SAN DIEGO — In the barren Southwest, a good portion of the 1,900-mile border between the United States and Mexico is being fortified with walls, electronic surveillance and trenches.

But in San Diego, private investors on both sides of the border are taking a different tack, building a bridge to Tijuana International Airport with the expectation of turning it into a low-cost alternative to San Diego’s boxed-in, one-runway airport.

Next year, if all goes according to plan, air travelers in this region will be able to park their cars in the United States and walk across an enclosed 325-foot passageway directly to Tijuana International.

The project would make Tijuana International a rare airport that would let passengers land in one country and leave in another. Two airports on the Swiss-French border allow for that, and a shuttle runs passengers with connecting flights between airports in Hong Kong and Shenzhen, across the border in mainland China.