IT WAS in 2008 that an out-of-work chef named Roy Choi began selling $2 Korean barbecue tacos from a roaming kitchen on wheels, tweeting to customers as he drove through the streets of Los Angeles. Mr Choi’s gourmet food truck has since inspired a reality-TV programme and a hit Hollywood film, and helped jumpstart a $1.2bn industry.

America now boasts over 4,000 food trucks. According to IBISWorld, a market research firm, from 2011 to 2016 industry revenue grew at an annual rate of 7.9%. But a patchwork of state and local regulations means that the trendy trucks are more popular in some places than others. Portland, Oregon, a city known for its vibrant culinary scene, has had small food carts on its streets for decades. After a study in 2008 by researchers at Portland State University concluded that the carts benefited residents, the city began encouraging the use of vacant land for food-truck clusters or “pods”. Today, Food Carts Portland, a website, reckons the city has over 500 carts and trucks.

In Chicago, where frigid weather discourages street sales for several months of the year, the relationship between local government and food truck owners is much chillier. The Windy City prohibits food trucks from setting up shop within 200 feet of a bricks-and-mortar restaurant or from parking in any one location for more than two hours. Vendors are required to carry GPS devices that record their whereabouts every five minutes. Such restrictions have stifled the industry’s growth. Despite being home to more than 7,000 restaurants and 144 craft breweries, Chicago has just 70 licensed food trucks.

Restaurateurs complain that food trucks threaten their business. The evidence suggests otherwise. According to the Bureau of Labour Statistics, counties that have experienced higher growth in mobile-food services have also had quicker growth in their restaurant and catering businesses. Since 2010, the number of restaurants in Seattle and surrounding King County has grown by 16% in spite of a thriving food-truck scene. In Travis County, Texas, which includes Austin, the restaurant count has jumped 18% even as food trucks have increased more than six-fold. Economic theory would not predict such a complementary relationship between competing industries. Perhaps there is such a thing as a free lunch after all.