I don't really see how this is an existential threat. Uber and Lyft have a simple option allowing each drive to set their own prices (presumably $ per expected mile and $ per expected time idling according to Google Maps traffic info). Uber/Lyft gets a defined cut. The user could then choose the closet or cheapest car, and you would have a self working market where the price is set by the drivers according to market forces. The main downside would be prices could fluctuate wildly if their are very few drives in an area.