The Delhi Metro, an intra-city electric rail system serving the National Capital Region (NCR), has been operational since December 2002. By March 2012, the Delhi Metro had an operational route length of 167 km.
While a key motivation behind building a mass transit system in Delhi was to ease traffic congestion, it wouldn’t have been hard to imagine that it may have a considerable impact on air quality as well. An improvement in air quality would presumably occur mainly because of the ‘traffic diversion effect’. This refers to the possibility that commuters who were earlier using personal transport such as cars and two-wheelers would switch to the Delhi Metro leading to net reduction in the level of vehicular emissions.
Investigating whether this actually happened becomes particularly important for Delhi because the city is infamous for its high levels of air pollution. On most days between 2004 and 2006, the average levels of nitrogen dioxide and carbon monoxide exceeded the permissible standards set by the Central Pollution Control Board (CPCB). Such high levels of pollution raise health concerns for the city’s inhabitants. The adverse effects of air pollution on health such as damage to the central nervous system, worsening of asthma and an increase in infant mortality rates, are well documented. Studies conducted by the CPCB find that high pollution levels in Delhi are positively associated with lung function deficits and with respiratory ailments. S.K Guttikunda and J.S. Apte in their book titled (Monitoring & Mapping Urban Air Pollution: A One Day Experiment in Delhi, India) found that about 10,900 premature deaths every year in Delhi occur due to ambient particulate matter pollution. In light of these facts, it is important to examine whether there has been any significant impact on air pollution in Delhi due to the operation of the metro.
Traffic diversion versus traffic creation
Based on transport economics theories, it is not possible to predict whether the net effect of the Delhi Metro on air quality will be positive or negative. The main argument is that along with the traffic diversion effect, there could be a traffic creation effect due to introduction of a new mode of transportation. The latter refers to new demand for travel generated by a faster and arguably more comfortable mode of transport such as the Delhi Metro. For example, new demand for travel could arise if, facilitated by the Delhi Metro, people decide to relocate to the outskirts of the city to possibly benefit from cheaper real estate prices, and then commute longer distances to work. If part of the increased distance is covered using pollution-intensive modes of transport (such as private cars), then this could negate any traffic diversion effect and could lead to an increase in overall level of pollution.
An added dimension that needs to be considered while studying the net effect is the presence of two coal-based power plants within the city limits that were operational during our study period (2004-2006). If operation of the Delhi Metro resulted in increased capacity utilisation of these plants in order to supply electricity for running it, then this could also contribute to higher overall emissions in the city.
Analysing the link between the metro and air quality
In our study, we examine the effect of the Delhi Metro on air quality using data obtained from the CPCB on four pollutants—nitrogen dioxide, carbon monoxide, ozone and sulfur dioxide, between 2004 and 2006. This data is collected at two locations in Delhi—ITO, a major traffic intersection in central Delhi, and Siri Fort, a residential locality in South Delhi. We obtained hourly data on temperature, rainfall, wind speed and relative humidity for Delhi from the India Meteorological Department.
In order to establish a causal link between the Delhi Metro and air quality it is important to compare pollution actually observed in the period after the Delhi Metro became operational with its correct “counterfactual”. This counterfactual refers to the level of pollution in the hypothetical scenario where all other factors that affect pollution remain the same as in the post-metro period, and the only difference is that the metro does not exist in the counterfactual. Any difference between the observed pollution in the post-metro period and the pollution in the counterfactual can then be attributed to the Delhi Metro. To do this, we estimate the trend (pattern over time) in pollution using hourly pollution data over a reasonably long time period which includes the date of extension of the Delhi Metro. If we detect a sudden change in the level of pollution at the date of extension of the Delhi Metro, then we attribute this change to the extension of the Delhi Metro.
Between 2004 and 2006, there were six extensions of the Delhi Metro rail network. At each extension, we examine the time trend for each pollutant separately. We identify the localised, short-term effect on pollution that can be attributed to each extension of the Delhi Metro by conducting this analysis separately for pollution data from the ITO (Income Tax Office) and Siri Fort. Our preliminary analysis shows a reduction in the levels of nitrogen dioxide and carbon monoxide at both locations. This reduction varies between 24-29% for nitrogen dioxide and between 26-69% for carbon monoxide. For sulphur dioxide, we find an increase of 90% at ITO, and a decrease ranging between 35-89% for Siri Fort. For ozone, we do not find a uni-directional effect even across extensions at a particular location .
Conclusions and caveats
To summarize, preliminary evidence points towards a reduction in the levels of nitrogen dioxide and carbon monoxide. Given that both nitrogen dioxide and carbon monoxide are important vehicular emissions, our initial findings suggest that the Delhi Metro has encouraged people to switch from private to public mode of travel resulting in positive effects on air quality in the city. In the light of our findings and given the existing evidence on the adverse health effects of air pollution, these indirect health benefits should be taken into account when urban policy makers contemplate setting up large scale intra-city transportation systems. We provide a rationale for subsidising these mass transit systems, such as the metro or dedicated bus routes, even when the direct costs do not show a net profit. These public transport systems should be considered seriously for other cities that face similar challenges in terms of vehicular congestion and health costs due to pollution.
Two caveats should be kept in mind while interpreting and understanding these results. First, the large number of missing observations in the pollution data makes this analysis particularly challenging. Further examination is needed to ensure that our results are not being driven by the pattern of missing observations. Second, for a few extensions, the magnitude of change in carbon monoxide and sulphur dioxide are very large to be driven solely by a traffic diversion effect. Also, ozone is created in the presence of sunlight and nitrogen dioxide through a complicated non-linear process. The results for ozone do not show a consistent pattern in our analysis. In the light of these facts, further investigation is needed to rule out the possibility that our findings are not being driven by chance or poorly measured pollution data.
Deepti Goel is an assistant professor at the Delhi School of Economics.
Sonam Gupta is currently working as an assistant professor in the Food and Resource Economics Department at the University of Florida.
This column has been reprinted with permission from Ideas for India www.ideasforindia.in