Delhi’s Odd-Even Rule Is At Odds With What’s Needed [Part 1]

[I have decided to provide an analysis of Delhi’s Odd-Even Rule in two parts for easier digestion: the first to provide some background on Delhi’s pollution issue, the rationale of the road rationing scheme and whether it’s working; the second to provide some further analysis, statistics and reflections on why it probably won’t have the level of effect Delhi desperately needs]

You’d be forgiven for thinking that Beijing was the world’s most polluted city—on the subject of air pollution, China’s capital consistently steals the headlines—but according to the World Health Organisation (WHO), Delhi actually claims the top spot. There are a number of important air pollutants (and Delhi scores badly on all of them), but that of most concern is PM2.5 (particulate matter measuring only 2.5 micrometres in diameter. PM2.5 comprises tiny particles of dust, soot, dirt, smoke, liquid droplets in the air. The size of these particles means they can easily penetrate the lungs, increasing the risk of a range of cardiovascular health issues including cardiovascular disease, lung cancer, bronchitis, and heart attacks. In many cities air pollution is a fatal issue, estimated to cut years off the lives of its residents.

Comparison of Air Quality in Delhi and Beijing 2014-2015

Delhi’s concentrations of PM2.5, like most growing cities, have been on the rise. The figures are pretty shocking. The city’s concentrations have recently been consistently reaching 400-500mg/m3—a staggering 16-20 times the WHO’s “safe” limit (25mg/m3). To try to combat Delhi’s air pollution issues, officials launched the trial of an odd-even road rationing scheme last week (starting January 1st and running for two weeks). This scheme, which has been tried in other cities, aims to cut road traffic by regulating private cars to travel every second day. Essentially, it’s implemented by your car’s number plate: if your plate ends in an even number, you can only drive on an even date, and vice versa for odd number plates. The rationale works on the basis that if you take a large proportion of emitting cars off the road, you’ll remove a large proportion of city’s particulates.


Has the scheme been working? The public response has been impressively cooperative (maybe not so surprising when you consider how debilitating the issue must be for Delhiites)—the number of drivers caught violating the restrictions has been in the 100s-1000s, which is small for a city with roughly 9 million vehicles. There have been a number of positive reports from locals on congestion and traffic improvements on city roads. But has it had an impact on air pollution levels? The government plans to publish full results at the end of the trial period, but from the reported numbers from various sources so far, the opinion was very much split. Some have reported that the scheme is working, others report that pollution levels are actually climbing.

The truth is that none of the comparisons are particularly valid. Air pollution levels are incredibly variable [see image above] and dependent on factors such as the season (winter is typically worse than summer), the meteorological/weather conditions (wind speed, direction, humidity, rainfall are all important), the time of day, the type of measure you’re using. One source reported a dramatic improvement…by comparing December’s peak reading to today’s average reading. That doesn’t tell us much more than the fact that peaks tend to be larger than, well, peaks. It’s data cherry-picking for a favourable headline. Other sources reported an increase since the restrictions were put in place, but seemed to forget that they were measuring during a period of very low wind and high humidity (particulates are basically clinging to water droplets in the air with no wind to disperse them). Their comparisons are no more valid.

In other words: air pollution is a complex issue that you can’t properly draw conclusions from using only a couple days’ data. To realistically evaluate the impact of the scheme, we’d need to compare like-for-like over a much longer-term dataset to average out the variability. What we can conclude, however, is that it’s probably not going to provide the scale of improvement Delhi desperately needs. To be debating whether air pollution levels have gone up or down a little when Delhi’s pollution is 16-20 times the recommended safe limit pretty much illustrates this point. However, we don’t need to wait for long-term direct pollution data to understand this—we can work out why that might be the case by putting some more general statistics and considerations into perspective.


One thought on “Delhi’s Odd-Even Rule Is At Odds With What’s Needed [Part 1]

  1. Pingback: Delhi’s Odd-Even Rule Is At Odds With What Needs To Be Done [Part 2] | convergence

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