[Report prepared for the American Public Transportation Association by Science Applications International Corporation. Read the full report in PDF format. -Ed.]

Executive Summary

"The scientific debate on the causes of global climate change is basically over – the focus has turned to action"
– Jonathan Lash, Director of the World Resources Institute

This report addresses four questions.

1. How much net CO2 is public transportation saving in the U.S. from the current level of services being offered?
2. How much additional CO2 savings are possible if incremental public transportation passenger loads are increased?
3. What is the significance of non-public transportation commuter use at a household level and what can households do to save additional CO2?
4. Are there favorable land use impacts that public transportation contributes to that result in positive environmental and social benefits?

Answers to these questions show that public transportation is a highly valuable asset for reducing global warming.

1. How much net CO2 is public transportation saving in the U.S. from the current level of services being offered?
Answer: Public Transportation is a net CO2 reducer; saving 6.9 million metric tonnes in 2005.

In 2005, public transportation reduced CO2 emissions by 6.9 million metric tonnes. If current public transportation riders were to use personal vehicles instead of transit they would generate 16.2 million metric tonnes of CO2. Actual operation of public transit vehicles, however, resulted in only 12.3 million metric tonnes of these emissions. In addition, 340 million gallons of gasoline were saved through transit’s contribution to decreased congestion, which reduced CO2 emissions by another 3.0 million metric tonnes. An additional 400,000 metric tonnes of greenhouse gases (GHG) were also avoided, including sulfur hexafluoride, hydrofluorocarbons (HFC), perfluorocarbons, and chlorofluorocarbons (CFC).

This study estimated the following benefits of public transportation in 2005 in reducing congestion and this nation’s transportation CO2 emissions:

The above referenced 6.9 million metric tonnes of CO2 exceeds the transportation CO2 emissions that exist in the sparsely populated states like North Dakota (6.3 million metric tonnes) and a more densely populated state like Delaware (5.0 million metric tonnes),1 (Environmental Protection Agency [EPA] 2007).

2. How much additional CO2 savings are possible if incremental public transportation passenger ridership is increased?
Answer: A solo commuter switching his or her commute to existing public transportation in a single day can reduce their CO2 emissions by 20 pounds or more than 4,800 pounds in a year.

An average private vehicle emission rate is about 1.0 pound of CO2 per mile. An automobile driven by a single person 20 miles round trip to work will emit 20 pounds of CO2. Thus, the savings by using existing service would be about 20.0 pounds of CO2 per daily trip. As passenger loads increase on public transportation, there may be only a slight increase in CO2, much less than driving to work in single occupancy vehicles (SOV). Over the course of a year, an individual could potentially reduce their CO2 emissions by more than 4,800 pounds (assuming 240 days of transit travel per year). This represents slightly more than two metric tonnes of CO2 or about ten percent of a two-car family household’s carbon footprint of 22 metric tonnes per year. In contrast, if one were to weatherize their home and adjust their thermostat the carbon savings would be approximately 2,800 pounds of CO2. Other comparisons include replacing five incandescent bulbs to lower wattage compact fluorescent lamps (445 pounds of CO2 per year), or replacing an older refrigerator freezer (335 pounds of CO2 per year.

3. What is the significance of using more public transportation at a household level and what can households do to save additional CO2?
Answer: Public transportation is also effective in reducing household CO2 emissions and cost.

One of the most significant actions that household members can take to reduce their carbon footprint is to use public transportation where it is available. The annual use of an automobile driving an average of 12,000 miles per year and with an average 22.9 miles per gallon (MPG) consumption emits 4.6 metric tonnes of CO2 per year (one metric ton is equivalent to 2,205 pounds). Households that have a sport utility vehicle (SUV) or light duty truck drive and drive an average of 14,500 miles per year with an average MPG of 16.2 emit 7.9 metric tonnes per year.

The carbon footprint of a typical U.S. household is about 22 metric tonnes per year. Reducing the daily use of one low occupancy vehicle and using public transit can reduce a household’s carbon footprint between 25-30%.

4. Are there favorable land use impacts that public transportation contributes to that result in positive environmental and social benefits?
Answer: Public transportation provides many benefits that go beyond energy and CO2 savings – as transit assets are being used to accomplish these important functions.

Investments in public transportation have the benefit of supporting higher density land uses that allow for fewer vehicle miles of travel. While it is difficult to precisely measure this impact, a number of studies have attempted to estimate the relationship between transit passenger miles and vehicle miles traveled (VMT) reduction as a proxy for this effect. The results range from a reduction in VMT of between 1.4 miles and 9 miles for every transit passenger mile traveled. The outcome would be more efficient use of roadways, reduced road maintenance, shorter highway commute times and reduced need for street and off- street parking.