Buseses on wheels are more flexible than trams or whatever on tracks, and can route around a collision.
Rails in the road are a severe hazard for cyclists and somewhat hazardous for pedestrians.
I'm glad you said this because it's a key point so many in this thread appear to miss: what works for one person or locale is not the solution for another, therefore cars aren't going away but this does not negate the legitimacy of bikes or trains either.
Self-driving cars would enable self-driving taxis which would be superior to both private vehicles and public transport.
Imagine if there was no need to park and you could summon a vehicle of the appropriate size at will.
* It could be even worse. Imagine the morning rush - thousands of taxis have driven to peoples' homes and then driven them to their work/school. Then most will have to park until the next peak of demand in the evening. Either they park in the city, or they drive to an out-of-city parking lot, then drive back in to collect, then drive people home, then drive somewhere else to park until the morning rush. What would have been two vehicle journeys using a private car became six.
One problem with buses is that paying the drivers is expensive. As a result buses need to be big to amortize the cost of the driver across lots of users. But big buses don't work well in low demand times of the day. The only option is to run them less frequently, which sucks for users. And big buses don't fit down small streets. And fewer larger buses means fewer routes. Self driving buses could solve all these problems.
I'd still prefer to walk or cycle, but I'm probably biased because I live in a warm, dry and flat part of the UK and don't have kids.