Theoretically, if the linear length l of number of cars in a given length of road r is less than r, then the cars could maintain ANY speed. After all, a train is really a bunch of cars moving at the same speed with minimal spacing.
What is key, obviously, is co-ordination between vehicles. So, what we need to think about is enforcing the laws that relate to signalling/communication.
That is, things like indicating. If all vehicles were fully cognisant of the actions and intentions of all others, speeds would be higher yet safety would be higher also; until we get that, indicators are what we have.
We also have to look at what causes congestion slowdown. again, theoretically you could drive at ay speed even in HEAVY traffic
My theory is the "dust speck" - congestion is called by drivers in the pre-peak hour slowing down excessively, causing tailbacks and a snowballing effect. So perhaps we need to enforce minimal speeds at certain times.
vadz is saying that if everybody drove perfectly and each individual had a high confidence in others behaviour, then it would be less risky (than it is now) to drive faster.
But there would still be risk eg of mechanical malfunction or of course as always the remote chance of human error - and then it would be very very bad.