Truck drivers and warehouse workers at the Ports of Los Angeles and Long Beach have gone on strike to protest XPO Logistics’ Cartage and Port Services divisions. The strikes at the two ports, the busiest in the U.S., are blamed on drivers’ wage classification and concerns associated with recently announced zero-emissions goals at the ports.

The strike began Monday at the Port of Los Angeles, with picketers from Teamsters union Local 848 spread across six or seven of its terminals. While the strike held up some traffic at the port, a spokesman for the Port of Los Angeles says its impact on cargo flow has been negligible.

Workers’ complaints stem from their status as independent contractors; claiming that they should be correctly classified as employees, they say they are not paid for all the hours they work and that they are required to lease their trucks at difficult terms.

The strikers have also issued a complaint regarding the ports’ zero-emissions goals. The mayors of Los Angeles and Long Beach signed a pact on June 12 to shift ports to zero-emission trucks by 2035. The union’s concern is that the pact does not make clear how the costs associated with the transition will affect truck drivers.