From a Worldwatch Institute paper (October 1990) by Marcia D. Lowe:

More than half of all U.S. work trips and nearly three-fourths of those in the United Kingdom are eight kilometers {approximately five miles} or less -- and commutes in more densely developed settings are even shorter -- but people prefer to drive rather than face dangerous streets without the protection of a motor vehicle. In Third World cities, where most people have no choice but to walk or cycle, the few people who drive are allowed to dominate the streets.

The most effective way to make cities safer and more convenient for walking and cycling is to keep motor traffic from commandeering urban space. Because of their mass and speed, cars automatically take over streets, intimidating and endangering people on foot or on bicycles. For cycling and walking to be viable means of transport, people must be able to move safely throughout the city without a motor. Ensuring that pedestrians and cyclists have a continuous route to their destination calls for separate lanes and paths in some situations (where motor traffic is heavy and traveling quickly), but more often, it requires making motor vehicles share regular streets with slower traffic.

It takes active restrictions on automobiles to make some streets safe for slower traffic. Particularly on residential streets, making cars slow down with speed limits and physical barriers can discourage drivers from "rat running" -- racing down side streets to avoid congested intersections and delays on larger roads. Many European cities use these restrictions, known as "traffic calming," to turn streets into places for people who live, work and shop there, instead of for drivers just passing through. ... {T}he scheme's chief contribution is to safely accommodate pedestrians and cyclists on city streets. ...