national speed limits

[pawwaw17]

why do signs for national speed limit have a black stripe and white background aposed to the the actual speed?

[Ethel]

Because the national speed limit is different for different vehicles such as cars towing caravans, hgvs and so on.

[Skids]

It saves on signage because the actual speed limit can change along a road depending on the road's configuration and its routing. For example, you join a dual carriageway that has NSL signs. The NSL here is 70mph. The dual carriageway turns into a single carriageway. The NSL is now 60mph. The single carriageway now goes through a built up area, like a village. The NSL is now 40mph.
These are standard speed limits (for cars) unless otherwise stated on numerical speed limit signs.

[Buenchico]

As Ethel says, there are different speed limits for different types of vehicles. For example, on a single-carriageway road, a car driver is limited to 60mph. Unless, that is, his vehicle is towing a caravan or trailer, in which case the limit is 50mph. However, he's still allowed to go faster than the big artics, which are restricted to 40mph:
http://www.highwaycode.gov.uk/09.htm#103

Chris

[Buenchico]

PS: I've just re-read Skids' reply.

While I applaud the idea of reducing speed while driving on main roads which pass through villages, the statement that the NSL (for cars) through built-up areas is 40mph is just plain wrong. If it's a single carriageway, with the NSL sign, the speed limit for cars is 60mph, irrespective of the local housing density.

Chris

[Grunty]

Another reason is that the national speed limit can change. I can remember it being 50 m.p.h. at a time of petrol shortage. I think it was introduced at that speed, but am not sure. In theory, it could be 50 or 90 next week.