Most motorists insist on bunching up together at red lights. Because of this, when the light changes to green, each car needs to wait for the car in front to get far enough away so he can start.
Cars waiting at lights are like cars in a train. In theory, when the light changes, all the cars should be able to move simultaneously, assuming they all accelerate at the same rate. By close observation, it can be seen that the reason this doesn't happen in practice is that they are way too close to the car in front.
If each car left about 5 - 6 feet between his car and the one in front, all cars could start moving at the same time when the light changes.
On rare occasions, the theoretical becomes reality and all the cars move at once, even the one in the rear of the line. When this happens, it always is surprising to that last driver in line, since he expects to be waiting a while longer before moving.