How 4 policemen wasted sh26,000 in 15 minutes

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

06 August 2006

I was sitting in a traffic jam in Nairobi the other day and I started wondering how much fuel we waste by leaving the engines running. It’s not an easy problem because all car manufactures only give the consumption rate in litres per kilometre.

Most modern cars can achieve about 15km per litre on the highway and about 8km/L in town. But when in jam, the distance travelled is zero thus a straightforward multiplication would yield a consumption of zero litres. This is obviously wrong since the engine is running!

A better way of tackling the problem would be to note that the 15km/L is achieved at a speed of about 100km/h. This works out to 6.67litres per hour (100 divided by 15). Now, most cars reach 100km/h with the engine running at about 2,500 revolutions per minute (RPM) the idling rate is usually around 500RPM. Thus the idling consumption rate per when is about a fifth of that at 100km/h (2,500 divided by 500 is 5).

Therefore, the consumption rate of the stationary car in a traffic jam is about 1.3litres per hour (6.67 divided by 5). What is the effect of this to the national economy? That is a difficult question, but we can get an idea from my traffic jam.

The hold-up was at the Uhuru Highway / Kenyatta Avenue roundabout. The traffic lights died a few weeks ago and the police are controlling the flow. Unfortunately, their method is not very efficient – they open each entry for about five minutes, thus it takes about 20 minutes to go round the four entries of the junction. The average waiting time is 15 minutes!

The result of these long waiting periods is that the tail back on the main highway reaches the next roundabouts on either side – a total distance of almost two kilometres. To find out the total number of cars held up, we need to divide total length of the jam by the average distance occupied by one car.

Now, you can get out of your car and measure this distance (and look silly!) or use a more elegant method: I counted the number of cars in the space between two streetlight poles and then measured the separation of poles using the mileage meter of my car (boredom can be fruitful!). The result was six metres per car.

Thus the tailback over the two kilometres was about 333 cars per lane. The highway has three lanes, thus there were 1,000 cars on that section alone! Each time the cop raised his arm, these 1,000 vehicles would stop moving and remain idling for about 15 minutes.

At the rate of 1.3 litres per hour each car consumed 325ml of petrol in 15 minutes. The total consumption is 325 litres every time the cop stops the cars. At current prices, this works to over Sh26,000. This was one section of one road, how much do you think it costs in the whole city? When I realised this, I turned off my car!