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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
Three Programme Vehicle ControlThe signals are vehicle controlled by means of magnetic detectors embeddedunder the asphalt. The detectors operate to all kinds of vehicles and aredirectional. The function of vehicle control—which must now be universallyknown—is that the lengths of the green periods automatically vary from onecycle to the next according to the existing volume of traffic passing the signalintersection, and that at maximum volume the green periods displayed to thedifferent streets are limited to a predetermined maximum time. In normalvehicle controlled signal systems the maximum periods can be adjusted bymeans of knobs, each street and phase having a "maximum knob".Experience abroad and in Sweden has shown that the maximum knobarrangement is usually adequate for the requirements both at peak hours andother times. But it has proved deficient at certain places where the morningtraffic differs markedly in direction and volume from the afternoon traffic,or where other special conditions prevail. Many different procedures havebeen tried out to overcome this problem. Since the method of variablemaximum green periods tried at Langholmsplan may be assumed to be entirelynew, a brief description may be of interest.Langholmsgatan has three maximum knobs, as also Hogalidsgatan. Thuseach phase has a "low maximum", a "normal maximum" and a "highmaximum"—programmes I, II and III. The change from one programme toa higher or lower takes place automatically under certain given conditions,as follows:An increase from programme I to programme II for phase A (Langholmsgatanphase) takes place on condition that the maximum period of programmeI has been taken out fully by the traffic in street A n^ times in succession. (n lcan be set on a special n 1 knob from f to 10 units.)An increase from programme II to III takes place under similar conditions.Here there is a n., knob which can be given values other than n .A decrease from programme III to programme II takes place on conditionthat the maximum period on programme III has not been taken out n., timesin succession."*> . ** f ffl 1 IFig. 2x8007Control cabinet and signals at the streetcrossingThe signal head in the centre controls thetramway traffic and in the photograph showsa white arrow. The sign on the signal maston the right indicates »No right turn».25
Page 3 and 4: ERICSSON REVIEWVol. XXXIV 1957RESPO
Page 5 and 6: ERICSSON REVIEWVol. XXXIV No. 1 195
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