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

[algy]

There was a study done some years ago that identified a ripple effect caused by traffic joining the motorway. Traffic moves at a steady speed, safely spaced out. Joining traffic from the slip road filters into the stream, reducing the distances between vehicles, drivers therefore slow down to match the new distances. As the distances are restored, the speed of the traffic flow is also restored.

[tinfoilhat]

There was a study done some years ago that identified a ripple effect caused by traffic joining the motorway. Traffic moves at a steady speed, safely spaced out. Joining traffic from the slip road filters into the stream, reducing the distances between vehicles, drivers therefore slow down to match the new distances. As the distances are restored, the speed of the traffic flow is also restored.

 

I think that's fine in theory but there is, based on my own experiences sat in plenty of queues in the last 6 weeks, there is a saturation point. No lane closures, no accidents just lots and lots of traffic.

[Daid]

Middle and outside lane hogs.

Scum of the earth.

 

I thought that accolade went to more deserving types like, well everyone that Sheffielders don't like to be fair.

[davyboy]

A lorry on fire.

 

A few weeks ago we were traveling to Cornwall via the M2 M26 M25 M3 etc

Spent 2 hours stuck on the M26,

The police turned us all round, so onto the M20 to get to the M25 further north. Stuck for 2 hours, as there were two lanes queuing to get on the M25.

Decided to call it a day went up the outside lane to the A20 turned round and went home.

6 hours in the car, 120 miles and never left our home county.

If we had been going to Gatwick or Heathrow we'd have missed our flight.

[geared]

There was a study done some years ago that identified a ripple effect caused by traffic joining the motorway. Traffic moves at a steady speed, safely spaced out. Joining traffic from the slip road filters into the stream, reducing the distances between vehicles, drivers therefore slow down to match the new distances. As the distances are restored, the speed of the traffic flow is also restored.

 

That'd work brilliantly if everyone operated like a computer, but we don't, everyone is unique and so is their driving.

 

There's no use modelling a traffic system which is based on everyone following rules, because they simply will not do it.

A more effective model is one that will accept that people are going to do what the hell they want and controlling everything they do is impossible.

[Xt500]

There was a study done some years ago that identified a ripple effect caused by traffic joining the motorway. Traffic moves at a steady speed, safely spaced out. Joining traffic from the slip road filters into the stream, reducing the distances between vehicles, drivers therefore slow down to match the new distances. As the distances are restored, the speed of the traffic flow is also restored.

 

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.

[DT Ralge]

That'd work brilliantly if everyone operated like a computer, but we don't, everyone is unique and so is their driving.

 

There's no use modelling a traffic system which is based on everyone following rules, because they simply will not do it.

A more effective model is one that will accept that people are going to do what the hell they want and controlling everything they do is impossible.

 

I think you'll find that motorway traffic models are built on actual traffic volumes and density, actual speeds at different times of the day.

The data for these models comes from the buried loops in the road surface roughly every 500 metres that make up the MIDAS system (Motorway Incident Detection Automatic Signalling).

[Bruno]

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?

 

Easy answer is braking, imagine a group march, you are leading then all of a sudden you stop, then Immediately continue! The person behind you has to stop also, then immediately continues, this carries on throughout the line, but further down the line one persons reaction may not be as quick as the one in front so the person behind him crashes into him.... You get the drift...... And that scenario happens on motorways, if you think about it, anyone of us could be the cause of an accident by driving too close to the vehicle in front then braking, it could be half hour after before an accident occurs.

[Cyclone]

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".

It's probably unrelated to that sign.

The 'stop start' traffic you find in busy traffic is as you suggest down to the weight of traffic.

One person brakes for some reason, the next person brakes more heavily, by the time this has happened for 20 cars, the one at the back is breaking to a 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?

 

The 40 might be a trigger for a few overzealous people to actually brake, but the problems it causes most likely resemble ripples of stationary traffic moving backwards. You could see it all from the air really easily.

 

If you're interested in detail you could google traffic laminar flow and see what studies pop up.

[DT Ralge]

It's probably unrelated to that sign.

The 'stop start' traffic you find in busy traffic is as you suggest down to the weight of traffic.

One person brakes for some reason, the next person brakes more heavily, by the time this has happened for 20 cars, the one at the back is breaking to a stop.

 

The 40 might be a trigger for a few overzealous people to actually brake, but the problems it causes most likely resemble ripples of stationary traffic moving backwards. You could see it all from the air really easily.

 

If you're interested in detail you could google traffic laminar flow and see what studies pop up.

 

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.