In Summary
  • I am sorry: the equations in last week’s article were messed up during transfer from MS Word, to the printing design software.
  • Superscripts became normal scripts rendering the formulae meaningless. I apologise.
  • Nevertheless, I have posted the article with correct formatting in my website www.figures.co.ke.

Our speed bumps are substandard and must be removed

A while ago, fellow columnist, Sunny Bindra quipped that Kenya is the only country in the world where a speed bump is erected on a road to reduce accidents and then removed a few days later to reduce accidents. I have also found that this is the only place where a speed bump will be erected to slow down cars and then a traffic officer is stationed at that spot to urge drivers to speed up! I see it every morning on my way to work.

The recent accident at Naivasha has put speed bumps to question on a national scale. Even the President Uhuru Kenyatta commented on the matter.

Erecting bumps is a science. You don’t just go and dump a lump of tarmac at the selected spot! The Kenya National Bureau of Standards (KEBS) published the design standard in November 2000 but it has largely been ignored.

The standard – KS774:2000 – says that the maximum height of a speed bump is 10cm above the road surface; but, in exceptional circumstances, the height can be increased to 20cm. In addition, the gradient must not exceed 1-in-40 (1-in-20 for exceptional cases).

In simple English, 1-in-40 means that for every 40cm travelled horizontally, the bump rises only one centimetre. So, the highest bump should ascend over a distance of 400cm and then descend for another 400cm along the direction of the road. In other words, the entire speed bump should be at least 8m!

Similar calculations reveal that the highest (20cm) and steepest (1-in-20) bump also straddles over a total of 8m along the road. Even the very long and very low Mercedes S-Class Pullman (4.315m between front and rear tyres and 15cm ground clearance) will go over such a bump without getting scratched underneath.

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