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Self-driving cars could form convoys to utilize slipstream. You could reduce the safety distance, because the reaction time compared to a human driver is massively reduced. Additionally, the driving style of the average driver leaves a lot of room for improvement.

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Relatedly, the same thing that gives you slipstream (lower following distances at a given speed) increases car density on the road at any speed, which increases road capacity, which decreases congestion and idling. My guess is that this effect, not the aerodynamics, is the biggest benefit.

Also note that self-driven cars can park themselves and shut off the engine to avoid congestion instead of simply sitting in a slow-moving queue at a "metering light".

How big are those slipstream effects compared to speed scaling? Drag power goes as the third power of speed; take average urban speeds from say 30 mph to 300 mph, and the drag goes up a factor of 1,000x -- to on the order of a megawatt.

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