It's a little crass to ask the question, I think. Widespread introduction of robotic cars would be one of the greatest humanitarian achievements in American history. You don't ask what's the profit motive in cancer research, you're just glad someone cares enough to spend the money.

Not that I blame you for being curious, of course.

2. Alternatively, if they don't want to get into the billboard business, sell routing influence. Bidders can pay to route cars past points of their choosing. They could both sell general place-based influence (bidders simply bid to have more cars come past their chosen point), or end-point based influence (bid to have people going to/from specific points routed past your chosen point).

For instance, suppose you have a new French restaurant, on a side street. You could pay Google to increase traffic down your street, so that more people will see your restaurant. Even better, you put out a sign announcing your grand opening, and prominently proclaiming "Higher Rated than Le Pretentious Bastard" (the top French restaurant in town), and pay Google to specifically route people who are going to Le Pretentious Bastard past your place.

3. Route based on conversation in the car. E.g., if the people riding in the car are talking about comic books, route past comic book stores (that have paid Google for this).

What was Henry Ford's options for monetizing horseless carriages in 1900?