This week it’s about speed limits, why we have them and how they help.

 Advisory speed limit

As the roads improved and vehicles got better things became faster and the need for speed limits became apparent. These limits are based around stopping distances and in the case of hitting a person, their chances of surviving the impact. Interestingly the 70 mph stopping distance is not too dissimilar to the length of most sporting pitches and the 30 mph stopping distance is just over the length between cricket wickets.


A problem with speed is, the faster the speed the greater the likelihood of death if you hit a pedestrian. At 40 mph studies showed that the chance of death is over 80%. At 20 mph it drops down to 5%. 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.

This brings us back to 30 mph, and it is all to do with people surviving their mistakes. Children do run out into the road, old people maybe can’t cross the road as quickly as others and people who have been drinking are less aware. At some point in our lives, we all make a mistake. Without a speed limit, these mistakes could be a death sentence.

The greater the difference between the speeds of the different road users, the more likely a collision and the worse the impact. So if a bike is doing 10 mph and is rear-ended by a car doing 30 mph. The difference is just 20 mph. Not nice but hopefully survivable. But if hit doing 30 mph was 40 mph the difference could well be life or death.


So don’t forget to check your speed and come back next week for more on our roads and how they came to be.