City buses (at least in europe) have roughly the same amount of standing vs. sitting people. If they have to use seatbelt, they could not allow standing passengers, reducing capacity and flexibility of use, thus making it less attractive and less financially viable. It would also be impossible to enforce the rule!
Intercity buses, where there is no space (nor is allowed) to stand, seatbelts are in fact provided and, in theory, compulsory. But in reality I never saw many people wear them.
Latest Answers