Why does traffic stop on the interstate?

For the same reason they answer questions on WikiAnswers. They are curious about things. When they look at something interesting, (an accident, a car parked by the interstate, a funny cloud, a plume of smoke, etc.), they slow down so that they won't run into somebody while they are busy paying attention to something besides their driving. This causes the people in back of the first car to slow down, so that they don't hit them. If a critical number of cars are attempting to pass through the same stretch of road at the same time, then overly cautious drivers will slow down even MORE than is needed in order to avoid an accident, (a very costly event indeed, in terms of wasted time and possibly even wasted money), and cause even MORE congestion which then results in a "snowballing effect". If the number of cars attempting to enter a congested section of roadway stays constant, then the resultant traffic jam will persist until enough traffic flow is eventually reduced to below this critical point. It is quite fun to watch traffic jams from the news helicopters, as it is quite plain to see that they persist LONG after the originating event has passed or been dealth with. If you want a traffic jam to end sooner, then simply refuse to look at anything else besides the distance to each of the 4 or 5 cars in front of your vehicle and keep your vehicle moving at an optimum rate of speed for safety. Bet you can't do it!