[photochemsyn]

The safest driving speed on busy roads is the speed everyone else is going. If everyone would drive this way, overtaking accidents (often very serious injury-wise) would be eliminated.

This means slow drivers are just as dangerous as fast drivers, and also points to why self-driving cars would greatly reduce highway accidents. Assuming they'd all be monitoring each other's speeds, they could coordinate like swarms of drones, and thus could drive safely at faster speeds.

To go even faster, the cars could link up into a single line, under coordinated control, and zip along like a high-speed train (which is why trains are the most efficient transport system, at a nice optimum balancing speed and energy consumption). However, this 'train of cars' has some advantages, as you could then just detach from the train and drive on independently to your local destination, avoiding the last mile problem.

[seabass-labrax]

In my country (the UK), the penalties for driving dangerously due to being too slow are usually greater than the penalties for speeding. Being unreasonably slow could make other drivers more likely to overtake and have a head-on collision, or hit other cars when re-merging back onto their side.

Single-track country roads are the most problematic, though, as everyone has a different idea of what the safe speed is - and the limit is as high as 60 miles/hour! I wouldn't be against simply reducing the national speed limit for roads without markings to 30, so that at least then people could be reasonably expected to drive at that speed, rather than at an unspecified speed anywhere up to 60.

[tonyedgecombe]

Most people have terrible overtaking skills, they sit too close behind the car they are going to overtake, are in the wrong gear and don't accelerate hard enough when they do go. Either that or are too timid and just sit behind slow vehicles causing a long tailback.

[jcgrillo]

Another possibility that's fun to imagine in a world where every car is self-driving and connected to all the other cars is traffic meshing at speed (say, 25mph) through intersections. You wouldn't need traffic lights anymore, because the vehicles would be able to set their spacing and velocity correctly to avoid colliding. Unfortunately, self driving cars can barely figure out how to merge onto a freeway so we may be some decades away still.

[tonyedgecombe]

>The safest driving speed on busy roads is the speed everyone else is going.

That's a recipe for pileups. People lose concentration in a convoy and as soon as something bad happens up front everybody piles in. People aren't computers, they need some variability to keep them alert.

[Nevermark]

"Trains" of linearly packed self-driving cars are an interesting idea. It would be fun to watch the emergent ripples of inevitable flow disruptions spread through such auto-mata.