“Typical” Public Mode Volumes for Large U.S. Airports A “typical” public mode volume for a large U.S. airport can be estimated from the available data. The steps to calculate an average daily ground access public mode volume are straightforward. 1. From passengers to enplaning passengers. The scale of an airport is generally categorized in terms of total annual airport activity. For example, in 2005, Boston is usually described as an airport of roughly 26 MAP. For the analysis of ground access, it is more useful to examine movements in one direction: Boston could be just as well described as an airport of 13 million enplanements. 2. From total enplanements to originating passengers. The most important step in observing the overall scale of the ground access market is to subtract the airplane-to-airplane connecting movements from the total enplanements. When these movements are subtracted, Boston airport in 2005 can be observed to have 10.4 million originating passengers, making it the ninth largest ground access market in the United States. As such, it can be used as a “typical” larger airport in the top 20 U.S. airports. 3. From annual to daily originating passengers. By dividing Boston’s 10.4 million originating passengers per year by 365, somewhat less than 29,000 airline passengers arrive at the airport on an “average” day. 4. From daily to hourly volume. Approximately 10% to 15% of the 24-hour passenger volume have been observed to arrive in a single peak hour, creating a peak-hour volume of between 2,900 and 4,200 airline passengers arriving by all ground access modes. 5. Peak-hour public transit volumes. In Boston, about 18% of arriving airline passengers arrive by some form of public transportation; thus, between 500 and 750 airline passengers arrive in the peak hour by rail, bus, and van combined.