Why do traffic jams happen?

June 18, 2004 11:44 PM

Why do traffic jams happen? [mi]

I can't count the number of times that I've been driving down the freeway, only to get snarled in some inexplicable traffic jam. After an indeterminite, frustrating amount of time, it seems to disappate, and I'm on my way again. I have never understood why this happens.

I'm aware that once and a while there are accidents, closed lanes, etc., and on those occaisions I mutter to myself "stupid [accident or closed lane or whatever]" and I move on. But most of the time, I can't figure out what the problem was.

I also may be putting too much stock in my fellow humans; my supposition is that if everyone is doing their best to travel at reasonable freeway speed, there is no reason there should ever be an extended traffic jams on the freeway. Please, someone explain this phenomenon to me.

posted by Yelling At Nothing to science & nature (10 answers total)

This is pretty interesting.
posted by interrobang at 11:54 PM on June 18, 2004

This probably has your answer as to why traffic jams occur, and why it takes so long for them to recover afterwards (which is why it's better to start work earlier than it is to start work later). It even has a cool little animation to show the effects.
posted by calwatch at 12:02 AM on June 19, 2004

Having observed traffic waves for a long time (as described in interrobang's link) my take is that traffic jams, when not obviously caused by too many cars, are caused by the imperfect reaction times of drivers. If drivers could respond instantly to the change in speed of the car in front (e.g. at traffic lights) then traffic would move much more smoothly and there would be less of the typical stop/start behaviour. Roll on computer-controlled cars!
posted by adrianhon at 1:08 AM on June 19, 2004

I'd guess there are many reasons.

Overall:
I'd say the major symptom is high traffic density that gets to a point that causes driver greed / impatience to push the system to critical failure. Said another way. The volume of cars increases at a rate where the new cars cannot get their average speed up before another new car enters the system. Average speed crashes to the point where it can drop no further.

Some things I see that cause this:

Merging from a ramp:
-Many slow drivers come in effectively knocking out one or two lanes. Drivers in the lane effectively stop to let new drivers in. Drivers in the lane being stopped try and move out of the way effectively stopping the next lane over. Repeat.

Merging from many lanes to one:
-Everyone wants to go as fast as they can right up to the point where they have to merge down. The result is that traffic basically stops at the point where everyone has to merge down. The stop back propogates.

Lane changing:
-Traffic effectively comes to a stop or very slow. Some lanes progress but drivers in the stopped lanes try to get in the moving lanes. This again causes the lane's average speed to drop. This doesn't help the problem.

Trying to drive to fast for the traffic volume:
-Accelerating behind a car infront only to brake again. Then accelerating only to brake again. Leads to a lower average speed than simply driving slow.

Drivers are greedy actors and do a poor job of managing the commons.
posted by rudyfink at 2:34 AM on June 19, 2004

From a 1999 Discover article:

"At certain traffic densities, small causes have large effects," Helbing says. "In particular, most types of congestion are avoidable--they aren't caused by overloading of the highway but by small disturbances that grow and at some point cause the traffic to break down."

This is pretty much what's said above about reaction times piling up until you have people coming to a dead stop.
posted by Nothing at 5:34 AM on June 19, 2004

Trying to drive to fast for the traffic volume:
-Accelerating behind a car infront only to brake again. Then accelerating only to brake again. Leads to a lower average speed than simply driving slow.
In the UK, the M25 around London has a variable speed limit to try and counter this problem. If there is a lot of traffic, the speed limit is reduced, to try and encourage a higher average speed. Unfortunatley, this same motorway has a higher than average number of tossers driving on it, so I'm not sure it has a chance to work.
posted by chill at 7:38 AM on June 19, 2004

Here's a Java applet that lets you play at being a vengeful traffic god. [joke © 2003 christian]
posted by Johnny Assay at 8:29 AM on June 19, 2004

If there is a lot of traffic, the speed limit is reduced, to try and encourage a higher average speed.

See, that is completely backwards.

Every road has a saturation level, at which point it no longer effective carries traffic. This saturation level is a function of many factors including size, terrain, obstacles... and speed. The higher the speed limit on a road, the more traffic can traverse it in any given time, the higher the saturation level, the fewer the traffic jams.

If you're having trouble getting N cars over a stretch of road at rush hour, let them go faster.

The problem often isn't the people driving fast -- it's the disparity in speed between the people driving the reasonable speed limit and the people driving the legal speed limit, with the latter bottling the system up for everyone.
posted by jammer at 9:24 AM on June 19, 2004

Here's a very interesting article from the New Yorker about the people who control Manhattan's traffic lights. The line between freeflowing traffic and gridlock is very small and depends on precise adjustments of the crosstown traffic's green light time.
posted by mookieproof at 10:01 AM on June 19, 2004

if everyone is doing their best to travel at reasonable freeway speed, there is no reason there should ever be an extended traffic jams on the freeway

There's one simple fact you can't get around. Take a mile of freeway. To fit more and more cars on that stretch of road, you have to squeeze the cars tighter and tighter together. You can't drive really fast when you're only 5 feet from somebody's bumper. So, as the number of cars on any freeway increases, the space between the cars must decrease. They become more densely packed. And as space between cars decreases, safety dictates that speed must also decrease, because of reaction time / braking distance.

The beauty of this point is that it doesn't require any complex thought about on ramps, turns, psychology, etc, all of which compound problems on top of it. But this effect applies just fine to any simple straightaway. It's just one thought, hardly the key to it all. But I think it says something interesting: if the number of cars is the problem, then you're part of the problem yourself. Why do traffic jams happen? Because people have places to go.
posted by scarabic at 6:32 PM on June 19, 2004