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The reductions in passenger vehicle emissions that have been achieved since the mid-twentieth century are a great environmental success story. Government regulation of tailpipe emissions and private investments in breakthrough technologies such as the three-way catalytic converter have reduced emissions of nitrogen oxides, carbon monoxide, and hydrocarbons by 75 to 90 percent at a relatively small cost to consumers. Since California first established emission standards for passenger vehicles in the 1960s, different regulatory approaches have been adopted by the United States, Japan, and Europe, and each has been emulated to some degree in other parts of the world. Significant work remains to replicate these successes throughout the rest of the global fleet.
The massive impact of passenger vehicles on climate also remains to be addressed. The transportation sector is responsible for about one-quarter of energy-related greenhouse gas emissions worldwide. Passenger vehicles account for just under half of this total, and will remain the predominant source of these emissions for the foreseeable future.
Since the early twentieth century, passenger vehicles have increased mobility and expanded economic opportunity for billions of people. The number of cars on the road has exploded from fifty million after World War II to over one billion today and is expected to reach two billion in just 20 years. The size of the global fleet and its rate of growth ensuree that passenger vehicles will remain the greatest single challenge for clean transportation policy.