I am in favour of speed limits. It makes sense to say to drivers in busy built up areas they should not go above 30mph when the road is clear, just in case something unexpected happens which requires them to brake or swerve. The national speed limit of 60mph is also a wise precaution on most roads where there is no lane segregation and no single direction access and exit from the road.
The old rules were fairly simple. If you were driving in a built up area with street lights, the speed limit was 30. You were told this on original entry in to the town or village, and told of its end on exit by a simple sign. The rest of the time commonsense told you it was 30mph from the surroundings and the street lights. If you started your journey within the built up area you knew it was 30 from the environment.
In recent years there has been a profusion of intermediate speed limits. There is no obvious logic to the choice of some of them. The other day I counted the number of changes of speed limit on a 15 mile journey locally. I counted 16 changes of speed limit. It varied between 20,30,40,50 and 60 mph. I did not go onto the motorway which would have added a 70. There were pieces of road with street lights where the speed limit was higher than 30mph, without frequent repeater signs on the lampposts to remind you of the different limit. Apparently similar roads through built up areas changed from 40 to 30 and back again.
Research shows that drivers are best at obeying the 60 speed limit on non dual carriageway roads. They regard 60 as the upper limit of what is wise on such roads, and normally drive well below 60 given the bends and hazards on such roads. When it comes to the 60 limit most understand it is an upper limit, which you do not normally achieve.
When I travel on motorways, seeking to keep to a steady 70, most traffic overtakes me. It looks as if most drivers judge around 80 to be a safe speed on motorways, and probably take the risk thinking they will be unlucky to be prosecuted if they stay below 80. There does not seem to be much driver buy in to the idea of 70. Motorways are our safest roads, because traffic travelling in different directions is kept apart and because vulnerable road users like pedestrians and cyclists are not allowed on them. In this respect motorways are more like railway lines, which are also segregated and do not allow any other users to share the route.
30 mph areas are more difficult. There is a stronger case for the 30 than for the 70 on motorways, and most people if asked would say they agree with the 30 limit in built up areas with single carriageway roads. Yet when it comes to driving modern cars with good brakes and steering, many drivers reckon they are safe at more than the speed limit and allow the speed to drift up. If speed limits are to work well, there needs to be general acceptance of them by the driving public.
Many drivers do think the profusion of differing limits and frequent changes of limit can be counterproductive. Drivers end up studying road signs and speedometers more than is healthy for keeping alert to what is going on on the road ahead. Traffic managers should be careful before introducing too much complexity. Speed control has to be part of helping get people to their destinations with safety for all road users, not part of a policy to stop people using cars or trying to make the motorists task too difficult.