Patent GB-985570-A (GB)
Title: Traffic density computer
IPC: G06G7/76
Filed: 1961-09-08
Abstract: 985,570. Electric analogue calculating; road traffic control systems. LABORATORY FOR ELECTRONICS Inc. Sept. 8, 1961 [Sept. 8, 1960], No. 32390/61. Addition to 949, 382. Headings G4G and G4Q. In the traffic density computer described in the parent Specification means are provided for computing traffic volume over a sampling time base which is variable inversely with the average speed output of the traffic speed computer so that the output of the traffic volume computer represents traffic volume for a constant distance of the roadway from a given measurement point thereon. Fig. 1 shows a modification of the circuit of Fig. 1 of the parent Specification wherein instantaneous traffic volume rate circuit 17 energizing linear cathode follower circuit 18 with feedback circuit 18 with feedback circuit 46, 47 feeds directly the division circuit 75, and volume and speed timebase correlation circuit 69 controlling speed averaging circuit 21, with a volume rate signal; the division circuit also receiving an average speed signal from the circuit 21. The averaging base circuit 19 comprising parallel circuits of variable resistances and opposed diodes defining the averaging time segment succeeds the division circuit 75 and feeds output amplifier circuit 20 to produce a signal representing computed traffic density by dividing volume by speed which by adjustment of the forward and reverse time constants of circuit 19 may be given a response which is more rapid for increasing density than for decreasing density or vice versa. The time constant of volume rate circuit 17 may be adjustable. Alternatively (Fig. 2) an additional averaging base circuit 19A comprising a variable series resistance shunt capacitance time constant circuit is connected between circuits 18 and 69, 75 while an averaging base circuit 19B similar to 19 followed by amplifier 20 succeeds division circuit 75 as in Fig. 1. The traffic density is averaged as in the parent Specification over a constant time segment and thus over a distance segment proportional to average traffic speed. In Fig. 3, series resistance 46<SP>1</SP> and shunt capacitor 48 constitute the averaging base circuit 19 establishing the averaging time segment which follows cathode follower 18. Resistance 46<SP>1</SP> is manually variable by limb 132 together with thermally variable resistance 58 of thermistor potentiometer 59 and the gain 133 of servo-amplifier 67 (see Fig. 1 of the parent Specification) whereby the time segment of circuit 19 is adjustable together with correction of the speed averaging circuit 21, so that the effect of the passage of a single vehicle is diminished as the number of vehicles passing during a particular time segment increases and vice versa. In a modification (Fig. 4) enabling traffic density to be computed for a fixed distance instead of a fixed time (i.e. a distance proportional to average speed) the time segment is made to vary inversely with average speed instead of being fixed. Servomotor 68 of speed averaging circuit 21 (Fig. 1 of the parent Specification) adjusts variable resistor 46<SP>1</SP> and the effect of the reduced time base and passage of fewer vehicles correspondingly corrects the effect of an individual vehicle average speed; the thermistor potential divider being corrected as before by volume and speed base correlator 69. Alternatively (Fig. 5, not shown) the thermistor potential divider may be controlled solely from the computed density output signal. A CR circuit giving a similar time constant for increase and decrease in volume rates may be placed between circuits 18 and 20 while a circuit similar to 19 giving variable and differing rates for increase and decrease of density variation may be provided at the density computer output. Fig. 6 shows in block form a complete system for determining traffic density over a constant distance base incorporated the above modifications;and Fig. 7 shows a modification for selectively switching the distance base to several alternative values wherein knob 134 selects alternative values of resistors R46<SP>1</SP> for co-operation with time constant capacitance 48 between cathode follower 18 and output amplifier 20 to vary the time base and corresponding values of R58 for co-operation with R59 in the thermistor potential divider circuit between leads 61, 62 in speed averaging circuit 21 to correspondingly vary the time base thereof so that the computed density is measured over the same time base;all such alternative resistors R58 being gauged for adjustment by servomotor 68 as above,while R59 is varied from either the volume output or density output. The thermistor potential divider R58, R59 may comprise a single thermistor in a temperature controlled enclosure, in series with an ohmic resistor,and heated by a control winding as in the parent Specification. Fig. 8 shows a traffic control system in block form incorporating two radar sensing units 10 responding to traffic in opposite directions (inbound Ib, outbound OB) feeding speed and impulse translators 11 as described in the parent Specification, operating traffic density computers of the kind described above. The outputs representing densities in opposite directions are applied to segmental scale circuits each of which provide selective outputs on one of six output circuits when the respective density signal falls within a predetermined segment of the full scale of densities. The segmental scale circuits operate a system selector operating to select a particular output when inbound density is substantially greater than outbound density, a second output when outbound density exceeds inbound density, and a third output or a combination of the first and second outputs when the inbound and outbound densities are similar. The selected outputs may supervise a traffic control system, or control the direction of flow in one or more reversible traffic lanes in accordance with the traffic density in each direction.