They been operational for years now,
there is meant to be one condition on placing a speed camera in that they are only meant to go at perseaved accident blackspots or sites of previous accidents.
they are not just allowed to wack them in anywhere.
to really confuse things on the M25 (a big circular motorway that rings london) there are "Variable speed limits" in that if its getting congested up ahead the speed limit is lowered behind to help stop more cars arriving so quickly and help to ease the congestion.
and you guessed it.. Speed cameras in every gantry overhead also linked to the variable speed limit!!
the worse cameras though are the portable units they erect on motorway roadworks, pretty tricky to spot sometimes one, sometimes 2.
Do they slow the traffic? well if you take the road i use to get to cornwall for a surf, I know the position of every camera so you just end up driving as fast as possible then braking for the camera then accelerating off again.
Plus I wonder how many people crash whilst emergency braking when confronted with a suprise camera popping up in front of them?