Module 7 : Traffic Signal Design

Lecture 38 : Coordinated Traffic Signal

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	Signal progression on one-way streets Determining ideal offsets Figure 1: Case study:progression on a one way street In Fig. 1 a one-way arterial is shown with the link lengths indicated. Assuming no vehicles are queued at the signals, the ideal offsets can be determined if the platoon speed is known. For the purpose of illustration, a platoon speed of 60 fps is assumed. The offsets are determined according to Eqn. . Next the time-space diagram is constructed according to the following rules: The vertical should be scaled so as to accommodate the dimensions of the arterial, and the horizontal so as to accommodate at least three to four cycle lengths. The beginning intersection should be scaled first, usually with main street green initiation at t=0, followed by periods of green and red. The main street green of the next downstream signal should be located next, relative to t=0 and at the proper distance from t he first intersection. With this point located, the periods of green, yellow and red for this signal are filled in. This procedure is repeated for all other intersections working one at a time. Figure 2: Time space diagram for case study Figure 3: Vehicle trajectory and green wave in a progressed movement Fig. 2 shows the time-space diagram for the illustration mentioned previously. Fig. 3 explores some features of the time-space diagram. Effect of vehicles queued at signals It sometimes happens that there are vehicles stored in block waiting for a green light. These may be stragglers from the last platoon, vehicles that turned into the block, or vehicles that came out of parking lots or spots. The ideal offset must be adjusted to allow for these vehicles, so as to avoid unnecessary stops. The ideal offset can then be given as: $$t_{\mathrm{ideal}} = \frac {L}{S} - (Qh + \mathrm{Loss}_{1})$$ (1) where, $Q$ = number of vehicles queued per lane, veh, $h$= discharge headway of queued vehicle, sec/veh, and $Loss_{1}$ = loss time associated with vehicles starting from rest at the first downstream signal. A note on queue estimation If it is known that there exists a queue and its size is known approximately, then the link offset can be set better than by pretending that no queue exists. There can be great cycle-to-cycle variation in the actual queue size, although its average size may be estimated. Even then, queue estimation is a difficult and expensive task and should be viewed with caution.