Car accidents cost over a million lives worldwide every year - 45,000 in the USA - and injure about 40 times that number. They are the leading cause of death for people aged 18-34. In the USA, the total cost of accidents is about $230 billion (1.7% of American GDP according to my calculations).
Robot cars will reduce car accidents, probably by a lot (human errors make up 93% of the causes of car accidents; 80% of accidents are due to human inattention). They'll make parking vastly more efficient (the car can drop you off and go and park in some optimal place). 50 billion person-hours are spent driving every year. In productive hours, that's worth a trillion dollars. Even if the hours aren't productive (watching TV or whatever), it's still worth a lot. Congestion would be reduced because robot cars don't need the follow space between cars that human reflexes require. It's like quadrupling the capacity of existing roads for free. Since the crash rate might be so low, cars might be ultralight rather than made of armor, so they'll use less energy and less metal.
I first became excited about robot cars from reading this essay: http://ideas.4brad.com/robocars-are-future