Actually, I know what holds up the traffic, in two words.

The road.

To be serious, though, there is a mathematical theory behind the way there seems to be a traffic jam, but with nothing at the front of it. It involves pulses and such things, and a brief explanation follows.

It all starts when a car suddenly brakes in the middle of the road. The car behind it also brakes, and the car behind that etc. Assuming there are no collisions, the first car moves away. The second car moves away after the short interval that it takes for the driver to notice. The third car moves away after a similar interval, but the short intervals are starting to collect up now. This means that by the time the sixth or seventh car moves off, the first car is already a few miles away. After a few hundred cars have passed through this pulse, there are serious tailbacks, and nobody knows why, not even the driver who started it all off, who now is probably at their destination, oblivious to the chaos they have caused in the simple act of depressing the brake pedal.