Learning this in Psych it is known as the 'Bottleneck Theory' limiting the amount of information that goes through the brain. Just like your brain can become congested with information, people are processing so quickly everything begins to slow down.
If everyone were traveling at the same speed 55-65 mph there would not be congestion unless of course there was an accident. Since people all process information at different intervals or become distracted their brains process at a slower rate.
Example: If the first car was driving at 55 than 5 to 10 vehicles would follow during rush hour. Vehicles 11-15 begin to become distracted and slow down. 16-20 begins to tap the breaks and the chain reaction takes place. Before you know it you are in a gridlock because the brain of the driver/driver's in front of you all moved at a different pace.