Will the expansion of Mombasa road ease traffic jams?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

08 February 2009

Several readers have asked me to comment on the repairs being done on Mombasa road and Uhuru highway in Nairobi. The main concern is that the former is being expanded by adding one extra lane while the latter will remain at the old width. “How will this reduce the traffic jams?”, the readers ask.

Before the expansion, this section of road had two lanes from the Jomo Kenyatta airport turn-off up to the Nyayo stadium roundabout. After that, it widened to three lanes heading into town.

With the increase in vehicle population over the last few years, the tailback of traffic heading into town has also grown to over two kilometres during the morning and afternoon peak periods. The waiting time on this queue can be more than one hour on most days…imagine that: driving at an average of two kilometres per hour!

Widening the Mombasa road side will certainly reduce the length of the queue. Ideally the peak hours tailback should shrink by the same factor as the increase in width.

The road widens from two to three lanes, therefore the queue shortens by a factor of two-thirds; that is, from two kilometres to about 1.3km.

It is easy to see why: with the extra lane, there will be more space to fit more vehicles per kilometre of road. The waiting time, however, is unlikely to change.

The reason is that the number of vehicles waiting to enter the roundabout is constant but, with three lanes on Uhuru highway, the rate of exit will remains as before. Thus the cars will spend the same one hour to cover the shorter distance of 1.3km. They will be slower!

Clearly, it would have been better to widen Uhuru highway and to leave Mombasa road unchanged. Adding one lane would increase the exit rate of vehicles from the Nyayo roundabout by 33 percent and thereby reduce the waiting time by at least the same amount. That is, from the current one hour to less than twenty minutes.

This must have been the logic applied in the original design where Mombasa road was two lanes wide and Uhuru highway three.

This case reminds me of something I observed at a certain hospital: when patients started filling the waiting room and “overflowing” outside on a daily basis, the managers increased the size of the room to fit more people, but they did not change the number of doctors and nurses!

The result was that patients still had to wait the same long hours to be treated.

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 As we mourn the victims of the Nakumatt fire disaster, I am worried that the next such incident will happen in a banking hall. These are crowded death traps with narrow doors. Some even have so-called “man-traps” which allow only one person at a time to pass through. Surely, it is more important to protect life than to secure money! I will write about the evacuation problem after this period of mourning.