When I commuted to Tysons from DC, I would have lots of time stuck in stop-and-go traffic to consider the reasons and causes for traffic slowdowns. It seemed that often, we'd all be slowing down and then speeding up again for no reason. I always speculated that sudden braking could cause a wave of slowdowns that would propagate backwards for hours/miles, leading to the annoying slow-down-speed-up cycle.
A team of Japanese researchers recently created an experiment to show that the effect of breaking can easily create traffic jams. It's all just physics. Their experiment shows that when the density of cars on the road passes a certain threshold, traffic jams will be caused simply by the multi-particle interactions of the system. In layman's terms, drivers inconsistently hitting their breaks will cause a wave of stop-and-go backups behind them.
Here's a video of their experiment showing that, indeed, traffic isn't always caused by an accident:
In the experiment, the cars were spaced out evenly around the track. The drivers were told to drive at the same speed and maintain a safe distance from the car in front of them. Traffic initially flows smoothly, but not every car drives at the exact same speed. Soon the vehicles are no longer evenly spaced out, and cars quickly begin to bunch up at one point around the track. When the bunching occurs, drivers have to hit their breaks and slow down, sometimes even stopping, before eventually speeding up again. Most importantly, the researchers found that the critical density was 22 vehicles. Bunch-ups occurred with 22 or more cars but not with fewer than 22 cars.
Here's a link to the full study in the New Journal of Physics: Traffic jams without bottlenecks — experimental evidence for the physical mechanism of the formation of a jam.