This week, address the speed bumps in your life and work

By Charles Kanjama

Recently I’ve been reflecting about roads in Nairobi, Kenya. Some leafy suburbs recently got new tarmac roads, built on new alignment and road reserves, which have opened up traffic in the area.

The development has been welcome, although the new roads have quickly attracted substantial traffic, so that the initial benefits of fast transit have already diminished. I’ve also wondered whether the roads reflect an inbuilt prejudice to improve high-value areas in the city, building up land values for landlords without commensurate recovery to the public.

To make matters worse for the hurrying motorist, speed bumps were then added onto the new roads to improve safety, immediately slowing the average rate of transit.

The basic rule of traffic management in urban areas is that traffic moves as fast as it is churned through the road bottlenecks. And the main bottlenecks are junctions and roundabouts, as well as any obstructions that reduce traffic flow. Speed bumps are a bottleneck, as are slow moving trucks and stationary vehicles.

To add a quantitative element to road traffic analysis, one can use a simple tool called ‘queuing theory’. This versatile tool can explain all traffic jams, including why roads clog up in wet weather. Simply stated, the rate of flow through a channel is determined by the rate of flow at the bottleneck. So to improve the flow-rate, one must focus on improving the rate of processing through the bottleneck.

For example, if a bank queue keeps building up, the bank manager will determine the key elements as follows: the rate at which people join the queue, the time people spend on the queue, and hence how fast the queue grows or diminishes. If the cashier can only serve one customer every 5 minutes, it means an inflow rate greater than 12 customers per hour, holding all things constant, will result in a queue lengthening at the rate of every extra hourly customer.

So an inflow rate of 20 customers an hour will cause the queue to grow by 8 customers an hour. If the inflow and outflow rates remain constant, after an 8-hour day, there will be 64 angry customers hoping to get served. And after a week, the angry customers will look for alternatives, including changing banks, to mitigate the never-ending queues.

So speed bumps create a bottleneck, and the rate of processing through the speed bump automatically regulates the maximum flow of traffic on a particular road. If the traffic inflow exceeds the outflow rate, then a queue will build up. Nairobi County has recently acquired sophisticated traffic lights with green and red countdown systems, as found in some Western countries.

Traffic lights only work well when information is fed into the traffic management system, to inform the timing of the red-green phases. At peak traffic, the green phase in a busy intersection should ideally last only as long as the vehicle flow in the roundabout is constant, full and unimpeded. As the green-time lengthens, the speed of vehicles approaching the roundabout increases, as does the distances between vehicles. Thus paradoxically the volume of traffic through the junction decreases.

So traffic lights should not remain green too long. Efficiency in using the bottleneck can be improved by turning the lights red and allowing vehicles from a different congested direction passage. Of course, a high frequency of red-green phases will also work a disadvantage since there is inefficient use of the intersection during every transition.

Our Nairobi traffic officers tend to be quite inefficient since they don’t seem to understand these basic rules of traffic flow. They love to allow vehicles from one side extended passage-time through junctions, and allow other roads to build up traffic. So I naturally rejoiced when the new traffic lights appeared in Nairobi. To my dismay, police officers are routinely overriding and ignoring the lights. The end result is Pavlov’s dog phenomenon, meaning that road users have been trained to disregard the lights, which have turned into a costly flop. This week, address the speed bumps in your life and work, and hope the traffic officers do likewise.

The writer is an Advocate of the High Court of Kenya