A (probably) silly question: What causes traffic jams on motorways?

[Eater Sundae]

These studys can generate what they are designed to and many do.There are bottlenecks being designed on any road they can get them on to hamper traffic flow and variable speed limits are just another.

 

There is only ONE way to ease a conjested road and thats to get rid of as many vehicles as possible in as little time as possible,variable speed limits greatly hamper that and often they are on when there is no need at all and the only thing stopping traffic doing 70mph is the reduced limit signs.

Its just a con but some will always believe what they are told even if it flies in the face of common sense,something that seems to be dwindling fast.

 

“The faster the vehicles are travelling then the more vehicles the road can carry” will only work if the space between each vehicle is correct. If everyone drove correctly, such that the distance between vehicles increased appropriately with increasing speed, then there wouldn’t be hold ups unless the road reached saturation. The problems come with some people leaving inadequate space, having to suddenly brake, and causing the concertina effect. A blanket 40 or 50 limit during busy periods allows more traffic than the 70 limit only because it keeps everyone at a similar speed, takes away any desire or opportunity to overtake, accelerate into a gap etc. There is less likelihood of anyone making the sort of sudden change that initiates the concertina. So traffic does not stop/start.

[Bonzo77]

I know the thread title asks a blindingly obvious question. Obviously the weight of traffic. (shall I close the thread now? )

 

Let me explain. I have cause to regularly travel south on the M1. Yes they're doing barrier renewal at the moment, and there's a 50mph limit. But it's the same when they're not working on it.

 

If one of the gantry signs says 40 mph. It 'appears' to me, that people take that as a signal to 'stop' The traffic comes to an abrupt halt, and very slowly builds up speed again. There's no accident's or obstructions. It just 'seems' like people think..."Oh!!.. 40 mph, I'd better stop".

 

I don't understand the mechanics of why the traffic comes to a standstill. I can kind of see the logic of a sort of domino effect in reverse. But I've never really understood the true mechanics of how it ends up being at a standstill.

 

To me, the simple logic would be...lets say I'm travelling at 65 mph, I see the sign saying 40 mph. I slow down, to 40, and presumably everyone else does too...But traffic should still keep flowing at that rate surely? Regardless of how much there is?....So why does it not happen like that? What causes it to stop?

 

Think of a contraflow like an egg timer. The grains of sand can't all fit through the gap at once, so they have to stop and join the queue.

[Xt500]

The stop/start or phantom traffic jam end-result of the ripple/domino effect of braking-heavier braking-even heavier braking stems from drivers driving too close.

The two-second gap that many drivers ignore and/or under-estimate should be considered as a minimum certainly when the view ahead is blocked by the size of the vehicle(s) ahead and/or by the straightness of the road. Thinking of such a gap as merely giving braking or stopping distance is not particularly good enough when you can't see what lies beyond the first vehicle. It is better to think of this gap as giving you the luxury of being able to see beyond the vehicle ahead in order to be "ahead of the game" and to know why the vehicle ahead will brake. Where you can't see anything ahead and where it is very likely on that full motorway that someone ahead of you will brake without you knowing about it is where you see the painted chevrons on the road surface. These encourage you to keep at least a two-chevron/second gap where there is long straight stretch of motorway that has a history of rear-end shunts involving drivers driving too close.

 

If only drivers kept space for themselves more consistently.

 

Gaps have nothing to do with variable speed limits whatsoever.

 

If you slow cars down from 70 to 50 with a sign the lead cars will slow down to 50 and everything behind them will also have to do the same it doesnt matter how they do it or how much gap every vehicle passing that sign will be doing 50 or less.

Take the sign away and the lead cars accelerate back up to 70 its not rocket science.

 

The arguement you seem to be making is how the vehicles behind the lead cars react may have some truth in it but if the signs went there neither would the problem be.

variable speed limits are a goverment measure for knacking up traffic flow and no more.

[RootsBooster]

I know the thread title asks a blindingly obvious question. Obviously the weight of traffic. (shall I close the thread now? )

 

Let me explain. I have cause to regularly travel south on the M1. Yes they're doing barrier renewal at the moment, and there's a 50mph limit. But it's the same when they're not working on it.

 

If one of the gantry signs says 40 mph. It 'appears' to me, that people take that as a signal to 'stop' The traffic comes to an abrupt halt, and very slowly builds up speed again. There's no accident's or obstructions. It just 'seems' like people think..."Oh!!.. 40 mph, I'd better stop".

