GB1204484A - Improvements in or relating to road traffic systems - Google Patents

Abstract

1,204,484. Detecting traffic flow. GENERAL ELECTRIC and ENGLISH ELECTRIC COMPANIES Ltd. 30 Nov., 1967 [30 Nov., 1966], No. 53595/66. Heading G4Q. A traffic monitoring system has means for directing a plurality of beams of electromagnetic radiation across a carriageway in such a way that the manner of interruption of the beam system by a vehicle depends on the particular lane occupied by the vehicle. Signals so obtained may be used to indicate occupancy or to control the traffic. In Fig. 1 beams of modulated infra-red energy are directed by sources 1, 2 towards sensors 4, 5 which feed receivers 6, 7 and Schmitt triggers 7, 8. The outputs of the latter are normally of such polarity that an AND gate 13 is disabled. A vehicle travelling in the fast lane interrupts the beam from source 1 and changes over the Schmitt trigger 8 to enable the AND gate. A short time later the vehicle interrupts the beam from source 2 and changes over the Schmitt trigger 9, this output being differentiated and applied to the AND gate which now triggers a monostable circuit 14 and lights a lamp 15 for a short time. A vehicle in the slow lane interrupts beam 2 first, and the differentiated output of Schmitt trigger 9 will have ceased before an output is obtained from 8, so that no indication is given. The differentiating circuit 12 may be in the other or both outputs if it is desired to indicate vehicles in the slow lane or both lanes. In the latter case, a second AND gate, monostable circuit and lamp are used. The lamp may be replaced by a counter or other monitor, or control circuit. Application to more than two lanes.-In this case the beams diverge towards the fastest lane, the transverse position of the vehicle depending on the time elapsing between interruption of the two beams. To compensate for varying vehicle speeds a third beam is provided, parallel to one of the other beams, and the time between interruptions of the parallel beams is a measure of the vehicle speed. The output signal of the system can then represent the ratio between the times between two pairs of beams. If the two parallel beams are perpendicular to the carriageway errors due to varying widths of vehicles are avoided. In an elaboration of this system (Fig. 2) four beams are used as shown, the outputs from the sensors S1-SA being applied to logic circuitry 20 comprising four monostable circuits connected in cascade by means of three AND gates. Only when the input signals are applied in a particular time sequence will all the monostable circuits and AND gates be operated, and this sequence is unique for a particular lane. By repeating such circuitry for each lane a lane indication can be given. The parallel beams S2, S3 also give a speed indication by means of a pair of start/stop gates 21. The first gate initiates a ramp generator which is stopped by the second gate, giving a voltage inversely proportional to vehicle speed. This voltage is converted to frequency in a unit 22 which is controlled by the second gate, and a counter 23 is calibrated to show the particular lane occupied. For discrimination only over half the carriageway the fourth beam may be omitted. Modifications using reflectors.-In the system of Fig. 1 the sources and sensors may be on the same side of the carriageway, with a reflector on the other side. For more than two lanes, reflectors are placed one beside each lane so that the number of beams interrupted indicates the particular lane occupied. Alternatively, the output of the sensor for the last lane is fed via an AND gate to which is fed the signals from the other sensors through a NOR gate. The reflectors are embedded in the road surface with an access channel for the incident radiation. For three lanes, the sources and sensors can be arranged so that the centre lane intercepts two beams while the two outside lanes intercept one beam each. Alternatively the beams are staggered along the carriageway, the first beam traversing one lane, the second two lanes &c., so that the first beam interrupted indicates the lane. In another version the beams are inclined at an angle to the horizontal so that vehicles in only one or some of the lanes can intercept the beam. This modification is useful in tunnels or under bridges. Sources and sensors.-The sources may be sub-lasing gallium arsenide diodes modulated by a transistor blocking oscillator and emitting a beam-width of one degree. The sensors are photo-diodes with transistor amplifiers. Both are as in Specification 1,115,802.