speed signal from detector D5 it applies a signal to timer 72. Timer 72 times a minimum duration for this condition and then turns on switching unit 74 applying a potential to line 75. Detector D8 applies a signal to timer 76 so long as a vehicle is within its zone. Timer 76 times a minimum duration for this condition and then turns on switching unit 78 applying a potential on line 79. Timer 50 receives a signal from the detector D2 when the vehicle speed is within the low speed range. If this condition continues for a sufficient period, the timer 50 applies a signal to switching unit 80 causing a signal to be applied on line 81. Similarly, timer 58 receives a signal from detector D5 when the vehicle speed is within the low speed range and the timer energizes switching unit 82 to apply a signal on line 83. The detectors D1 . . . D6 are connected to computers 84, 86, 88, 90, 92, 94, respectively, each of which generates an output indicative of the volume of traffic passing its detector. The outputs of computers 84, 86, 88 are applied to a summing amplifier 96, this amplifier, because of its inherent inversion characteristic, producing an output voltage representing the negative of the total upstream traffic volume demand on expressway 10 passing the vehicle detectors D1, D2, D3. Similarly, a summing amplifier 98 having its inputs connected to computers 90, 92, 94 produces an output voltage representing the negative of the total downstream traffic volume passing detectors D4, D5, D6. The output of amplifier 98 is inverted by an amplifier 100 so that the latter gives an output voltage representing the total downstream traffic volume, the outputs of amplifiers 96, 100 being applied to a summing amplifier 102 which gives an output voltage indicative of the difference between the upstream and downstream traffic volume. The output of amplifier 102 is applied to a level monitor 104 having two outputs corresponding to two ranges of the difference between the upstream and downstream traffic volumes. When the volume difference is within a first range the level monitor applies an output to timer 106 which times a minimum duration for this condition and then turns on switching unit 108 to apply a potential to line 109. When the volume difference is within a second range the level monitor 104 applies an output on line 110. The outputs of switching units 70, 74, 80, 82, 108 and of weighting units 64, 39 are connected to switches 118, 120, 122, 124, 126, 128, 130, respectively, the outputs of the switches being connected as inputs to a summing amplifier 132. The output of a scaling unit 116 is also connected to the amplifier 132, this output being a voltage representing an empirical value of traffic volume capacity for the expressway 10. The output of each of the switching units 70, 74, 80, 82, 108 and of weighting units 39, 64 is a voltage representing a parameter indicative of congestion on the expressway 10 which results in variation of this empirical value. Thus, for example, congestion may be indicated by the low speed of vehicles passing detector D5, whereby level monitor applies an input to timer 58 which turns on switching unit 82. Switching unit 82 then applies a voltage through switch 124 in its closed position to reflect this reduction in capacity of the expressway. Continued build up of congestion downstream of ramp 16 results in a decrease of the traffic volume passing detectors D4, D5, D6 relative to the traffic volume passing detectors D1, D2, D3. This difference in traffic volume is given by the output of summing amplifier 102 and level monitor 104 turns on timer 106 when this volume difference indicates that traffic congestion is commencing to build up downstream of the entrance ramp 16. If this congestion continues for the period for which timer 106 is set, the timer causes switching unit 108 to apply a voltage through the closed switch 126 to summing amplifier 132 to reflect this indication of reduced expressway capacity. If the downstream congestion continues to build up, the average speed of traffic passing detector D7, drops and the output of differentiator 35 is a negative voltage, the output being passed through inverting amplifier 37 to weighting unit 39. The output of unit 39 is applied through switch 130 to the amplifier 132 to reflect a decrease in expressway capacity. The speed of vehicles passing through the merging area 20, as sensed by detector D8, reflects the ease with which these vehicles are able to merge into the traffic on the expressway. If this speed is low it indicates that it is difficult for the vehicles to merge and thus a greater capacity should be indicated before merging is permitted. The vehicle speed in the merging area 20 produces an output from the monitor 44 having eight levels connected to the weighting unit 64. If the vehicles are passing rapidly through the area 20 unit 64 applies a -ve voltage through switch 128 to the amplifier 132 to increase the indicated capacity of the expressway. If the vehicles are passing with difficulty through the area 20 unit 64 applies a +ve voltage to amplifier 132 to reduce the indicated capacity. If the vehicle speed passing detector D2 is high while the vehicle speed passing detector D5 is low, congestion has commenced downstream of the ramp 16 but has not reached the detectors D1, D2, D3. In this condition level monitors 38, 40 apply inputs to AND gate 52 which turns on timer 72 to apply a voltage through switching unit 74 and switch 120 to the amplifier 132 to reflect the reduced expressway capacity. If the congestion reaches detectors D1, D2, D3, the speed of vehicles passing detector D2 drops to a low level and monitor 38 applies a signal to timer 50. The timer applies a signal through switching unit 80 and switch 122 to the amplifier to reflect the reduced capacity of the expressway. When the speed of vehicles passing detectors D2, D7 is low due to high traffic congestion, AND gate 48 turns on timer 68 which causes switching unit 70 to apply a voltage through switch 118 to the amplifier 132 to reflect a further reduction in capacity of the expressway. Thus the output of summing amplifier 132 represents the instantaneous traffic volume capacity of the expressway 10 and this output is applied through a selector 56 to an input of summing integrator 134. The other input of integrator 134 has applied thereto the output of summing amplifier 96 representing the negative of the traffic volume demand of vehicles passing the detectors D1,D2,D3. The output of integrator 134 is the time integral of the difference between the instantaneous traffic volume capacity of the expressway and the upstream traffic volume demand. Thus, the output of integrator 134 reflects the accumulated available unused capacity on expressway 10 and this output is applied via line 137 to a controller 136 controlling the traffic signal 24 via line 139. When the time integral indicates sufficient capacity exists on the expressway for a vehicle on the ramp 16 to mer
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