To avoid gridlock, everybody must obey traffic lights

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

13 January 2008

After the post-election chaos, the good news is that the usual Nairobi traffic jams are back! The most notorious ones are on Mombasa road, heading into the city centre. The notoriety arises from the fact that the tailback runs for several kilometers during both the morning and afternoon rush-hours.

Apart from the high vehicle density on this road (and don't confuse it with the Mombasa highway, which is deserted), the other contributing factor to this traffic jam is the manner in which the police control the flow at the Lusaka / Langata / Uhuru highway roundabout.

This junction has four inlets (technically, they are called “approaches”) and four exits. It is common for the police officers to open the traffic from each direction for about five minutes at a time.

While this might sound good to those who are moving, it is very annoying to those who are stopped. The accumulated stoppage time comes to 15 minutes – you have to wait for three inlets to be opened and each gets 5 minutes.

Now, if you stop the cars for 15 minutes, how far will the tailback stretch? Assuming a steady flow of vehicles moving at 80km/h, the rate of inflow then works out to about 13 vehicles per minute. In 15 minutes, there will be 200 cars waiting on each lane – stretching back for almost two kilometres!

Although it is annoying, it is not worrying when these cars are lining up on Mombasa road. But on the other approaches, the knock-on effect can easily cause a gridlock on the entire city road network...

The Lusaka road tailback, for example, will block the Car & General and        Enterprise road roundabouts. That, in turn, clogs the City Stadium junction, which blocks  Jogoo and Landhis roads.

The jam at Landhis road creates problems on Haile Selassi avenue which then blocks Uhuru highway. The tailback from Uhuru highway can easily stretch past Nyayo stadium all the way back to the Langata road roundabout – the same place that the police  are trying to unclog! Before long, the road network (or, grid) locks itself and movement becomes impossible.

So, how do we avoid that from happening? In my view, the first step in clearing these jams is simply to ensure that everybody obeys the traffic lights. That includes drivers, pedestrians and the traffic police! The last group are the worst offenders.

The traffic lights on all the Uhuru highway roundabouts operate on a 120-second cycle during rush hours. That is, all the approaches share the 120s in different proportions depending on the vehicle density. The shortest “Green time” is 15s , thus the longest stoppage time is only 105s.

If police follow the same restrictions, then the tailback on Mombasa road can be reduced to under 25 vehicles stretching for only 200 metres. This not only eliminates the risk of a gridlock but also reduces the stress caused on drivers.