Police; if you want smooth traffic flow, count the cars

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

25 April 2010

This was bound to happen: After seven years of writing this column, I finally made the mistake of repeating myself. Last week’s article was a re-run of one that appeared on 13th January 2008. I had intended to re-visit the traffic problem in Nairobi from a different perspective, but I ended up sending the wrong piece to the editor – ouch!

The idea came to mind while I was held up in the traffic on Mombasa road during the evening rush hour. The 10km journey from the South C flyover to Kileleshwa took two hours; in other words, I was averaging a speed of only 5km/h! Now that must be a record of some kind.

And to cap it all; the bulk of the two hours was spent between the flyover and the Langata Road / Uhuru Highway roundabout. That 300m or stretch took about one hour to clear. The average speed here was just 0.3km/h – I had enough time to read the day’s newspaper thoroughly.

The culprits were, of course, the police controlling traffic at the roundabout. The officers are not aware that stopping the flow for 15min at a time creates chaos due to the resultant tailback as discussed last week and two years ago. This also makes the drivers impatient and discourteous to one another.

This is the reason why traffic engineers will never hold the flow for more than about 90 seconds – one and a half minutes. That might seem too short, but when you are waiting, doing nothing, it can appear like a lifetime!

Thus Nairobi’s traffic lights and timed on either a 120s (during peak periods) or 90s (off-peak) cycle. In the rush-hour for example, all the approaches to a junction (vehicles and pedestrians) share out the 120s.

Therefore, if the police want to maintain smooth flow, they must “time” the duration that they hold the traffic to ensure that they never exceed 90s for any approach. But that might be impractical: can you imagine the officers going to the road with stop-watches to check how long they are opening and stopping each junction? Still, I have never understood why they control the flow at junctions that have traffic lights.

An easier way would be to count the vehicles passing through and allowing an equal number from each approach at a time. Counting, however, may not be as simple as it sounds: one can easily lose track especially when there are several lanes with fast cars and slow lorries all flowing through at the same time.

A more convenient method would be to mentally mark a particular location along the road. The officer can then decide that, when he opens the flow, he will stop it when all the vehicles between the junction and his marker have passed through.

Now, it isn’t difficult to ensure that the markers along all the approaches are an equal distance from the junction: a quick drive in a car that has a working odometer does the trick.

Once this is done, the only thing remaining is to follow the logical of opening the approaches; that is, in an anti-clockwise manner. The resultant flow might not be perfect but at least it will follow an objectively determined criterion.