Temporary Speed Limits - Why?

2006-07-20

I don’t own a car, and so don’t drive often. This means I usually avoid the passionate debates about speed limits, speed cameras, etc. But I think I’ve come up with a speed limit question that’s less contentious (famous last words). I almost missed a plane the other day when the coach I was on to the airport was held up in a traffic jam on the M25. What made it more frustrating, though, was the temporary speed limit signs (the ones that light up with a speed above the motorway). Either:

It’s impossible to go that fast anyway, which is the case 90% of the time, or:
The traffic jam is ending and everyone is so glad to be out of it they don’t honour the speed limit (and arguably the speed limit shouldn’t be there any more anyway).

So what exactly is the intended point of these? To pack more traffic on the same amount of roadway? Possibly, but if it’s impossible to go that speed, what’s the point? Have I missed something obvious? Is there some subtlety of queueing theory that explains this?

I assume these signs must be manually controlled - a person in a control room switches them on or off after observing the amount of traffic on a camera. Would they be more responsive if they were automatic?

Anyway, enough grumbling. I’d love to know how they are supposed to work though.

Comments

I think what you say is true - although the limit still typically persists for miles through the jam itself too. Again, I wonder if this is just slow reaction speed on the part of those who dictate the limits. I suspect more people don't get tickets for this as it's hard to prove the exact speed limit in that place at that precise time. Plus maybe the police are caught in jams themselves :)

— andrewferrier 2006-07-20 20:21

I would have thought they are supposed to slow traffic down before they get to the jam and reduce the risk of a collision and also allow the pile up to slowly clear. Then the speed limit can be removed again. This doesnt work. Mostly because everybody does 80 through the 30 temporary limil until they come round a bend and slam on the brakes and come to a halt in the queue. The theory is great, but until fines actually are issued from people speeding through them they are nothing more than a flashing light...

— Anton Piatek 2006-07-20 19:09