What's the point of the M25 speed restrictions?

Every time I venture onto the M25 I drive at a good speed of up to 70mph from junction 21A for miles past Watford, Rickmansworth and the M40 junction without any speed restriction, amidst lots of equally easily flowing traffic. Only after the M40 junction do the overhead gantries show a speed restriction of 50mph, when all the traffic immediately bunches up and the speed drops below fifty. Then the gantry signs show a 40mph restriction and the traffic bunches up even more, as speed drops to thirty and everyone is nose to tail. Eventually there is no speed restriction and traffic resumes normal speed with no traffic jam. Do these speed restrictions serve any useful purpose?

Asked on 22 September 2012 by DL, Ramsey

Answered by Honest John
There can be horrible congestion between J16 and J10 due to the sheer volume of traffic and the automated variable limits are supposed to prevent traffic piling into the backs of the queues. I think that to some extent this probably works. At other times the variable limits are plainly ridiculous. But when there’s trouble ahead I get off at J15, J14 or J13 and divert around the jam.
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