1,259,348. Road signals for controlling traffic. OMRON TATEISI ELECTRONICS CO. 12 Feb., 1969 [19 Feb., 1968; 20 June, 1968], No. 7666/69. Heading G4Q. A system for controlling a traffic signal at a street intersection comprises means 10 for detecting traffic congestion on at least one of the intersecting streets to produce an electrical signal corresponding to the degree of traffic congestion and control means 30, 60 including oscillating means 50, 51 responsive to the electrical signal to vary the frequency of the output signal of the oscillating means, the control means being arranged to control the unit extension and extension limit of the green signal displayed to the at least one street in accordance with the frequency of the oscillating means. Traffic congestion on the major street is detected by vehicle detector 11 and a congestion detector 12 produces an output dependent on the space occupancy on the major street. An input adjuster circuit 20 including a subtracting circuit 22, which subtracts from the input a voltage corresponding to the normal volume of traffic on the major street, and emitter follower 23 feeds the traffic congestion information from detector 12 via amplifiers 31, 33 or 32, 34 to control the frequency of oscillators 51, 52. If the major street is at green this causes signal 1G at 49 to allow only operation of amplifiers 31, 33. The outputs from these amplifiers 31, 33 are arranged to decrease with an increasing congestion input signal which above a certain value (P1, Fig. 4, not shown) decreases the oscillating frequencies of oscillators 51, 52 (curve B). Provided switch 61 is open (as is required for traffic control) and that there is no signal on line 62, the time taken for the unit extension setting counter 71 and extension limit setting counter 72 to complete their respective counts is increased so that the green unit extension and extension limit on the major street are increased to remove the traffic congestion. The outputs from the counters 71, 72 are used to operate a phase shifting counter 73 which changes over the right of way to the minor street in accordance with the unit extension and extension limit determined. The extension limit counter 72 is reset at R2 upon change of one phase to the next. The unit extension setting counter 71 is reset on detection of every vehicle so that it only gives an output when there has been a gap between detected vehicles sufficient to allow the counter 71 to complete its count. A signal occurs on line 62 on termination of the extension limit or when no more vehicles are detected during the extension to inhibit operation of INHIBIT elements 66, 67 and disappears on termination of the minimum initial green portion to enable these inhibit elements to be operated by a 60 c./s. signal via rectifier 70. If the green signal is given to the minor street then the signal on line 49 allows only amplifiers 32 and 34 to operate. These amplifiers produce an increasing output voltage as the congestion on the major road increases to increase the oscillators 51 and 52 frequencies (curve A) so that the counters 71 and 72 count up quicker to shorten the green unit extension and extension limit given to the minor street and effect change of the right of way back to the major street quicker. A level detector circuit (81, Fig. 1, not shown) may be added between the input circuit 20 and OR element OR8 so that when the right of way is being given to the minor street and the traffic congestion on the major street has reached a predetermined level the level detector produces an output to counter 73 which transfers the right of way from the minor to the major street. In a modification of this (Fig. 3, not shown), the minor street can be provided with a congestion detector (10<SP>1</SP>) similar to 10 and a level detector (81<SP>1</SP>) which operates to change the right of way from the major to the minor street when the traffic congestion on the minor street exceeds a predetermined level. In a further modification (Fig. 5) the congestion on both the major and minor streets are detected in detectors 10, 100 and are compared in comparator 110 which gives an output on line 111 or 112 if congestion is greater on the minor or major street respectively. These outputs are applied to subtracting circuits 113 or 114 which take the voltage of line 111 or 112 from the outputs of the corresponding detector 10 or 100. The outputs from the amplifiers 131-134 vary inversely with their input, like amplifiers (31, 33, Fig. 2). When the major or minor street is at green respectively, signals 1G or 2G cause amplifiers 131, 132 or 133, 134 to operate so that the green unit extension and final extension limit are controlled according to the congestion on the major or minor street in the same way as in Fig. 2 provided the street to which the green signal is applied has the greatest congestion. If however the minor street has the green signal and the major street has the greater congestion then the output from 112 is subtracted from the output of detector 100 to reduce the input to amplifiers 133, 134 and reduce the green unit extension and extension limit given to the minor street. Similarly the green unit extension and extension limit for the major street are reduced if the congestion on the minor road is the greater when the major street has the green signal. The congestion may be measured in the form of time occupancy obtained by integrating all periods of time obtained from a vehicle presence detector, space occupancy by having a loop of wire on the street, the output of which varies with the number of vehicles within the loop.