Cost-effectiveness analysis of compressed natural gas implementation in the public bus transit fleet in Delhi, India

Buses in Delhi have been fueled by compressed natural gas (CNG) since 2000, at significant expense, to improve air quality. We evaluate the emissions impacts and cost-effectiveness of CNG buses relative to diesel, in the early 2000s, and more recently - given stricter vehicle emission standards - in Delhi, and the Indian context. We also analyze the hypothetical use of cleaner diesel buses, instead of CNG, in the early 2000s. We consider health critical and greenhouse gas emissions, and capital, operating and maintenance costs, over the service life of buses. A scenario analysis is conducted to assess bus fleet emissions due to the higher costs of CNG buses resulting in some trips being shifted to diesel buses, or other modes. CNG buses in the early 2000s significantly reduced particulate and greenhouse gas emissions, relative to the diesel buses they replaced, and also cleaner diesel buses complying with more stringent BS-III/IV standards. However, cleaner diesel buses would likely have produced emission reductions with similar or better costeffectiveness ratios relative to CNG. The emission reductions due to low-floor BS-III/IV compliant CNG buses relative to their diesel counterparts, during 2010-2015, were lower, with worse or similar cost-effectiveness ratios, compared with CNG buses relative to diesel in the early 2000s. The scenario analysis revealed that, under a budget constraint, it is preferable to remove diesel buses, and allow other motor vehicles to meet any shortfall in CNG bus supply, strictly from a particulate and nitrogen oxide emissions perspective. So, allowing only CNG buses to operate in Delhi was an effective policy choice in the early 2000s for mitigating local air pollution. We finally explore the implications of our results for mitigating emissions from bus fleets in Delhi and other Indian cities going forward.