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Ramp Metering - Frequently Asked Questions
Ramp Metering - Frequently Asked Questions
1. Why are you putting lights on motorway slip roads?
This system, known as ramp metering, is a way to better manage traffic on the motorways and trunk roads to reduce congestion and smooth traffic flows.
During busy periods, signals are activated on the slip road and release just a few vehicles at a time. This prevents merging traffic and motorway traffic from bunching together and forming a bottleneck that delays everyone.
Sensors in the road surface continuously monitor the congestion and adjust the timing of the lights. They also monitor vehicles on the slip to stop queues forming that could spill over onto the local road network.
2. Where is it being implemented?
30 sites have already been implemented on motorways in the Midlands and the North. Up to 70 more sites will be introduced on motorways and trunk roads in these areas and also in the South East by March 2009.
3. When will the lights operate?
The lights are part-time and only operate when there is congestion on the motorway. The conditions that trigger the system depend on traffic speed and density and these vary between sites. Typically, the lights come on when the average traffic speed falls to 50mph.
4. Should drivers stop at the lights?
Drivers must stop at red lights, just as they would at traffic signals in urban areas.
5. How are sites selected?
We realise that the same solution will not work in every situation. Therefore pilots were carried out to help us understand the sorts of factors that affect the performance, such as the mix of vehicles, traffic flows and the length and angle of the slip road. Sites have been selected with these and other safety considerations in mind.
6. Does each site have the same design?
No. The design is tailored to the local circumstances and takes in account differences, for example, in slip road length, sight lines and the typical speed at which vehicles join the motorway. The lights are also specifically calibrated so ensure that the timings are optimised for the site.
7. What has been learnt from the pilots?
There have been pilots on the M3, M6 and M27. Sites were specially selected to help us understand how ramp metering works in different conditions. These crucial lessons led to the creation of the site selection criteria and national standards that are now being used. Some pilots were successful in reducing congestion and these sites have been retained, others did not generate benefits and these have been removed.
8. Can it be used with other traffic management measures?
Yes. The system can be used where there are signalised roundabouts, lane gains and lane drops, and two, separated slip road lanes.
A trial is underway to link the system and those used by local authorities to control traffic signals on local roads.
It may also be possible to install and operate lights at consecutive junctions and at motorway to motorway merges. These enhancements will be investigated in the future.
9. How long will it take to implement?
Sites typically take just a few months to implement, but this varies according to local circumstances.
10. How much does it cost?
The current project to implement up to 70 sites has a budget of £18m for site selection, design, construction and maintenance work.
11. What are the likely benefits?
- Reduced delays
- Improved journey time reliability
- Improvements in traffic flow
The before and after surveys at the 30 sites recently implemented in the Midlands and the North West showed that installing lights on slip roads has been successful in reducing congestion. The average motorway journey time through the junction has fallen by 9% and the average downstream speed is up 7.5%. The delays to vehicles on the slip road are relatively short.
Longer-term evaluations of overseas sites have also shown improvements in safety and air quality.
12. Does it affect the route traffic takes?
No. We have monitored traffic flows around the pilot schemes and the 30 sites implemented in 2006-07. This has shown that ramp metering has negligible impact on flows on alternative routes.
13. How long are the delays on the slip road?
The signal cycle is very fast and the lights change much quicker than those in urban areas. This means that drivers will not be delayed for long, and will spend less time in congestion when they join the motorway.
14. Does traffic back up onto local roads?
Sensors monitor vehicles on the slip road to minimise the possibility of queues forming on local roads. If a queue starts to form, the lights increase the green time and let more vehicles onto the motorway.
15. Who controls the lights?
The lights are controlled by the computer linked to the loops in the road surface. They can be over-ridden by those with appropriate authority on site, or remotely from control centres.
16. Is this a forerunner to charging for motorway use?
No. The sole purpose of this system is to reduce congestion and smooth traffic flow.
