M4 J8/9 Traffic Lights Whats all that about

[Moose]

Noticed some new (but not switched on) traffic lights have appeared at the bottom of the slip road at J8/9 of the M4.
How is that supposed to work??
Seems like a ridiculous idea to me unless someone here can convince me otherwise.

 

[Flash]

Saw a few of these up on the M62 three weeks ago and thought the same.

There is a lot of maintenance/upgrade work currently going on the M4 from Junction 6 to 8/9 and my gueses are that the lights would be activated possibly if there was an accident further up but I agree and it is pointless!

However, the upgraded big signs that advise of updates and time/distance to certain junctions I think are very useful.

 

[whitenemesis]

These are quite common in the States. They use them to help filter traffic onto the freeways at peak times. I think it stops the freeways from backing up as heavy traffic tries to join.

 

[MBManInKen]

Filtering traffic lights on slip roads exist in Belgium as well. As Whitenemesis says, they are to control the flow of traffic on the motorway at critical junctions, i.e. where potentially a lot of traffic might join an already busy motorway.

 

[Ted]

they have had them on M6 J5 southbound for years. They filter traffic onto a busy motorway very effectively.

 

[stevesey]

Part of what's known as "ramp metering" schemes. Idea is to control the flow of traffic entering the motorway to ease congestion. The HA has complex traffic monitoring and prediction systems that would allow these to function in a number of ways, at the simplest level they monitor the traffic upstream of the junction and prevent traffic joining if there is a "bulge" of traffic approaching, and then release it a so it reaches the bottom of the slip road at a more convinent time. Even when the motorway is busy these are still dense patches and less dense patches, so the idea it to release traffic into the less dense patches and keep the flow smooth.

The are more complex scenarios where these lights can be used to limit the traffic joining the motorway if it is at the point of flow breakdown. I think the M42 ones can to this as the whole M42 system is controlled by a complex traffic model that uses the traffic flows in and around the controlled section to try to adjsut the speed limits and lights to prevent flow breakdown.

 

[Moose]

Well, if it works, all well and good. Particularly as I don't get on at J8/9
My colleague who does, is pretty annoyed about it though. Anybody who has used the roundabout at the top of the slip knows how many traffic lights need to be negotiated will empathise with him I'm sure.