This is only an issue during peak times. Most of the time during the day, buses are fairly empty. If driverless car companies use something like uber's surge p

This is only an issue during peak times. Most of the time during the day, buses are fairly empty. If driverless car companies use something like uber's surge pricing, so that it is more expensive to travel 1/2hr before/after peak, people will respond, flattening out peak demand. Intelligent routing software will assist, allowing people to car pool to reduce the required number of cars. Finally, let's say if automated cars remove half the riders from buses, the costs of providing buses will need to be spread across half the number of riders, making the economics of running a bus system twice as expensive per passenger. At that point, it makes sense even more sense to replace the buses with cars. The only question will be how long it takes for people to adapt due to existing habits having momentum.