Thursday, July 14, 2011

Counting cars

Standing at the bus stop yesterday I found myself idly wondering if bright red is the most popular colour for cars these days. It certainly seems that way, but perhaps scarlet is just more attention seeking.

I decided to do a quick, unscientific survey.

What really astonished me was that in next to no time at all I had counted 100 cars passing (in all directions); I did check my watchwhen I got to 90, so I know that the last 10 passed in only 15 seconds.

And this in what we regard as a fairly quiet village, not on a main A road, at a relatively quiet mid-day, peaceful compared with school run time. I think if I had ventured a guess in advance I should probably have said about 10 a minute.

That reminded me of my very first solo ‘project’ as someone who was supposed to be an economist – advising on the level of tolls to set on a road.

I wasn’t starting from scratch – a report had already been produced by consultants with detailed estimates, but the revenues would not even have made a visible dent in the interest which would be payable on the soft loan granted at overseas aid rates, so I was asked to find a way to up the figures, with less than a week to do it.

At first I thought to redo the traffic estimates based on a gravity model of the population at both ends of the road, rather than the data on existing levels of travel (which, until the road was built, was by river or air). But population figures turned out to be unavailable.

It seemed completely hopeless until I noticed that the annual traffic levels quoted in the study translated into something ridiculous in terms of the number of cars per day using the road – I cannot now remember what it was but I would believe you if you told me it was no more than 10 a day.

So, with nothing much more than a wet finger to the wind & some plausibility checks based on population, car ownership & bus travel in parts of the country where there were roads already, I came up with something a little larger.

The first year estimates were not bad, but I prefer to draw a veil over what happened in subsequent years. But I did learn the important lesson that sometimes decisions have to be made at speed, in conditions of great uncertainty.

Oh – my unscientific survey showed that fewer than 10% of cars are red.