LOS ANGELES — Hungry Southern Californians will soon learn if their favorite food truck makes the grade, after Los Angeles County supervisors voted unanimously Tuesday to assign them the same health-board ratings that brick-and-mortar restaurants receive.

The vote gives county health inspectors the authority to conduct surprise visits to the trucks twice a year and give them A, B or C ratings, just as they do now with restaurants. Truckers must post their grades prominently for customers to see, just as restaurants do.

The new law takes effect in 30 days.

Among its supporters were many food-wagon vendors who say they believe it will give people more confidence in the cleanliness of their operations.

"I'm itching to get our grades on our trucks and quiet the skeptics that think we're somehow unregulated," Matt Geller, chief executive officer of the Southern California Mobile Food Vendors Association, has said.

Food trucks are already supposed to be inspected twice a year, once at the commissary where they are parked for cleaning and once in the field.

But they don't receive letter grades, and county health inspectors have said it's sometimes hard to locate them in the field. Under the new law, they'll have to provide schedules.

Dr. Jonathan Fielding, head of the Los Angeles County Health Department, said the explosive popularity of the trucks led officials to push for the new regulations.

More than 4,000 food trucks, selling everything from spicy Korean-barbecue tacos to sweet crepes to vegetarian hot dogs, are licensed to operate in the county. They will be subject to the inspections, as will thousands of pushcarts and other smaller chow wagons.

Fielding has said the new regulation won't result in an increase in fees for the trucks for at least the first year.