Transit and Suppressed Auto Trips
In a later study, Pushkarev and Zupan quantify the ratio of transit trips to suppressed auto trips, illustrating the dramatic effect that a high-density, transit supportive environment can have on auto usage. In a study of six metropolitan areas served by rail transit, they found that "the reduction of auto travel…is much grater than that attributable to the direct replacement of auto travel by rail travel," on the order of a reduction of 4 auto trips for every 1 trip by transit.
In further research on "transit leverage," John Holtzclaw found a reduction of VMT in San Francisco of 9 miles for every passenger mile of service. If a single passenger mile on transit equals multiple passenger miles in an automobile, then increasing transit use emerges as a substantial tool for greenhouse gas reduction.
Recognizing this, the American Public Transit Association calculates that, if only 7 percent of daily trips in the United States were shifted to transit, CO2 emissions equivalent to more than 20 percent those of the commercial sector would be eliminated. Taking the 1999 CO2 emissions from transit, APTA calculates what the equivalent emissions would have been had those trips occurred on other modes, and obtains a figure representing a near doubling of the transit value.