Self-driving cars provide an opportunity to make better use of roads at night. Already much haulage occurs overnight, but little passenger transport. This could change with sleeper cars - self-driving cars fully fitted with beds. A self-driving sleeper car of a personal rapid transport style fleet would arrive at your home the night before you needed to be somewhere. You'd get in, bed down for the night, and wake up at your destination.
The efficiency is twofold: firstly using roads at their lowest traffic load; and secondly combining travelling with sleep. (Although the latter is less relevant as self-driving cars will in general allow road travel time to be productively employed.)