Why traffic stops on the freeway

I just want to point out that the Science Hobbyist page has a really excellent explanation of this phenomenon. I had noticed his conclusions myself, but couldn’t put it into words before reading it.

I have actually tried smoothing out a lane of traffic (to get rid of the 0 - 30mph - 0 inchworm effect), and it WORKS. Unbelievable what a big impact on traffic a single car can make. And I did it on Route 17 in Paramus, NJ at 5:30pm. If you know the road, you know how amazed I was and that it’s a perfect proving ground for the theory. :slight_smile:

(I think the author is actually registered on this board, too)

My experience is not consistent with the theory provided in your link. At least, not to the great extent that the board author professes.

Consider there is a whole class of drivers who regularly follow the driving patterns indicated by the author - they have to. They are big truck drivers. Semis, dump trucks, garbage trucks, cement trucks - all have reduced acceleration and braking capability, so they have to maintain a large gap in front of themselves and drive slowly. Yet somehow they don’t manage to prevent traffic jams.

What happens? Every car becomes the guy who speeds around the truck and then merges into the gap to get as close to the front as possible.

Even if you do follow the practice and take your big gap into the jam and relieve it, unless that’s a one-time spot (such as accident site), the relief will only last until the next driver behind you doesn’t let the guy next to him merge. Poof - traffic jam again. Consider those regular jam locations, like merging lanes or on/off ramps. Those consistently develop jams. Sure, you may make a temporary dent that lasts a few cars behind you, but then it just reforms.

He’s correct that if more people drove that way, the situation would be better for all. However, that old rule about one car length for every 10 mph (or 2 seconds between you and the car in front of you) is rarely followed.

Now my comment about Cecil’s column. I’m curious about his numbers of 40 cars per minute, and 25 cars per minute. I wish there were an easy way to see just how many cars that is. Given Houston highway traffic, the cars pack pretty densely. The rule of thumb - if you leave more than a car length between yourself and the car in front of you, someone assumes that space is for them and pulls into it.

I’m backing Joe Cool. You can, individually, have a small effect by slowing right down, hanging back, and trying if at all possible to keep moving, even if very very slowly, so that you never actually have to stop altogether because of the jam in front. Obviously, if the jam is too bad, it doesn’t work for reasons such as those given by Irishman. You can’t hang back far enough to keep the flow, and/or the gap you need to create is so large that others move into it. As the author of the website to which Joe Cool links says

But the feeling you get when you look back and see you have partially evened out a jam is pretty cool.

I heard once that in Germany they had implemented a system of signs that could be turned on to temporarily lower the speed limit to allow a “rolling jam” further up the freeway (autobahn) clear. And apparently it worked beautifully, because Germans are law abiding, and are able to understand that if they co-operate it will be best for all, in the long term.

But when they tried the same system in the UK, it didn’t work because everyone ignored the lower limits.

Don’t have a cite though.

The New Scientist, had an excellent article on this a few years ago - I’ll see if I can find it on their site. I don’t think that one person can make as much difference as the OP suggests but if a few did I bet it would make a significant difference.

Slight hijack

I think (and it’s my two pet peeves)(about driving) that other causes of jams are :

People driving in the wrog lane ie people driving at 40 in the overtaking lane even if the slow lane and middle lane are clear.

People driving at the speed limit purely to slow down drivers that want to go a bit faster.

Sorry, forgot the link.

No stoplights on the freeway, so why does traffic halt?


I was going to post something about the ‘jam with no cause’ having been caused by a now-missing wreck, and the jam slowly ‘backing away’ from the original cause, but persisting nonetheless, but I had no link near as cool as yours.

The topic was covered on Scientific American’s ‘Frontiers’ program, with a computer simulation, and the simulation was an epiphanaic thing to watch. I haven’t gotten nearly as frustrated in traffic since I saw that.

The writers of this article have come to a different conclusion. Interestingly, they used " molecules in a gas" as the basis for a computer model of traffic.


that was a hella cool article (and i live in seattle so i know all the places he mentioned). i think ill try out his method and see if it makes a difference

It definitely is time drivers pass control over their cars to a supreme authority.

Imagine every car had a radio transmitter tuned to a certain frequency and equipped with a number. When you enter the freeway, your transmitter logs onto the network, and the car is teered by a central computer. Jams would be over, since this “accordion effect” (that’s how my driving school techer called it) would not occur any more - the computer would pilot all the cars over the freeway at constant speed. It would save us a lot of trouble, avoid accidents, and save time as well as gasoline, since the energy-consuming infinite play of braking, waiting, accelerating and braking again would stop.

Schnitte, the US department of transportation (DOT) is currently testing similar methods.
Intelligent Vehicle Initiative

From this article, written, I would guess, in October 1997:

I submit what I refer to as the ‘Slinky Effect’, in which the first car in line slows down to (for argument’s sake) 45. The next car in line must then slow down to at least 44 to increase the spacing. The third then must drop to 43 or less. Fourty cars later you’ve reached zero. Now the next car behind is also at zero, and must wait until the car ahead begins moving before they can also begin moving. The delay increases with each stopped car. To illustrate, streach a slinky across a hard floor and watch the wave patterns pass up the line when you introduce a wave. This is what a traffic 'copter must see from above.

Nope. The second car slows to 44 for a moment, but once they have created a gap they can then increase back up to the speed of the first car, 45.

But the time it would take for the knock on effect you suggest, right down to car 45 reducing to 0 is less than time it would take for the second car to slow, create a gap and then speed up again.

I think you may be on the right track generally, but the example you give is a crock.

Actually, I think Gregcdss is more or less right.

The above is only right if you pretend that people are accurate with their braking.

In reality, when the first car brakes to 45mph, the second car will slow to considerably less, especially if the gap between the cars is fairly close. When a car brakes in front of you, you slow to lower than its speed, then make up the difference. If you didn’t you’d hit it before you’d got the speed right!!

Therefore, the second car will slow to say, 40mph, or less, the one behind to 35, or less, etc. You only need 10 cars travelling close together to cause a standstill.

By example, I travel a little country road to work (in England). 1 land, windy, with a number of junctions off it. If a car pulls out of a junction into the road ahead, causing a car, say, 100m in front to slow, I will have to stop, despite the fact that none of us were doing more than 40mph in the first place. This is because, as above, the first car slows, the second slows more, and so on.


Like I said, [b[martiju**, I think the example given was a crock (the figures and time given were his not mine), although he was one the right track. As is clear from your examples, it depends on many factors, such as speeds, type of road, how cautious people are being, etc.

I realize everyone is talking about the compression waves that occur in traffic but the real source of conjestion in the first place was misleadingly presented by Cecil.

Cecil confused the concepts of Capacity and Thruput. The thruput on a lane of freeway is fairly constant until the speed starts to get fairly slow. This is because the separation rule is a linear function of velocity. Think about it … if every car is spaced by 2 seconds then there will be a car crossing a fixed point every 2 seconds regardless of speed. The only reason thruput reduces as speed gets slow is because the 2 second rule has to be defined from the back of one car to the front of the next car. Its the “extra” space taken up by the cars the reduces thruput. Another way to look at it is at very slow speed it is impossible to maintain 2 seconds of separation if the definition is front-of-car to front-of-car.

However, the real culprit in traffic is Demand on Capacity (cars per mile per lane). Using the 1 car length per 10 mi/hr rule (equivalent to a 1.2 second rule), the capacity on a road only increases significantly when the speed is considerably slower than 60 mi/hr. Because there are multiple entrances on most freeways the potential demand on capacity can be huge.

You just can not model it as one stretch of road with one entrance and one exit.