What is a variable speed limit scheme?
It uses variable speed limits to smooth the flow of traffic. Sensors calculate the optimum speed for traffic across all carriageways. This speed is displayed on electronic signs.
What does this variable speed limit scheme aim to do?
It aims to increase motorway capacity and smooth traffic flow for 13km between junctions 24 (The Coldra) and 28 (Tredegar Park) of the M4.
What will the benefits be?
This project aims to save money through fewer accidents and better journey-time reliability on this particular stretch of the M4.
Because of decreased congestion and heavy braking, there will be an improvement to the existing air quality management areas near to the M4.
Which sections of the M4 will be affected?
Junctions 24 (The Coldra) to 28 (Tredegar Park) in south east Wales.
Why this section of the M4?
Traffic flows are in excess of capacity and the accident rate is greater than the national motorway average because of non-standard gradients, bends with existing advisory speed limits and reduced visibility on a number of tight bends.
There’s also very little scope for widening this section of the M4 because of the Brynglas tunnels and the proximity of existing housing.
How much is being spent on this project?
This project will cost £7 million.
As a driver, what changes will I see?
The scheme will consist of electronic signs on gantries over each lane to indicate the speed limit.
There will be motorway indicator signs which will be mounted on posts above the carriageway each spaced 250m to 700m apart between new and existing gantries.
Enforcement cameras will be installed to ensure that drivers adhere to speed limits.
There will also be electronic indicator signs installed on slip-roads to advise motorists of the scheme. Signs will also be installed at the start and end of the 13km stretch.
An artist’s impression of the eastbound approach to the Brynglas tunnels with 50mph speed limit.
An artist’s impression of the eastbound approach to the Brynglas tunnels with lane closures.
What will the electronic signs display?
When activated, the electronic signs will display speed limits appropriate to the traffic level as well as directional signs, lane closures and carriageway closures.
- Blank Signal
Applicable to all lanes - normal motorway rules apply to this lane.
- Example of Mandatory Speed Limit
Applicable to all lanes - this will show the maximum permitted speed.
- White Arrow with Flashing Beacons
Applicable to all lanes - move into lane directed.
- Red Cross with Flashing Beacons
Applicable to all lanes - carriageway closed.
When does construction start and finish?
Construction will start in September 2009 and will finish by Autumn 2010.
Will the M4 be subject to closures during the construction phase?
There will be intermittent overnight carriageway closures whilst the work is carried out. On each occasion the closures will happen between 8pm and 5:30am with diversion routes fully signposted. Details will be posted on the Traffic Wales website, via its SMS and email updates (click here to register) and on roadside variable message signs. You can also find out more by listening to traffic and travel bulletins on the radio.
Who’s working on this project?
The Welsh Assembly Government is working with Traffic Wales, its traffic management service and contractors Balfour Beatty Civil Engineering.
Who can I speak to in more detail about this project?
You can contact the VSL Public Liaison Officer on 07798 731 791 or m4vsl@bbcel.co.uk
