Cargo could stop flowing at East and Gulf Coast ports, which handle most imports, if a union and an employers’ group can’t agree on the use of machines that can operate without humans.

The union representing 45,000 dock workers on the U.S. East and Gulf Coasts and their employers on Wednesday said they reached a tentative deal on a new six-year contract, averting further strikes that could have snarled supply chains and taken a toll on the U.

Dockworkers represented by the International Longshoremen’s Association negotiated to avoid a second port strike in a matter of months.