Peyote wrote:
I thought they were introduced because certain car companies were using Motroways as test tracks. AC Cobras and the ilk caning it up motorways at 140+mph was thought a bit unsafe at the time.
I've also heard that story, however whatever triggered it, it was brought in as a temporary measure - and is still with us four decades later.
Someone 'thought' it was unsafe at the time.

Quote:
Isn't this the principle that works quite well on the variable speedlimit sections of the M25? I was always under the impression that to get maximum capacity from roads (especially motorways) vehicles had to travel at approximately the same speed to counteract the ghost-traffic-jam effect (i'm sure we've all experienced this!) caused by wildly varying speeds.
Not in my experience they don't. I generally try to steer well clear of the M25 because every time I'm on that road I end up sitting in a queue.
Once I stood in one spot for the best part of twenty minutes, almost under gantry signs displaying '40' - and that was after spending a week driving around Germany and not being held up once, despite the traffic being very heavy.
I've spent a considerable amount of time trying to figure out how variable speed limits are supposed to smooth traffic flow - even to the extent of writing computer simulations, but I've gotten nowhere.
The theory of travelling at the same speed will only work if:
1) Everyone is doing
precisely the same constant speed for the entire distance
2) Traffic enters the motorway at only one point, and at a precisely constant rate
3) Nobody enters or leaves the motorway anywhere but at the start and finish.
4) Nobody ever changes lanes.
It will only work if all four of these conditions are satisfied. If not, then constant speeds actually make the situation worse.
Cheers
Peter