The short answer: braking and full roads. When there are between 10 and 15 vehicles on a one-kilometer stretch of highway and the front one hits the brakes, a "backward travelling wave" is created that can sometimes lead to traffic jams. As Dr. Gábor Orosz of the University of Exeter's School of Engineering, Computing and Mathematics, said in a statement: "As many of us prepare to travel long distances to see family and friends over Christmas, we're likely to experience the frustration of getting stuck in a traffic jam that seems to have no cause. Our model shows that overreaction of a single driver can have enormous impact on the rest of the traffic, leading to massive delays." He continued: "When you tap your brake, the traffic may come to a full stand-still several miles behind you. It really matters how hard you brake - a slight braking from a driver who has identified a problem early will allow the traffic flow to remain smooth. Heavier braking, usually caused by a driver reacting late to a problem, can affect traffic flow for many miles."
This seems like a problem with no solution. Not braking could lead to accidents, which certainly don't make the highways easier to travel on. And removing cars from the road would be appreciated by many, until public transportation becomes a better option, it ain't gonna happen. So, if you get stuck on the way to or from grandmother's house this year, at least you now kind of know why.
[Source: University of Exeter via Scientific American]