Motorway speed limits

 I often find myself traveling on the motorway, and I've noticed that the motorway speed limits are pointless, especially the local temporary speed restrictions.  Well when I say temporary I mean virtually permanent in Port Talbot, for some reason the M4 at Port Talbot isn't fit for high speed traffic, or indeed anything over 40mph.
 Why the government spent £2 million pound a mile building a motorway to speed up traffic and then have it only 10 mph faster than the normal road baffles me!  Probably the same reason the local council spent the best part of £9 millions closing a junction for 4 hours a day for 6 months for no useful reason other than the fact they are all barking mad.
 The problem with the motorway speed limit is nobody takes a blind bit of notice of it, and that makes it bloody dangerous!  Try driving along the M4 at 40 mph, it's bloody lethal, articulated lorries are passing me like I'm standing still, they are doing at least 70 mph, and the cars are doing about 90 - 100 mph.
 The reason is simple, the speed limit signs are on all the time. If they were only used when it was really risky, like when theres a broken down car, or fog or something people would slow down. But when they are on all the time people think theres nothing really dangerous, it's just the local police/council playing silly buggers, and ignore the signs, it's human nature, the boy who cried wolf.