Weekend post - The physics of road safety

I’m a little obsessed by road safety at the moment. This is partially because a very good friend of mine, a loving and vibrant young wife and mother, was killed a few weeks ago in a car accident. It’s also because I’m a driver. I spend up to 2 hours each weekday behind the wheel of a car, driving mainly around Gaborone. You can’t do that without spending much of the day considering matters of mental health. How did that combi driver not see me in my great big car or perhaps he just didn’t care? Why did that BX driver think it was acceptable to overtake in the face of oncoming traffic, crossing a solid white line and exceeding the speed limit? How did that person ever get a driving licence?

As well as questioning the mental health of drivers I often find myself wondering whether drivers studied any science at school. Don’t they know anything at all about physics?

Newton’s First Law of Motion says that the velocity of an object will remain constant unless a force acts upon it. The cup of tea on your desk isn’t going to move unless you pick it up. The ball you throw in the air will only change direction and speed because of air resistance and gravity. A passenger in a car will continue moving in a straight line within the car when the car hits a brick wall. If a car travelling at 60 km/h hits a solid object and suddenly stops the occupants will continue moving at 60km/h until something stops them as well. If they’re lucky it’ll be their seatbelt or an airbag that will slow them down as gradually as possible. However if they’re not stopped by something soft it’s likely to be something very hard indeed that does it. Something like the steering wheel, the windscreen or the wall the car hit after they’ve burst through the windscreen in a spray of blood and bone.

Let’s take a simple, not too scary example. Imagine a child is in the front seat of a car that’s just left your house. Let’s say it’s a perfectly average 9-year old girl who weighs 30kg. She’s not strapped in. The car is travelling slowly at only 30 km/h but collides with the back of a massive truck and stops almost instantly. Let’s say it takes a meter to come to a halt. In the 8th of a second the car takes to stop the girl becomes a flying object within the car. She’ll hit the windscreen at something close to 30 km/h with a force equivalent to a man weighing 117kg jumping on her. She’ll probably live.

The problem is that this isn’t typical. Who drives at 30 km/h? The other problem is that the amount of energy a moving object possesses is not directly related to it’s speed. The kinetic energy of an object is proportional to the square of its speed. Double the speed and you quadruple the energy. To make matters worse if you double your speed you don’t necessarily double your stopping distance. If the car driving that unrestrained little 9-year old girl hits the same truck while travelling twice as fast, at 60 km/h, it’ll probably still stop in a metre. This time she’ll hit the windscreen with a force of almost half a ton. She’ll probably die.

Let’s be completely depressing and double the speed again to 120 km/h. The same collision will hit her with a force of almost 2 tons. She won’t just die. Someone will be forced to reassemble her body so a relative can formally identify her. I know a very brave uncle who volunteered to identify the pieces of his niece’s body so his brother, the girl’s father, didn’t have to. He couldn’t bring himself to drive his car for weeks afterwards. Every time he got behind the wheel of his car he was plagued with the images from the morgue.

That’s why I lose my temper when I see loving parents driving their beautiful children to and from school each day and they allow their kids to stand up on the front passenger’s seat. I get even madder when I see an unbelted adult in that seat with a kid on their lap. You know what? It might not just be the windscreen that kills her, it’ll be your body hitting her from behind, crushing her from front and back. Could you live with the knowledge that your body, perhaps the body that brought that child into the world, is the thing that crushed her to death?

Maybe that’s what we need to teach in physics classes in schools. Instead of balls hitting each other or weights going up and down inclines we should use the idea of a little girl inside a metal box being driven by a psychopath. Maybe that’s the sort of science we need to cut down the slaughter on our roads.

Sources

For a quick summary of Newton's Laws of Motion see the Wikipedia page here. There's also a good explanation of the physics of car crashes here.

I'd like to say that I did all the maths in this piece myself but that would be lying. I did SOME of it myself but then was lazy and used the online car crash impact calculator here.