Traffic lights on motorway slip roads?

Discussion in 'The Intelligence Cell' started by bossyboots, Nov 20, 2006.

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  1. Not sure where to put this but here goes.

    I have noticed lately that on some slip roads leading ONTO the M1 there have been traffic lights installed ( not in use yet ) at the bottom of the slip road as you join the motorway.

    Can anyone enlighten me as to what thats all about? Surely it's dangerous to have traffic moving from a stationary position when the lights go to green onto a motorway of fast flowing traffic?

    And surely the traffic that has to stop for the lights will back up and cause more congestion on the roundabout at the top of the slip road?

    Does anyone know what its all about and how its expected to work?
  2. could be for future traffic management of road works
  3. I believe it is for when there is heavy traffic (therefore not fast moving) to control the flow of traffic onto the motorway and ease congestion... but.. I have yet to see it work.
  4. I can't see how it would work. The traffic on the motorway is already bumper to bumper, the traffic on the slip road and on the roundabout will be bumper to bumper as will the traffic coming off the motorway and onto the roundabout at the top.

    I'm baffled.
  5. They use it in germany when the traffic is slow on the autobans, fcuk knows how it works though. - :?
  6. I think it's known as 'ramp metering' and it's designed to drip-feed traffic onto motorways during busy periods. There are cameras aimed at the motorway and when they detect a gap the green light goes on for a very short time and then goes back to red again. As you quite rightly say, accelerating from a standing start onto a busy motorway doesn't seem quite right to me either.

    They did a trial along the bottom part of the M3 and the M27 a few years ago and now they appear to have ripped up the lights - so that probably means they'll put them back in at great expense.
  7. woopert

    woopert LE Moderator

    Used in the USA quite a lot during rush hour to ration the num,ber of cars that can get onto the M'way at any one time. The green light stays on long enough to allow no more than 2-3 cars to pull away, and as the traffic is moving slow enough as a consequence of congestion for you to be able to speed up to it on the slip road there's no chance of being pranged, plus there are warnings to traffic on the m'way as it heasd towarsd junctions with traffic management on them. It actually does work (in so far as not making you prang anyway)
  8. seen some just erected M1 north, junc 33 not being used yet though probably be a nightmare though
  9. That's why we have stupidly large engines in our cars now: To get from a standing stop to 60 mph in the length of our merge lane!

    Actually, the rule here in California is 'One car per green light' with one or two places where it's 'two cars per green light'. (signs are on the lights). The metering lights are usually only on at peak traffic times.

    They do sortof work. It's sortof like a similar concept to police cars running a traffic break: Keeping a backlog of traffic behind them doing only 50mph, keeps all the traffic moving smoothly (and thus faster overall) rather than letting people drive at their own speed.

  10. Theres also some at junc 40.

    I'm still convinced that all they would serve to do is to back the traffic up on the slip road even more resulting in more congested traffic on the roundabout, as well as more congestion for traffic wanting to get onto the roundabout.

    I can see road rage going wild and tempers at the tea table though. :lol:
  11. I had the misfortune of spending 45 minutes trying to get off the slip road from the M25 (clockwise) for the A1(M). This was caused mainly by people at the front of the queue not being able to see the traffic lights and those people already on the motorway ignoring there's! It was every man for themselves!
  12. They have had them on the elevated sections of the M6 in Brum for some time, (junctions 5 and 10). I am not convinced they make any difference.
  13. If its right what Woopert says then surely its just the same as without the lights? People pull onto the carriageway when theres a gap...surely you dont need traffic lights to tell you you :?
  14. It's an issue of capacity. A road is capable of dealing with X many cars per minute. If you go over that, you end up with congestion, even though there is no 'sensible' reason for the traffic to come to a halt. (eg lane closures for accidents and whatnot). We've all seen it, traffic blocks up, then for no apparent reason, clears up.

    The theory is that the metering lights keep the amount of traffic coming onto the roads close to the capacity without overdoing it.

    Additionally: For example, let's say that you have an on-ramp coming from a major A-road onto the motorway, and there are traffic lights on the A-road/onramp junction. As soon as the A-road light goes green, a massive pulse of vehicles is all going to go down the on-ramp at the same time. Metering light at the onramp mitigates the effect of that 'pulse'

  15. They've had those lights on the M6 in Brum for years. I have never once seen them used.