Actually, what we need is intelligent cars that CAN tailgate. You get much better fuel efficiency following an inch behind the car in front of you than 40 feet. In fact, all the stuff they teach you about not following trucks to closely is completely wrong when it comes to fuel efficiency. On the highway, tailgating that huge line of trucks is actually the most fuel efficient place you can drive.
Also, don't think that driving faster is necessarily less efficient. One road I drive on daily has the lights timed so that you hit every yellow light at 18mph over. I drive 20mph over the speed limit and make it through every light. Because 60 mph is also my cars peak fuel efficiency point, I get much better fuel efficiency than all those "intelligent" drivers driving 10 over and hitting every red light. I've found that driving at 40 also gets you through every light. However, because I drive 15 miles on the road, the extra gas used to accelerate to 60 is negated by the gas saved driving at a more efficient speed.
Maximizing fuel efficiency is a much more complicated problem than you think. Until we have computers that can drive cars that are touching bumper to bumper and can compute the most efficient speed based on fuel used to accelerate, and fuel saved over the driving distance, we will not come even close to peak fuel efficiency.