TedEd: What is phantom traffic and why is it ruining your life?

A TedEd by Benjamin Seibold

You’re driving along when suddenly, all the cars in front of you slow down. There’s no accident, stop light, no speed limit or road changes. So what’s happening? This is a phenomenon known as the phantom traffic jam. For this to occur, there must be a lot of cars on the road. This doesn’t mean that there are too many cars for the road - at least when everyone is driving at the same speed and maintaining spacing between cars. But all it takes is a small change. If one driver brakes a little, each following car will brake a little harder, creating a chain of stop and go. This can continue down a highway for many miles. When there are fewer cars on the road, minor changes in one car do not affect the others, because the others have room to adjust. However, when cars get less than about 35m apart, dynamic instability occurs. This effect causes natural phenomena as well, like raindrops. When a group passes a critical density, each additional object decreases the number of objects passing through a certain point at a certain time. This usually occurs because drivers brake too late and accelerate too quickly in traffic jams. This in turn causes waves of braking, and the cycle continues. Self driving cars can help solve this problem by taking data from other cars to maintain the proper speed, equal to that of the average flow. In the case where there is already a traffic jam, they could brake more softly than a human driver, decreasing the effect of the wave. And studies have found that it only takes 1 in every 20 cars to be self driving, so future innovations could save energy and make our commutes more efficient.