[TheCoelacanth]

The biggest component of the price is distance traveled so driving faster means you make the almost fare for a trip and finish the trip in less time so that you can complete more trips in the same amount of time. There is also a time component, but it is pretty small, so it wouldn't be worth the driver's time to take more time without also covering more distance.

Taxis often take this even farther and make the time component only include time where the taxi is going less than fixed, fairly low speed (in NYC it is 12 mph), so slowing down from 25 mph to 20 mph wouldn't gain them any extra money on the fare but would cost them extra time.