If you can walk/bike to your job without the threat of being run over, you are one less car on the road.

Thihk of it in levels:

1. People will skip work-commuting by car (including students) when there are other viable options that are not made life-threatening by other people in cars. Fewer trips= less traffic

2. People will avoid driving for errands when there is decent local public transit that lets them shop where they want. Even fewer trips = even less traffic

3. People will stop owning cars when there is decent local public transit and decent regional networks. Fewer cars = less traffic