Variable Motorway Speed Limits

[squirrel_hunter]

I was travelling along the M4 recently and the Active Traffic Management section of it by Bristol that has the variable speed limits indicated by overhead gantry's. It was rush hours and I was expecting the hard shoulder to be open for use due to the heavy traffic. As I approached, the variable speed limits brought us down from 70mph to 60mph to 50mph as there was an "obstruction". The hard shoulder was marked as closed.

Just before reaching the "obstruction" which was a broken down vehicle on the hard shoulder traffic speed was further reduced, but this is where it got odd...

The hard shoulder was marked as closed. Lanes 3 and 2 were 40mph but lane 1 was 20mph.

After then passing the broken down vehicle the limits on all 3 lanes returned to 40mph and then 50mph. But the fact that there were different speed restrictions on the same carriageway and in the same direction at the same time to me was very odd. So what is the org's take on this?

[Bibio]

safety for people in the hard shoulder. the 20 was indicated just in case the person in the hard shoulder wandered into the traffic or to give the breakdown service personnel safety when dealing with the recovery.

[Red ones]

If you filtered could you do 30?

[squirrel_hunter]

I understand that it is for those working on the hard shoulder. However 20mph is a very low number when you consider the lane next to it is running at 40mph. Quite a difference. And this is what bothers me, particularly when it is enforced via fixed cameras...

[Red Herring]

Personally I think if you were to drive at 20mph on a free running live motorway lane you're likely to cause an incident when the first foreign lorry comes along and drives straight up your a......

I live by the m42 where they first started using these things, they are a bloody pain tbh.
There is no consistency between them, you can go past one saying 60, next 40, then back up to 60, then 50, with no hazards obvious.

They are also slow reacting to things too, especially when the hazard has been cleared, sometimes taking over an hour.

I was on the m42 on Sunday morning at 9.30am and they were on for about a 5 mile section, going down to 40mph, it was very quiet so I just couldn't understand why, then in one gantries length it goes back up to national speed limit, so bloody frustrating.

[ophic]

Sometimes they say "warning - queueing traffic" along with a 40mph speed restriction, which is kind of a self fulfilling prophecy.

[andrewsmith]

Don't understand them in the slightest. Waiting for it to happen as part of the current A1 upgrade up here (not been mentioned, but its a prime area for it)

The Idea is that they smooth out traffic flow and prevent queues caused both by hazards and by traffic congestion. So if an area ahead of you is getting congested they will slow you to 50 or 40 to keep things consistent.

Obviously it doesn't work but I'm not sure whether that's because everyone ignores it or simply because it doesn't work.

[PyroUK]

Little bit of both prob Spank