How to create smooth traffic flow on Uhuru Highway

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

16 November 2008

The greatest mystery regarding the congestion of motor vehicles in Nairobi is why the police control the flow at junctions that have traffic lights. Is it because many drivers do not obey the lights? Not quite: If that was the reason, then the officers doing the controlling would also obey the lights, but as it is, the don’t!

Whatever the reason, this action is not improving the rate of traffic flow. But many motorists wonder whether there is a magic formula that can clear the traffic jams. The simple answer is NO! But here is an idea worth trying: what if the signals were coordinated such that when vehicles leave one junction, they find a green light at the next intersection and continue straight through without stopping?

The first step in doing this is to decide the appropriate speed in a stretch of road. Consider Uhuru highway in Nairobi, for example. It has three roundabouts in the vicinity of the Central Business District, namely, the University Way, Kenyatta Avenue, and Haile Selassie Avenue junctions.

Even though this section of the road boarders the CBD, it would be unreasonable to limit the speed of vehicles to the standard 50km/h for built-up areas. Many motorists cruise at about 80km/h on this stretch.

But the vehicles don’t just take off and get to 80km/h instantly! The acceleration can last about 15 seconds before attaining the cruising speed. During this initial period, the car will have covered about 170 metres.

Now, the distance from University Way to Kenyatta Avenue is about 500 metres and from there to Haile Selassie Avenue is another 700m. Suppose the car is travelling from the University side of the highway. It will cover the first 170m in 15s and the remaining 330m in 15s. The total time taken is about 30s.

A similar calculation can be done for the 700m section from Kenyatta Avenue to Haile Selassie Avenue, but with slight modification. Since the cars will already be in motion (they did not stop at Kenyatta Avenue), they will not need the 15s for acceleration. Thus the time taken would be about 32 seconds.

Thus, if the green light at Kenyatta Avenue comes on 30s after the one at University Way, the cars will go straight through without having to stop. Similarly, the green signal at Haile Selassie roundabout can be timed to come on 32s after Kenyatta Avenue and the vehicles will also pass this junction without stopping.

But this only happens if the drivers obey the 80km/h speed limit. Any car that exceeds the limit will get to the Kenyatta Avenue roundabout before the 30s are over and find the lights at red. The net result is that, regardless of the speed at which the cars are driven, they would all take about one minute to get from University Way to Haile Selassie Avenue.

What about the heavy trucks that cannot accelerate to 80km/h over such a short distance? Well, they will take much longer than the 30 seconds allowed for cars. Therefore, they are likely to find the lights red by the time they get to the roundabout. They are effectively slowed down to 40km/h, but that is not a major problem, is it?