Research topics below this in the hierarchy
- Freight (General)
- Passenger traffic (General)
- Urban transportation
- Transportation geography. Trade routes
- Traffic engineering. Roads and highways. Streets
- Water transportation
- Railroads. Rapid transit systems
- Stage lines
- Ferries
- Express service
- Postal service. Stamps. Philately
- Pneumatic service
- Telecommunication industry
- Telephone industry
- Cellular and wireless technology
- artifical satellites
- Signaling
- Messenger service
- Air transportation. Airlines
- Automotive transportation. Including trucking, bus lines, and taxicab service