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

[PeteMorris]

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?

[Ousetunes]

If there was one subject I was bobbins at at school and still am, it's science.

 

But surely, if everyone is whizzing down the M1 at 80 mph and suddenly they drop their speed to 50 mph, the traffic will 'compress' and in effect, begin to back up? Those at the back are joined by other motorists coming from a 70 mph zone into a 50 mph and the result is traffic slowing to a point where it occasionally grinds to halt?

 

I'm guessing, of course I am, but that's how I see it.

[willman]

In my experience on motorways especially between 31 and 27 of the M1, it's slow traffic.

Slow moving traffic in lane 1 forces vehicles moving SLIGHTLY faster to pull into lane 2 for a 15 minute overtaking manouevre. Everyone else then has to visit lane 3 to get past.

[Annie Bynnol]

A group of vehicles travelling at 80mph break hard when the traffic in front travels at 60.This group will slow to below 60 because they panic see red brake lights etc.

The group behind these slow even more and progressively the speed decreases. This is sometimes referred to as a standing wave.

To overcome this problem speed limits and spacing chevrons are used to smooth the flow more traffic can move.

As yet it has not been possible to explain to drivers how travelling at 65 mph on long unbroken stretches of Motorway enables more traffic to get from A to B.

Same principle applies to connected traffic lights in urban areas.

[tinfoilhat]

A group of vehicles travelling at 80mph break hard when the traffic in front travels at 60.This group will slow to below 60 because they panic see red brake lights etc.

The group behind these slow even more and progressively the speed decreases. This is sometimes referred to as a standing wave.

To overcome this problem speed limits and spacing chevrons are used to smooth the flow more traffic can move.

As yet it has not been possible to explain to drivers how travelling at 65 mph on long unbroken stretches of Motorway enables more traffic to get from A to B.

Same principle applies to connected traffic lights in urban areas.

 

Pretty much on the money. Also known as phantom jams. For the best example observe most motorways on a Friday afternoon or sections of the m25 at nearly anytime.

[smithy266]

Not keeping to the nearside also helps. Outside lanes get crowded, then braking as has been mentioned. Drivers not keeping pace with the car in front when it speeds up is another.

[medusa]

This is basically a product of just too many cars on the road at certain times of day. Statistically the traffic flow is less impeded by the same number of cars travelling a steady 40mph than the same number of cars attempting to travel at 70mph and having to frequently brake and accelerate because the road is too full for that to happen.

 

Some clever boffin worked out that (for instance) 100 cars travelling at 40mph without the constant start/stop will clear the stretch of road faster than 100 cars doing the starting and stopping, so when the road is really busy 40 is actually faster than 70

 

When the road is very full one car will change lanes or do some other manoeuvre which then causes the car behind them to touch the brakes. This causes the people behind to touch their brakes too in a ripple effect out behind them. When the cars are driving with proper stopping distances between them this stopping distance absorbs the braking of the first few cars and the effects of the braking go no further, but when cars are driving really close together like when the road is really full the effects are very different.

 

What happens when someone brakes when the road is full is that the cars get ever closer together when under braking and that means that hundreds of yards or even miles behind the car that originally braked the effects can still be seen. When cars have to brake to a standstill on a busy road but there's no visible reason for it when you get to the front of the queue, the usual reason is that someone braked gently a few miles ahead 15 minutes ago.

 

It is this cycle that driving more slowly gets around (statistically, at least). If the speed limit is 40 and the start/stop cycle is still happening then that's a sign that the road is too full even for it to function at that speed.

[Xt500]

Rubber necking.

Variable and unessercary speed limits.

inapropriately lowered speed limits

Bad attitude from lane hoggers

over braking

and micra types who seem to think they are in the Asda car park

 

---------- Post added 22-07-2013 at 16:51 ----------

 

oh and of course them that miraculously manage to crash while driving in a straight line!

[DT Ralge]

It's down to inefficiencies in the queue - drivers too close, nervous braking, drivers changing lanes causing others to brake.

Mathematicians have modelled this inefficient queue and have expressed in a formula, I believe.

The model recommends lower enforced speed limits in congestion - that works well to keep us all moving by shrinking the difference between the fastest and slowest, by allowing more vehicles to move together and more closely packed at the lower speed in a moving queue.

Furthermore, as regards drivers chopping and changing lanes, you'll note the overhead signs telling you "Congestion, stay in lane!"

[Xt500]

It's down to inefficiencies in the queue - drivers too close, nervous braking, drivers changing lanes causing others to brake.

Mathematicians have modelled this inefficient queue and have expressed in a formula, I believe.

The model recommends lower enforced speed limits in congestion - that works well to keep us all moving by shrinking the difference between the fastest and slowest, by allowing more vehicles to move together and more closely packed at the lower speed in a moving queue.

Furthermore, as regards drivers chopping and changing lanes, you'll note the overhead signs telling you "Congestion, stay in lane!"

 

Variable lower speed limits some how speeds up traffic is nothing but a con!