This last section on speed limits concerns that national speed limit, there are actually two. The first national speed limit is 30 mph in a built up area, but it is the second one that I want to explain here.
The first thing to notice about the national limit sign, shown below, is that there is no number on the sign. Also you will see this sign on a variety of different roads, from fast dual carriageways to narrow country lanes. This national speed limit demands a different limit for different vehicles, even a car that tows a caravan will need to observe a lower limit.
When I see this sign, I think it means use your “common sense”. The theoretical limit for a car on the road below is 60 mph, but that would be crazy on this narrow lane – 25 mph maybe too fast. As always you need to drive to the conditions that are apparent. Most of my pupils are very surprised when they enter a road like this one and see this sign. They assume that since they are encouraged to drive reasonably close to other speed limits that I would expect the same here. So I explain that if the county and borough councils had to specify exact speed limits for every road, the cost would be prohibitive. This is because most of the roads in the UK are national, but most of the roads that we regularly drive on are signed up with specific limits. The cost of mounting signs on all of these type of roads would be enormous. Each would need a lamp, therefore an electricity supply, someone would need to maintain them, replace bulbs, clean them etc.
When you join a national speed limit dual carriageway, you should try to get up to about 60 mph (as long as the traffic will allow this. This is so that heavy goods vehicles (which are speed limited) will not catch up and overtake you. If you are driving 50 mph on a clear road, the truck driver will be “forced” to overtake you. The truck will take a while to do this since it is limited to 56 mph (probably), and both lanes of a fast dual carriageway will be blocked (not a good idea).