Be Careful On The Road, 80km/h is Not A Safe Speed


The Sunday Nation


06 February 2005


Thanks to transport Minister John Michuki, buses, Matatus and other commercial vehicles will never exceed 80 kilometres per hour. But is this a very safe speed? Should we throw all other caution to the wind since the vehicles are now moving slowly? What would happen if two vehicles collided head-on at 80km/h?

Suppose you are in a car moving at 90km/h and you approach a “Michuki-compliant” bus travelling at 80km/h. As you overtake, you will notice that you pass the bus very slowly even though your speed is quite fast. Indeed, to the passengers in the bus, your car will seem to be cruising at 10km/h only!

On the other hand, if the bus were approaching from the opposite direction, it would appear to you to pass extremely fast. If you had a speed gun in your car, you would record that the bus was moving at 170km/h (the bus’s 80km/h plus your car’s 90km/h).

Therefore, if two buses moving at 80km/h collided head-on, the combined impact velocity would be 160km/h. That is, the collision would be equivalent to ONE vehicle hitting a concrete wall at a speed 160km/h. Would any passenger would survive?

Let’s put this in proper perspective by comparing it to a falling object. The velocity of a falling object depends on the height from where it is released. If a stone were released from a height of 5 metres it would hit the ground at about 10 metres per second (36km/h). At 10m high, the landing speed increases to about 50km/h.

The next question is: from what height would an object need to be dropped for it to hit the ground at 160km/h? The answer is 99 metres. That is approximately the height of the Kenyatta International Conference Centre (KICC). In other words, the impact of a head-on collision between two vehicles travelling at 80km/h is the same as that of a car dropping from the top of the KICC – head first! Can anyone survive such a drop?

Furthermore, if a bus collided with a car, the impact on the car is much greater than that on the bus. This is because of the difference in the mass of the two vehicles. A bus weighs about ten tonnes while a car is only one tonne in weight. Therefore, the momentum of the bus is ten times that of the car.

At 80km/h, the impact from a bus is equivalent to that of a car moving at 800km/h! Now imagine the effect of a 40-tonne trailer… It is no wonder that, even at the reduced speeds, buses and Matatus still killing people, albeit in fewer numbers. So, even though Michuki has done his part, we must do ours and exercise extreme care on the road.

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