Traffic lights on motorway slip roads?

#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?
 
#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.
 
#6
bossyboots said:
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?
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
bluearmy said:
They use it in germany when the traffic is slow on the autobans, fcuk knows how it works though. - :?
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.

NTM
 
#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 that....do 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'

NTM
 
#17
traffic studies over the years have shown that when a certain level of car capacity on a major throughway reaches a certain point the " septentine Effect ' kicks in.. that is cars begin to slow down, bunch up and then speed up and stretch out in a measurable rhythm..traffic boffins attribute this to use of brakes and the cumulative effect of the cars slowing at natural locations on highways... curves, bridges, etc.. where the visual cues change..

as most rush hour trips run at near capacity for the roads, the only way to break this stop-start cycle is to regulate traffic flow by liminiting more cars coming on to the lanes - metering - this allows new cars to slide into/merge at speed [ really does work ] without causing the flow to diminish from speeding cars breaking to allow entry.


and, actually studies found that those waiting on ramps for the green lights, actually wait less [ in real time ] than those who back up on on-ramps under unrestrained circumstances [ don't know the science but it works ] even if perception is otherwise..also, those drivers who find this delay [even if its only seconds ] annoying/disturbing, etc..usually seek alternative routes and thus lessen congestion further..

University of California and similar studies in Toronto [ where camera monitoring of roads has been in place for over 35 years ] have shown this system markedly improves traffic flow when roads are at or near capacity..rest of the time it does nothing..
 
#18
Well practiced in the art of using these things whilst driving in the USA.

It works there, but I suggest it only works well at US traffic speeds when highway is congested (= slow or stationary, DC Beltway excepted where they all go mad anyway :wink: ) As previously noted by CT, infinitely preferable to trying to join the traffic flow via a 100 yard merge lane that often doubles up as the exit lane!

US road design, esp major junctions, much influenced by early GE autobahn design. The very short ramps / slip lanes worked at 40-50 mph, bloody lethal at 60+ and scare the living doodahs out of Management, my nervous passenger!
 
#19
California_Tanker said:
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.

NTM
That is caused by congestion and people driving too close to one another. The guy in front brakes (disengages his cruise control perhaps), the second brakes harder, the third harder still, etc. The wave, for that is what it is, builds to a crescendo and the traffic affected by it has to stop. Then it restarts and the guy at the front, who caused it all, is still travelling at 50mph, but everyone else can now drive like a dingbat to catch up with him..... and the whole process starts again.

The solution is for us all to drive sensibly with a gap between us that is large enough to give us all some thinking time. But life's too short and I want to get to my destination faster than the guy in front of me.....

Litotes
 
#20
Ramp Metering limits the amount of traffic entering a motorway from an entry slip road, so as to control the flow of vehicles and minimise traffic overload. It is achieved by electronically monitoring the main carriageway traffic flow by use of speed and flow loop detectors located upstream and downstream of the slip road and on the slip road itself.

Traffic lights using conventional signals of standard minimum stage lengths and amber periods on the entry slip road, are used to manage the slip road traffic.

As well as controlling traffic joining the motorway from the slip road, a second aim is to assist recovery of traffic speed through the merge area when it falls below preset thresholds. It is also vital to ensure that queues forming on the slip road by the signals do not spill onto the roundabout and interfere with local traffic.

There has been a trial in the Birmingham area since 1986, it lasted for ten years until the equipment was worn out and irrepairable, (this is why stooge hasn't seen them operating lately), the results from the trial showed an increase in traffic flow through the bottleneck of 172 vehicles per hour, and an overall journey time benefit. at first glance the idea looks daft but it does work.
 

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