Patent GB-560488-A (GB)
Title: Improvements in traffic control apparatus
IPC: G08G1/08
Filed: 1940-10-28
Abstract: 560,488. Traffic control systems. AUTOMATIC SIGNAL CORPORATION. Oct. 28, 1940, No. 15760. Convention date, Nov. 20, 1939. [Class 118 (ii)] In a traffic-control system in which the arrival at an intersection of each vehicle in the open lane prolongs the right of way by a small time interval or " vehicle-interval," means are provided whereby this interval is progressively reduced (a) if a fleet of closely-spaced vehicles passes the crossing and optionally (b) in accordance with the time the leading vehicle in the barred lane has been waiting as claimed in Specification 560,527, and preferably also (c) in accordance with the number of vehicles waiting in the barred lane as described in Specification 493,854. These controls act independently-not cumulatively. Feature (a) in conjunction with vehicle detectors at a greater distance than usual from their intersection produces a satisfactory co - ordination between the signals at consecutive crossings without any link between such signals. The whole of the right-of-way period may be made up of ''vehicleintervals," the early ones being comparatively long and the period being reduced at first rapidly and then slowly in accordance with the discharge rate of a condenser. In such a case the vehicle detectors should be close to the intersection. Means may be provided for reducing the vehicle - interval almost to zero either to make a maximum time unnecessarv or in response to a demand by a privileged vehicle. In the system described, which, apart from the provision of the features of the present invention, is the same as that shown in Fig. 4 of Specification 493,854, the sequence of operations is looked after by an 8-position control-switch. In position 1, the signal changes over in favour of the NS lane and the switch remains in this position for a period which may be fixed but preferablv depends on the amount of waiting traffic. In the absence of a demand from the EW lane, the switch remains in the " rest " position 2 indefinitely if the system is set by manual switches to operate as a floating or an NS arterial system, but moves to position 3 after a fixed interval if the system is set to act as a reverting or an EW arterial system. In the two earlier cases, a demand from the EW lane causes the switch to move on after an interval or, if the demand occurs in position 1, with no substantial pause. The duration of the stay in position 3 is under the control of a normal vehicle-interval timer, a variable vehicleinterval timer operated in accordance with the features (a), (b) and (c) referred to above, and a maximum timer, the particular timer to be effective to terminate the interval depending on traffic conditions. Position 4 is an amber position and positions 5 ... 8 correspond to positions 1 ... 4 but with the EW lane holding the right of way. Timing is done by discharge tubes which have their anodes brought to the breakdown potentials by the charging of condensers. In the case of the variable vehicleinterval, the grid potential of a tube is gradually adjusted by the periodical partial discharge of a condenser to vary the breakdown point. Detailed description. In positions 1, 2, 4, 5, 6 and 8, of the control-switch, the system operates as described in Specification 493,854 and the operation of the maximum timer and the normal vehicle-interval timer in positions 3 and 7 is the same also. The condenser KVV of the variable vehicle-interval timer, however, controls not only the discharge tube VVT, which breaks down at a voltage determined by the number of waiting vehicles in the barred lane, but also tubes DT, ET connected in parallel with VVT. The breakdown point of DT depends on the voltage of its grid-condenser KD which has a permanent charging circuit over RZ21 but is partially discharged. (in position 3) into a condenser KDNS1 (supplemented if necessary by KDNS2) at each operation of the NS road-pad Z51. As a result, the vehicle-interval is reduced in accordance with the frequency with which vehicles are passing the crossing. The breakdown point of ET depends on the voltage of its grid-condenser KE which is charged in position 8 and subsequently, if relay BM remains energized, and begins to discharge when BM falls on the arrival of a vehicle at the intersection in the EW lane. A manually-operated switch 334 may be opened so that the condenser begins to discharge as soon as AR pulls up in position 1 whether or no there is any traffic held up in the EW lane. By this means the vehicle-interval is progressively reduced either after the arrival of a vehicle in the EW lane or throughout the NS green period. As soon as the interval between two vehicles exceeds the shortest of the times determined respectively by the circuits of the tubes VVT, DT, ET, the corresponding tube breaks down and VVR steps the control-switch to position 4, the stepping magnet discharging the timing condensers KY, KVV, KMX. If the step to position 4 is caused by the variable vehicleinterval timer or by the maximum timer, the relay AM is released to register a claim for the return of the right of way when the EW traffic has been dealt with. When it is desired to give a progressive reduction of the vehicleinterval almost to zero, KW is discharged at each resetting operation to a point determined by the position of a tap 311 on potentiometer PZ7 considerably above the full discharge point. A sudden reduction of the vehicleinterval can be brought about by a privileged vehicle which controls an additional relay (not shown) to connect KE to a very low bias point on potentiometer PZ6.