The self-driving cars may be available to give rides 24/7, but if 90% of the demand is 0800-0900 and 1600-0200, they may end up sitting idle for twelve hours each day anyway.

You probably need over 100x the vehicles at 5pm as you do at 4am. You have to find a balance of how much CapEx is worthwhile -- probably more than you need to cover the 4am shift, but far less than needed for the 5pm shift.