Driver Knowledge Tests

Vehicle metering on motorway on-ramps: why are there traffic lights?

Ramp metering is when traffic lights are used on motorway on-ramps to restrict the number and timing of vehicles allowed to enter the motorway. Traffic is held back in order to break up platoons of vehicles which make it difficult for other vehicles to merge without causing braking which contributes to traffic slowing down.

The benefits of ramp metering

Research has shown that when ramp metering is properly implemented the benefits are:

  • Overall speed increase on the motorway
  • More vehicles can pass through the motorway in a given time period (throughput is improved)
  • Merging safety is improved for other vehicles entering at the on-ramp
  • Priority can be given to other types of vehicles where desirable/possible, such as trucks, vehicles with multiple occupants, etc
  • Increase the life of the motorway network by delaying gridlock
  • Fuel consumption is decreased as more and more vehicles are hybrid or have automatic stop/start technology.

The biggest advantage of metering is to drivers who get on a motorway at ramps with very little traffic and travel a long way on the motorway due to the overall speed increase. Drivers that enter ramps at busy intersections may experience longer delays, and this is a desired side-effect of the system: to discourage motorway use if possible.

Disadvantages of ramp metering

  • The public can sometimes perceive ramp metering as disadvantaging them, especially if it appears to be in operation for no reason, causing them unnecessary delays.
  • Holding traffic on the ramp with traffic lights half way down reduces the distance vehicles have to get up to speed on the ramp, and if drivers can’t match their speed with vehicles already on the motorway, this contributes to congestion as those vehicles have to brake to let entering vehicles merge into the stream of traffic.
  • Traffic lights on on-ramps need maintaining and the use power
  • The phase isn’t long enough to let a long vehicle through
  • Metering can cause queues to form on roads feeding the on-ramp
  • Filtering motorcyclists at the front of the queue confuse other drivers over who is allowed to go if two drivers can go at the same time.

How is ramp metering controlled?

New South Wales uses the SCATS program. Ramp metering systems can use a number of ways to control the traffic. They can be continuous, i.e. in operation all the time, come on at pre-timed intervals, or be activated in response to certain levels of traffic. When they are in operation, the timing of the lights can be affected by a number of parameters.

Timing intervals

  • Pre-timed metering rates have a set time based on historic data, e.g. 6 seconds per phase.
  • Traffic-responsive timing rates adjust the phase length based on how much traffic there is right now, using sensors in the motorway and arterial roads
  • Restrictive metering rates limit traffic to below non-metered rates (i.e. the level at which the lights turn on) to try to reduce traffic on the motorway
  • Non-restrictive metering limits traffic to the same as the average non-metered demand for that ramp.

Queue management

  • Less restrictive: if there’s a queue on the ramp which is spilling back into arterial roads, or a queue from the motorway back to the lights, the maximum metering rate is enabled to try to clear the queue as there’s no benefit in holding the traffic up
  • More restrictive: if there’s a queue from the motorway back towards the lights, the phase is lengthened to allow the queue to dissipate
  • Upstream integration: the ramp lights are integrated with traffic lights in the immediate vicinity that feed traffic onto the ramp. These lights are then changed to prevent as many vehicles from entering the ramp.

This video shows how SCATS works

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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