You can have smooth-running traffic or a traffic jam on the same road with the same number of cars.
The difference is following distance. People tailgate, and it causes a stop-start traffic jam. They rush up to fill any gap, only to brake and wait for the car in front to proceed.
If everyone maintains a good distance, everyone can move along a steady pace. This is one of the big benefits we will get from self-driving cars. They don’t have an emotional need to compete on the road.
From a recent Wall Street Journal article on the phenomenon:
There is a growing body of research finding that an individual driver, by preventing bottlenecks and maintaining a steady speed, can sometimes single-handedly ease or break up a traffic jam.
Mind the Gap
An individual driver with a calm, steady style can smooth waves of stop-and-go-driving before they expand into a traffic jam.
Disarm Aggressors
Keep a gap open in front of you so that when a road rager races to fill it, you won’t even need to tap your brakes. This prevents aggressive lane-changers from triggering jams.
Ease Exits
Encourage other drivers to merge into exit lanes in front of you, so they won’t have to slow down and block adjacent lanes waiting to merge.
Prevent Bottlenecks
When entering a congested zone where lanes are merging, stop pushing ahead Instead, open a wide gap and allow other drivers to merge into your lane without stopping.
The WSJ story is behind a paywall, but you can Select All and copy the text out of the page anyway.
Here is the guy who wrote the tips above. He’s obsessesed with the subject, and has some cool animated illustrations of the concepts. – William J. Beaty – Traffic Waves