20 mph Limit

Yes, it’s that time of year to hammer home Sir Isaac Newton’s message about “half emm vee squared”

The inflammatory quote from BBC news is “Push bikes go past you when driving at 20 mph!” The article’s headline is “Wales' 20mph speed limit: How has the first month gone?”

Two points:-

(1) If you are driving a car and a push-bike hits you at 20 mph or more, it is the cyclist who will be injured, not you. When you are driving a car at 20 mph or more, the cyclist will be injured or killed; not you.

(2) That someone else is breaking the law is no excuse for you breaking the law. In particular, it is herd mentality that leads us to the stupidity of shouting a conversation at cocktail parties. (remember them?)

OK, a third point:-

(3) A speed limit of 20 mph is a law devised and designed and implemented by lawyers and politicians. In other words, human beings (I am being generous here, but the point is that these laws are made by Homo sapiens. The laws of the universe (see “Sir Isaac Newton”) are separate from and pre-date the laws of Homo sapiens.

The laws of the universe include the law/formula for the kinetic energy of a moving body.

Note that “velocity SQUARED” term.

Driving to Clarenville I am doing 80 Km/hour; you go past me at 100 Km/hr. Your velocity is 1.25 times my velocity. 1,25 squared is 1.5625. Assuming that the mass of our cars is the same (we are both driving a 2019 Subaru), then when you collide with a tree, you have one and a half as much energy to dissipate as would I if I hit a tree while trying to avoid hitting a moose.

That means that your body – seat belt or not – has to absorb 1.5 times as much energy as does mine.

You die; I suffer a sore neck for a fortnight then get back to my stamp collection.

It gets worse. The distance traveled while braking is proportional to the square of the velocity, so at 100 KM/hr you are more likely to lose control when swerving to avoid hitting a moose than am I. You can see that in the limit if I am doing only 10 KM/hr I can easily avoid the moose. At 100 KM/hr you are much less likely to avoid hitting some massive object.

Again, you die.

And insurance rates go up. For everyone.

Search