Why was the 70 limit introduced?

[BliarOut]

My local Scameraship have a new advert on the local busses. It says "speed limits are set for your safety" Now I thought the 70 limit was introduced to conserve fuel and was simply never repealed. Does anyone know categorically why the 70 limit was introduced?

I just need to know, as if they are wrong I need to drop the ASA a quick line... After all, the law is the law

[BliarOut]

Well if anyone can see a hole in their statement I'm quite happy to run it by the ASA If anyone can find some limits that were set for other reasons I might still have an "in"