US Interstate 5 becomes Mexican Federal Highway 1 (aka the "Transpeninsular Highway") at Tijuana, and runs down the Baja peninsula to Cabo San Lucas. At a length of 1056 miles, it's considerably longer than I-5's 796 miles. Just as the Interstate Highway System extends, through connection, up through Canada, it extends by connection into Mexico; and the two systems are actually interconnected at many points along the border.

You seem to be making the somewhat chauvinistic assumption that Mexico is a more primitive country than it actually is. It already has an interstate highway system compatible with ours--no need to extend ours into Mexico, thank you, you Yankee Imperialist running dog, very much.