An expressway can carry 67 cars per lane mile (or 78 feet per car), without delays, but after that traffic slows because there isn’t enough space between cars, and to maintain stopping distance, motorists must reduce speed.

 

So a 6-lane expressway can carry 200 cars per mile at any particular time in one direction, smoothly. Any more then that, and traffic slows to maintain distance between cars and crashes become much more likely. Crashes tend to take lanes completely out of service for several hundred feet, which cause cascading reductions in capacity.

 

At 185 cars per lane mile (28 feet per car, or 20 feet plus 8 foot buffer), traffic comes to a complete stand still, as no more cars can fit on the highway until some motorists are able to exit the highway or move to another mile point.