Nairobi Expressway is nearing its maximum carrying
capacity
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
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 247kmlength.
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 27kmexpressway. 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
gridlock 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 threesecond 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.
