There is more to speedbumps than meets the eye

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

15 January 2023

What is the major cause of road accidents in Kenya? By looking at how the traffic authorities react, it seems to me that they blame it on speeding. And therefore, the solution is to erect speed bumps. Thus, we spend a lot of money building smooth roads only to make them uncomfortable by placing bumps!

Among the general public, everyone has an opinion about the major cause of accidents. My suspicion is that they happen because we have too many short-sighted drivers who don’t wear eyeglasses. Think about it: pick any sample of adults and you will find that about 20 per cent of them wear glasses. But have you ever seen a truck or bus or matatu driver wearing spectacles? I have seen only one bespectacled matatu driver in my entire life!

Any way, back to speedbumps: in 2016, I wrote about the science of building them. It turns out that there is in fact a law in the Standards Act stipulating the maximum dimensions. KS 774/2000 says that the highest bump should not exceed 20cm above the road surface and the gradient must be less than 1-in-20.

Using these limitations and assuming that the bump is built with flat plane surfaces (not curved), I found that, the steepest one should be at least 8 metres wide (along the direction of the road). Going up gently four 4m and descending over another 4m. Unfortunately, there are only a handful in the entire country that conform to the standards.

The reality on the road is that speedbumps are not made with plane surfaces. The are curved into all manner of shapes – sine wave, gaussian bell curves, etc. The most common however, is the arc of a circle. This curve starts off with a steep gradient which reduces rapidly to zero at the top and then increases on the way down.

I have done some revised calculations to find out the dimensions of a circular bump that is within the stipulated standards. It turns out that it should run over a length of about 16m along the road – 8m on the way up and 8m going down. Have you ever seen a speedbump built like this?

It is unfortunate that our traffic authorities do not take such matters seriously. Hence, they end up doing funny things like posting a police office at speedbump to urge drivers to move faster over the bump!