Problems with speed.
A problem with speed is the faster the speed the greater the likelihood of death if you hit a pedestrian. The source of this idea is two studies: one UK and the other Australian.
The UK study found that the chance of death from being hit by a car at 30 mph was 45%. And the Australian study reports that the chance of death from being hit at 30 mph was 37%.
At 40 mph both studies showed that the chance of death was over 80%.
And at 20 mph it dropped to 5% in both studies.
The reason it jumps from 5% at 20 mph to over 80% at 40 mph is down to the laws of physics. As the impact speed doubles the energy imparted to a collision quadruples. Blame Sir Isaac Newton for that one.
A more interesting question is why the difference between the UK and Australia. A 45% chance of death in the UK, compared to 37% chance in Australia.
The Australians might believe that this is because they are tougher, where as the English could retort that this is because drunks sustain less damage. A more likely explanation is that with wider roads and pavements Australian children are less likely to be run over.
All this brings us back to why 30 mph, and it is all to do with people surviving their mistakes. Children do run out into the road, old people wander out and drunks stumble out.
At some point in our lives we all make mistakes. Without a speed limit these mistakes could be a death sentence.
