Multimodal

• National Transportation Statistics, Chapter 1D: Travel and Goods Movement (BTS)
• Border Crossing/Entry Data (BTS)
  Persons, vehicles and containers entering the US from Canada and Mexico (monthly)
• National Household Travel Survey (FHWA)
  2009 Survey of daily travel behavior, including trip purpose, mode, trip length, household data, etc.
• Statistics on Travel to/from the US (Office of Travel and Tourism Industries - DOC)
  US citizens traveling abroad and foreign citizens traveling to the US; included tourism/spending data

Airlines & Airports

• Passengers, Flights, Freight and other Air Traffic Data (BTS)
• Guide to US Air Carrier fare data (BTS)
• Air Travel Consumer Report (OST)
  Monthly and 12-month totals for delays, complaints, mishandled baggage, etc.
• Passenger Boarding and All-Cargo Data for US Airports (FAA)
  Passenger (enplanement) and cargo data extracted from the Air Carrier Activity Information System (ACAIS)
• Aerospace Forecasts (FAA)
  Projections of airline traffic and financial data 20 years into the future
• Airlines for America (Trade Association)
  Traffic and economic data on US air carriers
• Airports Council International - North America (Trade Association)
  Traffic data and economic studies

Highways

• Highway Statistics (FHWA)
  Vehicles, drivers, and vehicle miles
• Traffic Volume Trends (FHWA)
  Monthly vehicle miles traveled
• Annual Energy Outlook (DOE)
  Projections to 2035: Includes vehicle miles traveled

Maritime

• National Census of Ferry Operators (BTS)
  Includes information on routes, terminals and facilities, vessels, etc. 2010 data is available in a new interface.
• Cruise Ship Statistics (MARAD)
  Number of cruises and passengers by port and cruise line
• Cruise Lines International Association (Trade Association)
  Industry data for cruise lines

Rail

• Amtrak
  Passenger statistics by state and station (last 2 years)

Transit

• Intermodal Passenger Connectivity Database (BTS)
  Data on the availability of connections among the various scheduled public transportation modes at each passenger terminal
• National Transit Database (FTA)
  Ridership statistics at the national and transit agency level
• American Public Transportation Association (Trade Association)
  Another source of transit passenger statistics