Using seat belts reduces traffic injuries and fatalities. There are no observational studies comparing use of seat belts by front and rear seat adult passengers in the United States (U.S.), and no comparisons of rear seat belt use by adult passengers in private, taxi, and rideshare vehicles. This observational study of drivers and adult passengers in two cities, one with a primary and the other with a secondary seat belt enforcement law, found the laws affected seat belt use by drivers but not by rear seat passengers, and that rear seat passengers wore seat belts more in private vehicles than in taxis. The latter result was not because of lack of seat belt availability in taxis. Results also showed that modeling of seat belt use by drivers had little effect on seat belt use by rear seat passengers. Results from rideshare vehicles was mixed. Implications for future research and countermeasures are discussed.