3 into 2 - Marcus
We've all experienced it.
3 lanes of fairly light motorway/dual carriageway traffic travelling at near enough 70mph. One lane is coned off, with warning signs miles beforehand, and inevitably a huge traffic jam happens. This occurs even though there is ample capacity in the 2 open lanes to carry all the traffic.

Now we all know that the traffic should merge into 2 lanes at speed well before the closed off lane - the zipper effect. That never works in the UK. Either someone will not leave a gap for a car to slot into, or an opportunist cannot resist overtaking a few more cars and the merging point gets nearer and nearer to the cones.

Now I have always thought that the jam is caused by cars in the closed off lane having to brake to a standstill(for the reasons above) when they reach the cones. This causing the lane next to them to stop and let them in with the knock-on effect going back miles.

A pal saw a programme on TV about this and feels my theory is too simplistic. So anyone else have a theory why this phenomenon occurs? And how can it be prevented.
3 into 2 - Andrew Hamilton
It also intrigues me how cars jam up before road works so the speed through roadworks is low. Often emerging drivers often keep this slow speed for some time before they wake out of trance! Even in my slow van I look back to see the gap growing rapidly as I come out of the roadworks.
I think people just loose concentration because visualy most motorways are so boring that any distraction helps such as radio, mobile phone or newspaper!
3 into 2 - JamesH
I agree with Marcus's theory.

I think zip merging is the best way to deal with these situations. However, we are taught (including by the Highway Code) to move over early.

The problem is then caused by different people using either method but they won't work together.

With perfect zip merging only, the traffic would be like this

* * * * * * *
* * * * * *

and fit together well.

By doing both, the traffic could be like

* *** * ** **
* + * +

and the '+' cars would find it harder to merge in.

James (apologies for crude graphics)
3 into 2 - JamesH
Sorry, my crude graphics don't appear as I typed them. Hope you get what I was trying to show (leading and double spaces taken away).

Teaches me to use preview next time.

James

3 into 2 - SteveH42
I think the trouble here stems from the usual pattern of lane usage in this country. Usually these merges are right lane in to centre lane, and as the centre lane is usually the busiest due to people being very reluctant to move over, you end up getting a significant proportion of traffic trying to use just one lane. Some people will pull in to the left lane, but many others don't like being surrounded by lorries so will not.
3 into 2 - No Do$h
I think Steve has hit the nail on the head. In these situations you are likely to have at least one motorist of the "blinkers on, I'm going 57 and I ain't moving for anybody" school of motoring. Put them in a 50 limit, make "their" lane the overtaking lane and close down the space around them and you find you have someone doing 35 with white knuckles and a 1000-yard stare, whilst HGVs pass 12 inches from their NS wingmirror as they overtake on the inside.
3 into 2 - Raymond
Marcus
It has always puzzled me too. Your explanation seems reasonable but if it requires co-operation from all motorists to prevent the jam, you can forget about it happening. You will always get someone race up the closed lane and dive in at the last minute.
It will not matter to him that he is the cause of a jam as he will be on his way. The 'me first' culture.
Ray
3 into 2 - Raymond
Marcus
It has always puzzled me too. Your explanation seems reasonable but if it requires co-operation from all motorists to prevent the jam, you can forget about it happening. You will always get someone race up the closed lane and dive in at the last minute.
It will not matter to him that he is the cause of a jam as he will be on his way. The 'me first' culture.
Ray
3 into 2 - Raymond
I only sent it once Honest
2 into 1 - Biglig
I have a favorite game I play every morning. On my commuting route I come off the M40 at Uxbridge, and there's a stretch of road more or less as you describe; 2 lanes for half it's length and then down to 1 lane. It backs up every morning without fail.

My game is simple; I get into the inside lane, and leave a 2 car width gap infront of me as we crawl along, so anyone who wants to can pull into the inside lane and thus avoid having to do it at the last minute.

It's a rare day when anyone takes advantage of the gap, and the most there has ever been is three.
2 into 1 - edisdead {P}
Traffic flow modelling is very similar to fluid dynamics. My theory is that for a reasonably busy motorway, 3 into 2 lanes will ALWAYS cause some slowdown, regardless of how fast and where on the road everyone drives.

Think of the 3 into 2 lanes scenario like a pipe of decreasing radius. The rate of flow of three lanes of traffic travelling 70mph (ie. the number of vehicles passing a fixed point per second) has to decrease as the "volume" descreases, unless we all drive nose to tail to maintain that rate of flow. Thankfully most people are sensible enough to slow down on the approach to allow safe stopping distances between vehicles. This is what causes the domino effect we have all experienced.

Taking the nerd hat off for a moment, I agree with all the points above and generally speaking I imagine traffic could flow much more smoothly in these circumstances, if only a little more courtesy was applied by some motorists.
Ed.
2 into 1 - Marcus
Ed,
A couple of points on your post.

If the 3 lanes are full to capacity and all travelling at 70mph, then your theory is accepted. To get the same amount of cars past a given point the options are for the cars to speed up when they come to 2 lanes or reduce the gap between vehicles. In practice of course this doesn't happen.

However I did say that this phenomenon also occurs when the 3 lanes have fairly light traffic. e.g. there is sufficient capacity on 2 lanes to carry all of the traffic at 70mph.

Taking your 'pipe of decreasing radius' analogy. If the water in the large diameter did not fully fill the pipe, when it came to the narrowing of the pipe the water flow would not be affected.

Marcus
2 into 1 - jeds
I don't think this has anything to do with merging. It is surely a result of the reduced speed limit that goes with the lane reduction. Usually from 70 down to 50.

 

Value my car