Why are there traffic lights before getting on a motorway?

Vehicle metering is what it’s called when traffic lights control the flow of traffic onto a motorway. It turns out that humans are quite useless at merging, despite calls to ‘merge in turn’ or ‘merge like a zip’ which means alternate vehicles merge..

Motorway traffic is kept flowing much more smoothly if vehicles are metered onto the motorway at regular intervals as it gives drivers that are already on the motorway time to adjust their speed and position to allow just one or two vehicles in at a time, rather than a steady stream.

When too many vehicles try to enter the motorway at the same time it slows up the left-hand lane. Those drivers try to duck across into the middle lane which causes panic braking in the middle lane, slowing that up. Drivers from the middle lane then move to the overtaking lane causing drivers in the overtaking lane to brake and suddenly everything slows to a halt.

The advantages of ramp metering

• More traffic throughput on the motorway – according to Highways England it generates an average of 13% savings in travel time
• Vehicle ‘platoons’ are broken up, allowing drivers to overtake and change lanes more easily
• Merging safety is improved
• Priority can be given to vehicles such as buses or carpool lanes
• It delays the onset of gridlock which improves the life of the motorway
• As overall throughput is increased, pollution is reduced.

The system benefits drivers who drive for long distances on motorways (primarily truck drivers and people driving long distances for work) and discourages local commuters who may have better public transport options.

The disadvantages of ramp metering

• Poor public perception is the primary disadvantage of ramp metering, especially when the ramp continues to meter traffic when the motorway is ostensibly clear.
• As traffic is held half way down the ramp it has less time to get up to speed to merge safely.
• Extra traffic lights need maintenance and power and are a hard cost to traffic authorities
• Metering can cause queues in adjoining streets that risks gridlock in suburban areas and localised areas of high levels of engine fumes.

How does ramp metering work?

Ramp metering can be controlled by sensors, timers or human controllers. Ramp metering can be set to start at a certain time, or it can be in response to building traffic on either the slip road or the motorway. Traffic controllers can instigate ramp metering in response to incidents on the motorway.

If you want to know the equations and algorithms for ramp metering, this presentation outlines Brownian network models that try to establish fairness to motorists while maintaining optimal flow, and this Highways Agency document outlines the ramp metering strategy in-depth.