Buckle Up?

When I drive and when I’m a passenger is someone’s car, I always buckle up.  However, during a recent trip where I rode on buses I where buckling up was not an option, I began to think “why is it ok to not wear a seatbelt on buses and other transportation options like trollies?”  Is it any safer to ride in a vehicle such as a bus?  It’s one of those things in life that I take for granted.  I’ve rode on hundreds of buses in my lifetime and never gave it much thought, until now.  Occasionally, a bus will offer seat belts, but I’ll rarely use them out of habit.  Since I only ride buses or other forms of public transportation when I’m on vacation or a business trip, I don’t think about it often.  I wonder if anyone else has given much thought to this?