Strikes by transportation workers tend to be some of the most disruptive, as they're responsible for transporting people and cargo by air, rail, water and highways.
A recent eight-day strike shut down seaports in Los Angeles and Long Beach, Calif., a key entry point for trade to and from Asia. The strike could lead to some delays on hot items in stores this holiday season.
Nearly 4.3 million U.S. workers are involved in transportation and warehousing. Of those, about 900,000 are unionized.
Union employees earn about $215 more per week, or 30%, than non-union workers.
Major unions: International Brotherhood of Teamsters; International Longshore and Warehouse Union; Air Line Pilots Association
