Can lowering the speed limit increase accidents?
The Sunday Nation
Nairobi,
20 January 2013
When I was a young boy, I used to think that the “80KPH” written at the back of light commercial vehicles (so-called, “Pick-Ups”) was an indication of the distance that it would cover if driven at its top speed for one hour. Of course I was wrong; very wrong! As I later came to learn, the number indicates the maximum speed that the vehicle is allowed under the law.
Upon learning the meaning of the numbers, the next question that came to mind was; why do they make vehicles that can do 140km/h but then limit them to only 80km/h? This puzzle has remained in my mind for the last several decades and, even today, I still haven’t figured it out.
I have also never understood why they make a car that can only do, say 130km/h but then install a speedometer that reads up to 180km/h. Why add the extra 50km/h on the display yet the pointer will never reach that area?
These questions came back to my mind recently while I was driving on the new 12-lane Thika highway. Before this road was expanded, it was a four-lane dual-carriageway with a speed limit of 110km/h. After rebuilding into a quadruple-carriageway, the limit was reduced to 100km/h!
It defeats logic. You can comfortably cruise at 140km/h on the
450km-single-carriage
Perhaps the lower speed limit was an attempt to reduce accidents on the road. If that was the case, I would be interested to know if it has borne fruit. I am curious because the assumption that slower vehicles cause fewer accidents can be challenged.
There is an interesting branch of physics known as statistical mechanics. It is concerned with the interactions of randomly moving bodies – how they collide with each other and with the confinement walls – for example, gas molecules.
The analysis reveals that the probability of collision depends directly on the average speed: the faster the bodies are, the more likely they are to collide. That is obvious, but it is not the only factor.
It also turns out that the probability of collision is dependent upon the variance of the speeds. That is; if all bodies moved at the same speed in the same direction, they would never collide – obviously! But when they are moving at different speeds and directions, the chances of colliding are much greater. No wonder we have more accidents on two-way roads than on one-ways.
But the greatest factor is the concentration of bodies – the number per unit space. It turns out that the probability of collision increase with the square of the concentration. That is, if say, the concentration is doubled, the chances of collision will quadruple!
Now slowing vehicles down has the effect of increasing their concentration. That’s how we get traffic jams whenever there are bumps on a busy highway. Therefore, lowering the speed limit might actually increase the number of accidents. Perhaps the ministry of transport should do a detailed study to establish whether this is so.
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