Toward a Full Accounting of GHG Emissions in US Urban Transportation

Accounting for greenhouse gas emissions (GHG) is no simple matter in urban transportation. It is not as simple as the GHG emissions from the vehicles themselves. For example, there is the vehicle manufacturing process. There is also vehicle maintenance and the energy used to keep stations and administrative facilities open. There is simply no central source of information for such data, a situation that is a serious “GHG omission.”

Three decades ago, BART, the San Francisco area rapid transit system, published estimates of the full energy requirements for operating transit and cars, including such factors as vehicle maintenance, right of way maintenance, and stations. Generally, the analysis found that the rail mode required 41 percent more energy than is consumed in traction (transportation), buses 37 percent and cars 22 percent. These factors may be old, but they may be the only ones available (and it is possible that they are still valid).

If we assume the BART factors, then the comparison of GHG emissions between transportation modes in the United States is even more favorable for cars. The average 2006 car would emit 374 grams of GHG per passenger mile, compared to 322 for transit and 418 for transit outside New York.

The best hybrids could best transit in New York.

All of which points out the needed for objective, comprehensive analysis.

The previous post on GHG emissions by mode is shown below. The information in the previous post does not include any adjustment for vehicle maintenance, right of way maintenance, and stations. Its calculations apply only to transportation.


    Despite perceptions to the contrary, there is little difference in greenhouse gas emissions between cars and public transport. On a per passenger mile basis, cars emit nearly the same GHGs per passenger mile as all public transport outside the New York urban area. Hybrid automobile technologies are already producing GHG emissions lower than the New York public transport figure.

Demographia has posted greenhouse gas (GHG) emission data for US public transport by mode and for personal mobility mechanisms (cars and SUVs). The data is for 2005 and is calculated using US Department of Transportation, US Department of Energy and US Environmental Protection Agency data.

The results may be surprising to any who have assumed that public transport is inherently less GHG intensive than cars.

    The average 2006 car emits 307 grams of GHG per passenger mile in urban driving. This is approximately 30 percent more than the average for public transport (233 grams).

    Virtually all of the public transport advantage is due to the New York urban area, where 133 GHG grams are emitted per passenger mile, 57 percent less than the average 2006 car.

    Outside New York, public transport and the average 2006 car emit have similar GHG emissions --- cars 307 and public transport 303.

    Cars are becoming more fuel efficient, which is indicated by the hybrid and hybrid diesel data. Toyota’s Prius emits 147 GHG grams per passenger mile in urban driving, 10 percent more than the New York public transport figure of 133 grams. Hybrid diesel cars just entering the European market emit 101 GHG grams per passenger mile, 22 percent less than public transport in New York.

    SUV’s are considerably more GHG intensive than both cars and public transport, emitting 443 GHG grams per passenger mile.

These estimates include the GHG emissions from electricity consumption and fuel refining. A full life-cycle analysis would be preferable, which would include GHG emissions from construction of public transport and highway systems, construction of vehicles, extraction of fuel for electricity generation and refining, disposal of vehicles and other materials, vehicle maintenance and administrative support.