The traffic heading south on Interstate 5 was heavy, as usual. Every day of the week thousands of cars and trucks drive from the United States into Mexico through the crossing at San Ysidro, the last little community before the border.
All manner of merchandise goes over into Mexico by truck. A much narrower range of cargo comes back the other way.
The United States Border Patrol has the unpleasant and futile duty of trying to stop the flow of illicit drugs and other contraband that spews across the border by the truckload every day. Their best tools in this struggle are highly trained dogs and years of experience in spotting drug mules—the the people who attempt to cross into the U.S. with bundles of narcotics strapped onto, or ingested into, their bodies. They get caught at the border with stunning regularity. The drug wholesalers who send them don’t seem to care, because they know that even the small number who do squeak past the dogs and the eagle eyes of the Border Patrol make it an incredibly profitable method of transport.
As a result, the crossing at Tijuana is one of the most heavily-monitored international borders between two countries that aren’t actually shooting at each other, although that is starting to happen as well.