Shipping companies expressed caution about using the shorter route between Asia and Europe that many ships have avoided for more than a year.

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.

Dockworkers and employers on the East and Gulf Coasts had until Jan. 15 to reach a new agreement that settled differences over automation.