When Bolivia banned all non-emergency vehicles from city streets for a day last week, air pollution levels dropped by an astonishing 60-70 percent. The difference in air quality was also noticeably clearer, mostly due to the fact that 70 percent of air contaminants in the country come from vehicles. Hosted annually since 2011, the vehicle-free day is dedicated to encouraging pedestrians to get active and has become so popular that some cities ban vehicles two to three times a year.