Compressed Natural Gas.
Living in Delhi, India in the 90s meant a lot of things, but one that left an indelible impression was the air pollution. One remembers disembarking at the Indira Gandhi International terminal after having been seated in cooled and filtered air conditioned comfort on a journey [from the US] that took about 24 hours, only to be greeted with a hot and sooty blast of air in the face with the very first step off the plane.
While unpleasant, it was simply the way things were – Delhi was a rapidly growing city, the capital of the country, a place of business and commerce and wealth. This brought with it increasing numbers of vehicles for personal transportation, in addition to an ever increasing fleet of commercial vehicles that plied the streets – trucks, buses, taxis, auto-rickshaws [3 wheeled taxis]. Vastly inefficient by current standards, they were apparently the prime source of air pollution in the city.
A lot was done to analyze and rectify the problem [very successfully, I might add]. It is presented in this UN document, which is quite a good read. Even more impressive is the timeframe within which this was achieved. I distinctly remember breathing in the air 6 months after CNG conversion of commercial vehicles and immediately noticing a very significant difference [at this time, I did not know of the CNG conversion program]. Of no less significance is the fact that this was accomplished in the world’s largest democracy, a system under which terms such as ‘action’ and ‘change’ are generally little more than rallying cries during [and immediately preceding] election year.