 

I don't understand the mechanics of why the traffic comes to a standstill. I can kind of see the logic of a sort of domino effect in reverse. But I've never really understood the true mechanics of how it ends up being at a standstill.

 

To me, the simple logic would be...lets say I'm travelling at 65 mph, I see the sign saying 40 mph. I slow down, to 40, and presumably everyone else does too...But traffic should still keep flowing at that rate surely? Regardless of how much there is?....So why does it not happen like that? What causes it to stop?

 

I believe fracking is the cause

[PeteMorris]

Well...Unfortunately, this is the first time I've been able to revisit this thread since starting it, but I've kept up to speed (see what I did there?) on all the fascinating theories and replies.

 

Someone mentioned keeping the right gap between cars, and that's a very valid point. The problem of course, is that it never happens. I 'always' leave a sufficient gap, but inevitably someone who's perhaps moving slower in another lane decides to fill the gap, so I have to end up dropping back from that vehicle. Personally in heavy traffic, I try never to apply the brakes, with leaving a decent distance between me and the car in front allows the car to simply slow to match the speed required without the need for braking. But I know, I'm probably in a minority.

[DT Ralge]

Gaps have nothing to do with variable speed limits whatsoever.

 

If you slow cars down from 70 to 50 with a sign the lead cars will slow down to 50 and everything behind them will also have to do the same it doesnt matter how they do it or how much gap every vehicle passing that sign will be doing 50 or less.

Take the sign away and the lead cars accelerate back up to 70 its not rocket science.

 

The arguement you seem to be making is how the vehicles behind the lead cars react may have some truth in it but if the signs went there neither would the problem be.

variable speed limits are a goverment measure for knacking up traffic flow and no more.

 

I think you need to spend less time dreaming up govt conspiracies and a short while in a traffic control room to see and understand why and when lower limits are applied to some parts of the m/w network.

[Cyclone]

The stop/start or phantom traffic jam end-result of the ripple/domino effect of braking-heavier braking-even heavier braking stems from drivers driving too close.

It's related to it, but not strictly just a consequence of it.

You can drive closer if you can look ahead of the immediate car, if you are observing traffic 3 or 4 cars in front of that, it gives you ample time to react by backing off the accelerator. If you can't see that far, then it becomes progressively less safe and heavy braking becomes more likely.

But driver capability is also important. Many drivers see a red light in front of them and immediately brake too hard themselves. Some people seem to drive along constantly braking for no apparent reason (very few to be fair).

The two-second gap that many drivers ignore and/or under-estimate should be considered as a minimum certainly when the view ahead is blocked by the size of the vehicle(s) ahead and/or by the straightness of the road.

Yep.

Thinking of such a gap as merely giving braking or stopping distance is not particularly good enough when you can't see what lies beyond the first vehicle. It is better to think of this gap as giving you the luxury of being able to see beyond the vehicle ahead in order to be "ahead of the game" and to know why the vehicle ahead will brake. Where you can't see anything ahead and where it is very likely on that full motorway that someone ahead of you will brake without you knowing about it is where you see the painted chevrons on the road surface. These encourage you to keep at least a two-chevron/second gap where there is long straight stretch of motorway that has a history of rear-end shunts involving drivers driving too close.

 

If only drivers kept space for themselves more consistently.

 

If only other drivers didn't see that space as a way to get another car ahead...

[WeX]

Im not sure if anyone else has posted this but jams for no obvious reasons are caused by shock waves in speed caused by bad drivers either cutting others up or breaking for no reason.

 

See this:

 

---------- Post added 23-07-2013 at 20:41 ----------

 

Middle and outside lane hogs.

Scum of the earth.

 

I like you dislike these people but I know why they do it. Its down to ease of use of the road. Now if the roads is clear up to the horizon and they still insist on driving in the middle lane they deserve to die along with their family and everyone they know, but when the road is in moderate use, its easier to travel at a constant speed in the middle lane for two reasons. Lorries and other traffic. Lorries cause you to need to overtake and other traffic that refuses to let you in and in some cases will speed up to prevent you from changing lanes. It doesn't take much of this for people to say 'screw it, i'll stay in the middle'.

 

The real problem is our insistence on having the system where the left hand lane is slower than the right. It doesnt work, and middle lane drivers are an example of how it does not work. To complain about it but stick to it because its how we do it is retarded. We should be more like the Americans who allow you to over take on either side. They have highways 7 lanes wide and it works there, why not here?

[Cyclone]

They don't really encourage changing lanes in order to 'overtake' though.

[Temuchin]

The Daily M**l has some insight on this matter.