Unrestricted crossings cause excessive delays to major flows

Submitted by john on Sun, 15/12/2019 - 16:14
excess stops

When there is metering, traffic signals will be linked for a green wave. This diagram and the following diagram for mini-linking show the progress of one platoon of traffic from each end of the Hoddle St route, nominally at the speed limit, but refined to provide a smooth flow for the most traffic. The direction with the highest flow gets priority and so defines the relative signal timing between intersections. Observe the smooth flow from each end towards Victoria St in the AM peak. This sets the timing for signals in the other, counter-peak, directions. All the signals must operate with the same cycle length so that the same timing happens next cycle. Only one platoon of traffic is shown in each direction for clarity.

Once traffic passes Victoria St, it is running counter-peak and some, or all of it, is delayed by minor intersections or pedestrian signals. Delays caused to the counter-peak flow, far exceed potential delays to pedestrians if they were to be staged, and delays to minor flows if they were to turn left and then U-turn.

Average travel time is 6 minutes each way for the Excess Stops figure, compared to 4-5 minutes with perfect linking between major intersections in the Linking & Mini-Linking figure (next screen). Both these diagrams are for the short, 60 second cycle time possible with two-phase intersections. For the existing cycle time of 160 seconds, the delays would have been much longer, particularly for the excess stops case. The problem is not so much the trip time but the frustration of the stop-start operation with many stops. Emissions are also an issue.

Excess stops is also described as stop start traffic, restricted crossing u-turn, RCUT, or superstreet. These references treat all the movements, but separate consideration of the U-turns is proposed to be done on a needs basis as there are many other nearby intersections. Staged pedestrian crossings are part of the picture.