WORLD OF FIGURES: There are standards on the shape and size of speed bumps

A car is towed after it overturned upon hitting one of the six bumps built by angry Mario-Inya Townships residents on the Nyahururu-Nyeri road on March 28, 2016. Motorists have protested the construction of the unauthorised and sub-standard speed bumps. PHOTO | STEVE NJUGUNA | NATION MEDIA GROUP

What you need to know:

  • 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.

So far, we have assumed that the speed bump is formed from two inclined straight surfaces – one ascending, the other descending. But, normally, they are in the shape of an arc of a circle. To stay below the maximum limit of the gradient, the curved profile requires an additional 2m along the road bringing the total size to about 10m.

Speed bumps are so-called because they are intended to reduce vehicle speeds. Can the Kenyan standard do that work effectively? The answer is yes; research in other countries has shown that the 10cm bump cuts the average speed to between 50km/h and 60km/h – just what we need!

In a nutshell, all the speed bumps in Kenya are substandard. They cause accidents, they cause traffic jams and they damage cars. They a good for nothing and they must be removed!

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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.

 

www.figures.co.ke;  Twitter: @mungaikihanya