What Causes “Phantom” Traffic Jams?

Photo by michaelrperry on Flickr.

Have you ever been cruising blissfully down the highway only to find yourself stuck in massive traffic jam? You wait, and wait, and wait, wondering what on earth could be causing such delays. Is it a crash? Construction activities? Rubbernecking from a big fire off to the side? Finally, you get to the end of the jam… and there’s nothing there. Transportation engineers call these delays “phantom traffic jams”, and they’re remarkably common.

The reason we have traffic jams in general is a concept called roadway capacity. Just like how a straw’s diameter limits how much liquid you can suck through it, there’s a limit to how many cars can be squeezed down a given roadway based on the number of lanes and other local conditions. For instance, under ideal conditions on a highway each lane can handle about 2,000 cars per hour.

When we think of traffic jams, things like crashes, construction, and police activity that constrict the overall width of the roadway usually come to mind. These impediments are called bottlenecks, and as you’d expect they cause the road’s traffic to funnel down into a narrower stream. So if one lane out of two is closed on an interstate highway, the road’s capacity is effectively cut in half. Any amount of traffic above the capacity of the bottleneck (in this case, above 2,000 vehicles per hour) is stopped up and begins to form the queue that we see as a traffic jam.

However, the vast majority of traffic congestion isn’t caused by bottlenecks. Most traffic jams are caused by there being simply too many cars on the roadway, resulting in a condition called oversaturation. Oversaturated conditions are typical in my adopted hometown of Washington during rush hour since there are simply more drivers who want to get downtown than the bridges across the Potomac can handle.

Since these oversaturated conditions are the real cause of our “phantom” traffic jams, there’s no specific, single cause that we can blame for these events. Just like how everyone is statistically equally responsible for taking the last Oreo from the package, each vehicle on the roadway is equally responsible for the oversaturated conditions simply by being in that location at that specific time.

Before we get into the science behind phantom jams, let’s take a look at a short video by some researchers at the University of Nagoya in Japan that actually shows a phantom jam forming under controlled conditions.

Without getting too much into traffic flow theory, it’s important to realize that there’s a relationship between travel speed, traffic volume, and the spacing between vehicles on a road. As more cars try to use a given stretch of highway, the space between each of the cars decreases. Eventually, this space – called the “headway” between the vehicles – decreases to the point where drivers can no longer behave independently of each other. At this point, reaction time becomes a significant factor and drivers must slow considerably in order to provide enough headway and therefore enough time to react to the actions of the preceding vehicle.

This occurence is what we see happening in the video. Although the instructions are simple – drive in a circle at a steady speed – at some point one of the drivers is going to have a lapse in concentration and will slow down for some reason, causing all of the following drivers to slow down and start a chain reaction of delay. You can actually see this process start at 0:08 into the video when the white car at the bottom of the screen slows down and the black car following it must brake hard to avoid a collision because its following distance  is no longer sufficient given the high volume of traffic on the “road”.

Once this process of hard braking begins, it is very difficult for conditions to return to normal. Because it takes longer for a car to accelerate up to speed than it does to brake to a stop, traffic volumes must decrease before conditions can improve. What’s more, once a road reaches oversaturated conditions, the volume flowing into the jam must decrease to about 80% of the roadway’s capacity in order to account for the inefficiencies of driver reaction time and resolve the jam.

So next time you’re driving along and come upon a traffic jam, don’t necessarily start cursing the actions of some impatient driver who got into a fender bender. In all likelihood, you’re stuck in a phantom traffic jam caused by nothing other than congestion – and the traffic jam is your fault too.

Now, you may be thinking to yourself, “if only they’d build more lanes, then we wouldn’t have as many times where roadways go over capacity.” In my next post, we’ll talk about why adding more lanes doesn’t help things as much as you’d think due to a concept called Induced Travel Demand.

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About Peter Kauffmann

I'm a transportation engineer living and working in the District of Columbia. By day, I analyze traffic patterns and create multimodal transportation plans for new and exciting developments in the Greater Washington region, and when I'm not on the job I enjoy backpacking, biking, and classic movies.
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2 Responses to What Causes “Phantom” Traffic Jams?

  1. George Huffman says:

    I’d heard that these waves of slow-down propagate upstream; it’s really cool to see it in a controlled test! However, once this starts, my observation is that people create the dangerous accordion stop-and-dash pattern by rocketing out of each slow-down, only to slam on the brakes a few hundred yards down the road.

  2. The phantom of phantom traffic jam has been unmasking. Looking for wavedriving in youtube.
    Please, visit http://www.youtube.com/watch?v=I7mjIdQh4Yw,

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