Constrain traffic by limiting the major flows to 90% of capacity

Submitted by john on Sun, 15/12/2019 - 16:06
metering control

It has long been held that practical capacity is 90% of absolute capacity and that has been adopted here. The need to meter to exactly 90% of capacity can be modified empirically by adjusting the metering until it produces a satisfactory green wave, trading off higher capacity against smoother flow.  
But a platoon will disperse because traffic is variable, so the faster drivers then learn that they are held up by red lights and the slower drivers will miss the end of the green and learn to keep up. The signals engineer needs to remove any obvious causes of turbulence, such as lane changing, and then adjust the time offset between signals to keep the platoon flow as lumpy as possible. This will be near the speed limit. Drivers should be advised of the ideal travel speed.

The figure illustrates the process of metering where traffic volumes entering the length are limited by the upstream signals. The traffic signal system records each vehicle at the stop line, noting the time gap between vehicles. Spare capacity is calculated at the downstream intersection and used to limit the flow at the upstream intersection. The objective is to provide spare capacity of 3 right and 7 through at Victoria Pde. This 10% spare capacity requirement only applies to the critical intersection in a control group. Less critical intersections will require greater spare capacity, governed by an intersection further downstream. The simplest control is just of the 81 in the figure.
Easier access and shorter trip times will make trips by car more attractive, and they will increase in number, including from sources in the Hoddle St vicinity. So when the 1, 4, 2 & 7 in the figure grow significantly, they will need to be moderated. Options are to meter them, to toll them, or ban them completely, or by time of day. Such moderation requires endless maintenance. The growth just doesn’t impact the flows, it also impacts car ownership and parking availability.

If 3 right spare to the city are not available, and right turn queues block the through lanes, a city entry toll for all right turns may be required, perhaps for the whole morning peak, but free entry to the city is still available via a right turn at Gipps St, Johnston St, or even Albert St. The only requirement here is that the intersection at Victoria St is not congested.

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