What makes traffic backup?

Question

Why is it that on an expressway you go from 70mph to a complete stop, then back to 70mph within a mile for no reason? There are never any accidents, it wasn't raining or snowing.. People were just stopped. This happens to me every day on 696 (Michigan). If everyone is driving at a reasonable speed, why do we stop?

Answer 1

slow dirvers, sun glare, a duck crossing the road...could be a ton of different things..

Answer 2

It can be difficult to tell an exact cause, because there is a good chance that traffic will have returned to normal wherever the cause was by the time you pass under the kind of circumstances you mention.

If you look at traffic on a large scale, you can almost apply fluid dynamics to how it works. Traffic back-ups behave like crystalization, with the front of the slow down being a bottleneck that eventually causes the fluid to backup and stop (crystalize).

If the leading edge of the backup is exactly equal to the rate of oncoming traffic, you'll have a case where there is an equal slow down for each car, necessitated by the slowing speed of the cars in front, and then the speed will increase back to normal. However, this would be very uncommon as all it would take to break this back to a normal flow is a small number of cars travelling at a slower rate of speed.

More likely is that you have a congestion where the leading edge evaporates more slowly than cars coming into it, creating an increasing backup. At some point the trend will reverse and the leading edge of the backup will begin to break and return to speed faster than cars are adding to it... and it will slowly work back along the length of the back up until it all returns to normal.

In high density traffic, the cause for the leading edge to form can be many. A few cars travelling too slow combined with a couple cars travelling too fast could easily create a back up, where the faster cars run quickly up on the slower cars (rather than just gradually moving to match speed) and then must put on their brakes, causing them to slow more than the already slower cars in front. This in turn will require the cars behind them to slow even more - and you can see how this pattern will repeat under high density conditions. Once the cars at the front (the leading edge) begin to move, the cars immediately behind will pick up speed, but this is not at the same instant as the cars in front. It will be a few seconds later. The seconds add up, and you have a back up where cars will be slowing down well after the reason for the original slow down has moved on (be it that transcient situtaion mentioned above, or something longer lasting such as an accident).

Answer 3

Actually, it has alot to do with the way the city has the lights at the bottom of the ramps set up on their timing. Also, you are looking at the fact that you have too many cars and not enough space. Plus the fact that everyone is in too much of a hurry to do nothing more than to go to their graves.

Answer 4

this is a chain reaction from a slow down at an earlier time. It would take a break in traffic that lasted longer than the original incident to get traffic flowing normally again

Answer 5

Because typicaly people can't drive! If one person shows their break lights its like a chain effect and then God forbid there happens to be a real accident along the line you have have the "rubberneckers" that cause things to slow down to a crawl.
Frustrating.

Answer 6

Too many distractions like cell phones, music, people looking at things outside (accidents) and some people are tired (snoozing at the wheel), in early morning rush hour, I see people who look like the have not woken up yet...and they are the ones driving really slow!

Answer 7

Because no one knows how to drive and when they all come together weird things happen.

Answer 8

because if one person slows down to get off ramp,,everyone else does also,,and then you have to re-fain your speed back,its that way here in the south too,it just takes time for everyone to react,,and by that time,everyone has slowed down,,it gets worse too,,the more drivers there is,,the worse it will get,good luck i hope this help,s.

Answer 9

General slowing due to fear of accidents.

Answer 10

just the idiots in front of you