The tragedy that left 10 undocumented immigrants dead and dozens of others hospitalized after being crammed into the back of a hot 18-wheeler, highlights the reality of human smuggling.

We know there are an estimated 11 million undocumented immigrants living in the United States, with the majority of them coming from Mexico and Central America.

So where do they usually go once in the United States?

While it’s nothing new that undocumented immigrants would pack into a truck, it just so happened that the operation and tragedy was exposed in San Antonio.

That’s likely because the Alamo City is where Interstate 35 and Interstate 10 meet. I-35 going all the way to the Midwest and I-10 stretches from California to Florida.

“Unfortunately San Antonio and other places in Texas are on those smuggling routes and some vehicles probably are getting through undetected,” Randy Capps said.

Capps is the Director of Research for U.S. Programs at the Migration Policy Institute, a non-partisan, non-profit organization that studies immigration patterns worldwide.

Of the estimated 1.5 million undocumented immigrants that live in Texas, only about 71,000 have called Bexar County home.

About 86 percent of those are from Mexico.

Harris County has the most undocumented immigrants, followed by Dallas County, Hidalgo County and then Tarrant County.

Even El Paso County and Travis County have undocumented immigrant populations similar to Bexar County.

So where are undocumented immigrants choosing to go once they cross the border and why?

“Immigrants really go where the most dynamic economies are. In the largest numbers anywhere in the biggest cities. In the state of Texas, that’s Houston of course, followed by Dallas. They go to L.A., they go to the east coast cities in large numbers,” Capps said.

And data shows there’s been a dramatic drop in people coming across the border. One reason for that is tougher security, but people will continue to try to get into the U.S. and will continue to go to great lengths to do so.

“When you have the border patrol successfully apprehending more people, it’s harder to get through, people are more likely to risk getting into a truck or getting into a car and hiding that way,” Capps said.

Capps said it’s important that people realize while California and Arizona used to be hot spots for people to cross the border, that has changed and it’s now south Texas, especially the Rio Grande Valley because of the transportation network.