Traffic on motorways can slow down for no apparent reason. Sudden changes in speed by one or two drivers can create a chain reaction that causes a traffic jam for the vehicles that are following. This kind of phantom traffic jam is called a "jamiton" and the article discusses some of the ways in which traffic engineers produce mathematical models that seek to understand and, if possible, reduce this effect. Ways in which jamitons can be simulated in the classroom are also discussed. (Contains 16 figures and 1 online resource.)