Principal aims of coordinated ramp signals

1. To improve safety

This is achieved by assisting merging and minimising traffic instability from lane changing and other causes of turbulence in traffic flow reducing the potential for incidents caused by braking and congested stop-start traffic flow.

2. To optimise throughput, travel speed and travel time reliability

This is achieved by minimising the possibility of flow breakdown and the development of congestion on the motorway.

The main ways coordinated ramp metering signals work are:

Localised operation at each ramp

• Making merging easier by dispersing ramp traffic to achieve an even flow of vehicles into the merge area, i.e. local benefit that a driver can see.
• Managing the amount of traffic in the merge area to maintain stable flow conditions, particularly when the motorway is near capacity.

System operation (using multiple coordinated ramps)

• System management of all critical bottlenecks along the motorway - this system takes a big-picture view to identify all areas where traffic conditions need to be managed to keep traffic flowing, i.e. to provide benefits in areas that the driver cannot see.
• Control of access to the motorway to ensure that entering traffic is managed within the overall capacity of the motorway system.

Further detailed reading is also available in the various traffic engineering manuals.