Speed Calculator
Some thoughts about Relative Speed
As advanced drivers, we all try to learn about stopping distances - thinking and braking distances, Highway Code tables, all rather confusing and hard to remember. I recently came across a different way of thinking about this, and it made a big impression on me.
It goes like this. Imagine two identical cars travelling side by side on a dual carriageway (or, more safely, on a test track or a disused runway). One is moving at 50 mph and the other one is overtaking it at 70 mph. At the instant that they are side by side, they both carry out an emergency stop. Obviously the slower car stops first. The question is: how fast is the second car travelling when it reaches the point at which the first one will come to a halt? Have a guess!
To work it out, you have to use those speed-distance-time equations that you probably learned at school in Maths or Physics and then promptly forgot. It boils down, in this example, to the square root of (702 - 502), which is a whisker under 49. So at the point where the first car, moving at 50 mph, has stopped, the second one, originally travelling at 70 mph, is still doing 49 mph. Makes you think, doesn’t it?
It's even more scary at lower speeds. Let's assume our two cars are now doing 30 mph and 35 mph respectively when they brake. The square root of (352 - 302) is about 18. So, where the first one stops, the second one will still be moving at 18 mph. Imagine that, if they were in a 30-mph limit zone and both braked to avoid a child running into the road. One stops safely; the other hits the child at 18 mph. Even if the speeds are 30 and 32 mph, i.e. the second car is exceeding the speed limit by only 2 mph, it will still be doing over 11 mph, or 16 feet per second, when it passes the point where the first one has stopped. That’s probably not enough to kill the child, but it’s enough to do a lot of damage all for a measly 2 mph over the speed limit.
If you have a calculator with a square root key, you might want to try this for other speed combinations. You can also use this link [calculator] to try it. But, whether you do that or not, next time you are driving THINK about the effects that only a little extra speed can have on your ability to stop in a safe distance.
Thanks to David Skinner of the Newbury Group of Advanced Motorists for contributing these thoughts.
Speed Calculator link © 2002 Andy Sharp, Newbury Group of Advanced Motorists (www.niam.org.uk)