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Fig. I X7712Sketch of siteA signal-controlled street crossing (I) and twosignal-controlled pedestrian crossings withpush buttons (2 and 3). The signals are interconnected.sented to Langholmsgatan at 2 immediately before it is presented at /. Afterthe pedestrian phase at 2 and D phase at /, the light for Langholmsgatan returnsto green simultaneously at the two installations. The reason for theinterconnection is primarily to avoid traffic from the bridge having to queuemore than once per vehicle and to avoid vehicles queuing in the middle ofthe pedestrian crossing at 2.If there is no traffic on Hogalidsgatan, and consequently no need ofright-of-way for that street. 2 changes on operation of the button withoutwaiting for any length of time for impulses from /.Installation 2 may be said to be placed in front of the master installationin the direction of traffic movement. Thus the problem of warning 2 of thechange in the master installation "that has not yet occurred, but which is tooccur" has been solved without difficulty. Although the interconnectionbetween / and 3, the latter being placed behind /, should be simpler, anotherfactor complicates the problem in this instance. This is that 3 is passed bytraffic from two directions, by northward traffic on Langholmsgatan (phaseA) and by traffic from Hogalidsgatan (phase D). Since queuing in front of 3,caused by one of these two streams of traffic, interferes with the other, it wasconsidered advisable to preclude such queuing altogether. On the assumptionthat a pedestrian request has been made at 3, the interconnection comes intoeffect in changing / from two-phase to three-phase. The third phase C (about7 sees.) consists of "all red", that is to say the red aspect is presented to alldirections of traffic. During this comparatively short phase there is time fora vacuum to form at 3, i.e. the accumulation space 3 is empty of vehicles. Thephase sequence in this instance is such that the last vehicle to pass northwardsfrom Langholmsgatan immediately before the signals switch to the thirdphase—red in all directions—has just time to pass 3 before the pedestrianphase at 3 appears. After the all red phase at / has disappeared, the greenaspect is given to Hogalidsgatan, so that the first vehicle arrives in reasonabletime to be met by green on signal 3.24