The analogy is a good one, but sadly I can't see the government ever adopting the position that limits are guidelines. I view them as such, personally. Yet the reality is they earn far too much money from rigid enforcement. We have limits set artificially low on many roads which tells you what the powers that be think of sensible limits never mind letting people view them as guidelines, There are no 'sell by date pressure groups' akin to Brake who would ban yoghurt for being too dangerous and have us all eat muesli instead. Muesli that must be of no more than 1 day old and, to comply with guidelines, must be spoon fed by an official appointed by the government to ensure we don't cut our mouth with the spoon or bang it off our teeth or eat in a manner contradictory to safe eating practice guidlines.
Sell by dates are not political. Limits are set by local councils/officials. These people often look to make a mark. Either by lowering limits and visible enforcement (scameras, etc.) so they can be seen as someone who cares about the young and elderly in their community and persumably progress politically/career-wise. Or they are afraid should they raise limits or relax enforcement to sensible levels that if there were a spate of accidents or deaths that they would get the blame.
There is also the fact that government generally views us all as erant children in need of authority, strict monitoring and control. To suddenly reverse that and admit that the vast majority of us can think for ourselves would go against everything they stand for. Besides, if limits were relaxed and some kid got killed by some berk doing 95mph in a 20 mph limit outside a school then limits would immediately be enforced once more to the letter. The fear of one person dying as a result of any of their policies or relaxing limits is what keeps this health and safety obsessed, nannying culture alive. I remember the debate about lowering NSL to 50 mph (a policy a lot of councils seem to have adopted anyway

) and how it would save 250 lives a year. Well, that is great, I for one do not want to see 250 people dead. I am not a callous, uncaring individual. Yet it was pure speculation and I am sure a lot more people would die in road rage incidents, lack of observation or just plain shoot themselves from boredom.
Look at our death tool. In 2009 it was 2538 - which, according to my calculations, is 0.0041% of the population. Assuming the population is 61 mil and my maths isn't as dodgy as it was when I was at school. Yet it becomes an international issue where Britian seeks to compete against other countries to see who has the safest roads in the world and one death is too many. I doubt we will compete with people like the Swedes on yoghurts.