A high percentage of drivers don't stick to 70 on the motorway, I was cruising at 70'ish today and was being overtaken almost constantly by motorists going in excess of 80+ so even if 60mph limit was introduced it wouldn't slow people down, perhaps it would generate more cash for the greedy government by way of more speeding fines
IMO 70mph motorway limit is outdated anyway it was introduced back in the 1970s when most cars were probably only just capable of reaching that speed and had milk bottle top brakes..... The stopping distances of modern cars are much much less than when the motorway limit was introduced back in the 70's so with the more efficient cars of today plus the fact there are more motorways with more lanes we should follow suit of the German autobahn and up the limit, just my opinion