A change in one constraint might allow you to move to a different point in the design space, but it doesn't change the physics by which the system must abide.
More concretely, replacing a human driver with an automated one isn't going to change all the physical constraints that make it inefficient to have cars that cruise at 100+ mph: http://energy-ecology.blogspot.com/2010/05/optimal-vehicle-s.... Whether you're in a Honda Civic or a Ford Explorer, doubling speed from 90 kph to 180 kph triples fuel consumption.
Even if self-driving cars can be a little lighter, that's not going to change the shape of the curve.
More concretely, replacing a human driver with an automated one isn't going to change all the physical constraints that make it inefficient to have cars that cruise at 100+ mph: http://energy-ecology.blogspot.com/2010/05/optimal-vehicle-s.... Whether you're in a Honda Civic or a Ford Explorer, doubling speed from 90 kph to 180 kph triples fuel consumption.
Even if self-driving cars can be a little lighter, that's not going to change the shape of the curve.