MEXICO CITY—You can’t get something out of nothing: this is common sense, not to mention a principle of physics and mathematics.

Yet the amazing science of Mexico City’s real estate development obeys no such laws.

Urban planners here, in one of the world’s most populous and crowded cities, have found a way to add hundreds of square metres of new commercial and recreational space. And it isn’t costing local government a cent.

Their gambit is called Under Bridges (Bajo Puentes) and it’s a simple idea: convert the vacant, trash-strewn lots beneath Mexico City’s overpasses and freeways into shopping plazas, public playgrounds and outdoor cafes.

“This used to be a dark, dirty place where vagrants slept, and now look at it,” said Mercedes Campos, the proprietor of a new Hawaiian-themed salad bar nestled under an expressway in the Coyoacan neighbourhood.

Campos was admiring the brand-new food court outside her shop, where outdoor tables were filled with a lunch crowd of high-school kids, hard-hat workers and taco-munching businessmen in suits.

“There’s a terrific atmosphere here,” Campos said. On a hot, smoggy afternoon, the area was cast in cool shade by the six-lane roadway, and there was no discernible noise from the traffic overhead.

A gentle breeze tickled, channelled through by the massive concrete pilings holding up the freeway.

City officials say they have developed four Under Bridges zones and have plans for 20 more.

“These were spaces that generated no benefit and had been illegally appropriated as dumping grounds for trash or as homeless campsites,” said Eduardo Aguilar, an urban planner for the Mexico City government who helped design the program.

“They were spaces that cost the city to maintain and were a drain on resources.”

Aguilar said he and other planners made up the Under Bridges program as they went along because they knew of no other city attempting to build cafes and shops under its freeways.

Persuading business owners to move in at below-market lease rates was not nearly as big a challenge as kicking out the unlicensed parking attendants who had commandeered the areas to operate private lots, he said.

The Under Bridges program stipulates that 50 per cent of the land remain as public space, with playgrounds, exercise areas, greenery and picnic tables; 30 per cent is commercial and office space; and the remaining 20 per cent is reserved for parking.

Aguilar said the city didn’t need to spend money to develop the land, granting concessions to private developers instead. Those developers shoulder the cost of cleanup, construction and maintenance while leasing the commercial and retail space to businesses approved by city officials.

“These are places that improve public safety,” Aguilar said. “They have pedestrian crosswalks, they are well-lit at night and, most importantly, they attract a lot of people — which, in turn, brings more security.”

In a city flooded with street vendors but short on affordable retail space, the program is also designed to move some of Mexico City’s snack stands and other impromptu eateries into more formal and hygienic quarters. The Under Bridges areas have bathrooms, running water, electricity and outdoor lighting.

At one Under Bridges innovation, Roberta Gutierrez stopped on her way home from a job as a cleaning lady to try out a new set of free outdoor exercise machines.

“This is great,” she said, working her abdominals as cars whizzed by. “Not everyone can afford to join a gym.”