Jacqueline, this is not a silly question and I maybe able to answer it for you.
Before the mid 60's there was only one motorway, the M1, and as far as I know there was no speed limit. Now one day some geezer was testing his car, an A.C. Cobra, as he was going to use it in the French 24 hour Le Mans race. I can't quite remember his actual speed, but it was around 300 kph (near 200 mph). The motorway police didn't like the look of that, it was dangerous, but they couldn't catch him at that speed so they just waited until he ran out of petrol!!! They worked it out that, at that speed, it would take a car some 5 km to stop safely and that on the M1 you couldn't see that far ahead. So although they couldn't nab him for speeding, they could for dangerous driving. It then only took a short time (a week I think) before they put a speed limit on the motorway of 70 mph. Then there was no "oil crisis", so that was not the reason. Speed limits on the Belgian motorways, 120 kph, came about through the "oil crisis" in the 1970's.
http://en.wikipedia.org/wiki/1973_oil_crisisAt that time there was a battle going on between British Rail, with the 'Inter City' trains and the bus company's 'motorway express'. The bus companies had built buses with Rolls Royce Spitfire engines in them, they had a cruising speed of some 106 mph!!! God knows what top speed these buses had, but when they overtook you, you were almost pushed off the road by the air pressure. You needed an E-Type Jag just to be able to catch em! I don't know what happened to those buses, after the 70 mph speed limit was imposed, as I had waved England farwell by 1967.
I know this all to be fact as I was there and the A.C. Cobra was, and still is, my favourite dream car.
I hope that this answers your question?