"The Bureau of Transportation Statistics (BTS) Border Crossing/Entry Data provides summary statistics for incoming crossings at the U.S.-Canadian and the U.S.-Mexican border at the port level. Data are available for trucks, trains, containers, buses, personal vehicles, passengers, and pedestrians. Border crossing data are collected at border ports by U.S. Customs and Border Protection. The data reflect the number of vehicles, containers, passengers or pedestrians entering the United States. Customs and Border Protection does not collect comparable data on outbound crossings. Users seeking data on outbound vehicles may therefore want to review data from individual bridge operators, border state governments, or the Mexican and Canadian governments."
Bureau of Transportation Statistics: http://www.bts.gov/