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8.7.2 Management of Exit Ramp Interfaces

The most problematic issue at exit ramp/arterial road sites is long queues forming on the exit ramp when there is inadequate green time provided at the adjoining arterial intersection to allow these vehicles to exit the ramp. Aside from causing long delays for these exiting vehicles, when exit ramp queues build up such that they overspill onto the motorway, they block the left lane of the motorway to traffic progressing beyond the exit ramp.

Even before queue overspill occurs, long queues on an exit ramp have the effect of reducing the available stopping distance on the ramp, meaning exiting vehicles must begin to reduce their speeds on approaching the exit ramp in the left lane of the mainline. This slowing down of exiting vehicles in the left lane impedes other vehicles in this lane travelling beyond the exit ramp and increases the likelihood of rear-end crashes.

On approaching the ramp, many vehicles move into the left lane to exit the motorway. At the same time, other vehicles travelling beyond the exit ramp move out of the left lane to avoid slowing down, further disrupting traffic flow which increases flow turbulence and the risk of a crash on the motorway.

A variety of treatment can be applied which involve increasing the queue storage capacity of the off-ramp, such as use of the emergency lane. Other approaches would involve increasing the discharge capacity of the exit ramp, either through minor civil works or through adjusting the traffic signals to prioritise exit ramp movements.