The biggest problems motorised road users have now adays is that speed limits for roads are not being set by the engineers that design the roads (before a road is opened the engineers examine the road and advise the ruling authority what speed is safe for motorised vehicles), but by politics. This tends to make motorised road users start to simply look at a speed limit and think "that's wrong" and go thier own way. Motorised road vehicles have been designed to safely travel at much faster speeds (mine does 70mph at 2000rpm).
Now I may be wrong because I don't live or travel in a built up area, and the roads I drive on really do not warrent anything under 30mph. Most of them are 60mph, and I commute on push bike along those same roads. Without fear, as nearly all the motorised road users give as much room as they can, and in return I give them as much room as is safe, so we share the roads.
Four years ago after a motorcyclist was killed by a car pulling out from a side road in front of him. They reduced the speed limit for a section to 40mph, which was purely political because at 40 he still would have been killed. The only time people do 40 is 6am Christmass and Boxing day morning when the Police sit in the layby, even the Police do not do the speed limit. Here is why we have issues with speed limits.