Common sense dictates that a period of grace exists between the new limit being posted and the enforcement cameras being activated for that speed. The usual method is to display limits quite far in advance. The major problem is that the overhead gantry signs are not always accurate, hence motorist tend to treat them with suspicion. I have seen them display warnings for heavy rain in bright sunshine. Someone obviously forgot to switch them off. It does give them a poor reputation. Shame really, used correctly they should be a useful tool in controlling traffic flow.

Cheers Concrete

There is a 60 second grace period before the cameras start enforcement. This is as stated by Highways England to prevent vehicles having to brake sharply for the lower limit.

Thank you but I could not find that in the HC book.

Please direct me to it.

Thank you.

Many thanks.

If there was no limit shown and a speed limit sign did suddenly appear on the "screen", it would most likely be the 60mph one first, rather than 40mph and people slamming their brakes on and rapdily slowing from 80mph to 40. Dropping 10mph shouldn't be an issue.

Then as you proceed further towards the accident or congestion you'll find the next gantry showing 10 mph less and so on.

Exactly what I saw on M25 yesterday, down to 40 in steps. Limit tended to be adhered to and anyway we were soon crawling. Arrow to leave lanes 1&2 and move to 3&4 was widely ignored as were red wigwags and lane closed cross. Copper or Highways Officer standing in road was having to direct them to move right immediately ahead of accident.