Nairobi Expressway is nearing its maximum carrying capacity


The Sunday Nation


15 October 2023


Jasper Ongechi referred me to a report that was in the Business Daily stating that the Nairobi Expressway is now handling up to 70,000 vehicles daily. He did a quick calculation and found that this translates to about 49 every minute. The he asked me: “Suppose the claim is factual, how does it work out?”

From his question, I gather that he doesn’t believe the numbers; especially after working out the 49 vehicles per minute – which is slightly less one every second! However, Jasper did not take into account the fact that the Expressway has 17 entry gates and 14 exits along its 247km-length.

Thus, the average number entering at each gate comes to about 2.88 vehicles per minute (which is equivalent to one vehicle every 20 seconds). We obtain this figure by diving the 49 by the 17 entrances. By similar calculation, we find that an average of 3.5 vehicles get out at each exit point.

These are quite high traffic numbers and it is not surprising that it has now become common to see a traffic jam at the exit points. Indeed, the video accompanying the Business Daily report showed a very long queue at the Museum Hill exit – it stretched back beyond the filter area and into the main carriageway, thus blocking one lane. If this situation continues, it will no longer be beneficial to use the Expressway!

Perhaps the question that Jasper had at the back of his mind is whether these vehicles can fit in the 27km-expressway. Let us find out…

The maximum capacity of any road is reached when the vehicles maintain at least three seconds from each other – regardless of their speed. Anything shorter than that will result with a spontaneous phantom traffic jam. This is the kind of grid-lock that occurs out of nowhere for no apparent reason.

With time separation, it turns out that the maximum carrying capacity of any single lane of a road is 20 vehicles per minute. We get this number by dividing the 60 second in one minute by the three-second time gap.

The Nairobi Expressway has a total of four lanes, hence its carrying capacity comes to 80 vehicles per minute, or 115,200 per day. It is therefore conceivable that 70,000 vehicles are using this highway daily. But, as noted earlier, if the usage continues to rise, we will soon see a situation where there is as much traffic congestion on the elevated deck as on the lower one.

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