LOS ANGELES—Truck drivers who want to pick up cargo at the Southern California ports are going to have to make an appointment.

Beginning next year, appointments will become mandatory for import container pick-up at 10 of the 13 container terminals at the ports of Los Angeles and Long Beach, a group representing the terminal operators said Thursday. The companies said a schedule system adds predictability to the supply chain, ensuring that truck lines don’t get too long at busy times of the day. The ports are the busiest in the U.S. by container volumes.

Five of the Southern California port terminals have already started using appointments with some regularity, and they say they’re working.

“We have seen a marked decline in queuing at our gates and better turn times for our truckers,” said Mark Wheeler, general manager of West Basin Container Terminal in Los Angeles, in a statement.

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But trucking companies said the systems used to schedule appointments differ from one terminal to the next, with varied degrees of success.

“There’s no consistency,” said Weston LaBar of the Harbor Trucking Association, a group of Southern California trucking companies. Members of his association are concerned the new mandatory program could do more harm than good if it’s not coordinated port-wide, he said, and they want to be involved in planning it out.

“A uniform port-wide system that has reasonable appointment windows could be beneficial, but the trucking community must be at the table in developing that,” he said.

Congestion issues aren’t unique to Los Angeles and Long Beach. As cargo volumes have grown—driven by the improving economy and the advent of mega-ships that unload thousands of containers at a time—marine ports in the U.S. and Canada have experienced bottlenecks at the entrances to port terminals, with trucks idling and precious hours ticking away. Once inside the terminal, it can take hours for truck-drivers to navigate through check-points, drop off empty containers and retrieve full ones from the stacks.

Appointment systems are a common solution put forth by port operators. Several ports in the U.S. and Canada have launched or are trying out appointment systems, including Vancouver, Oakland, New Orleans, Norfolk and New York-New Jersey.

Later this year, GCT Global Container Terminals Inc. is launching an appointment system at its terminal in the New York-New Jersey port—the first of its kind at the East Coast’s largest port complex. GCT runs an appointment system at its terminal in Vancouver, which has received positive feedback from truckers.

The Port of Virginia has been working on its own appointment system, holding several meetings this summer with contractors, drivers, and logistics and trucking companies to develop the program.

“It has to benefit us and provide a benefit to the motor carrier as well,” said Virginia Port Spokesman Joe Harris, adding that getting it right won’t be simple.

“The truth is, we have to cut down on surges and smooth the flow of trucks throughout the day—from the time we open to the time we close,” he said.

Write to Erica E. Phillips at erica.phillips@wsj.com