Not everyone, because it's too cost prohibitive. They will select people coming through the line, but they'll be tested for both drugs and alcohol. It's not every person who comes through.
The buses do 50,000 drug tests, and the other 50,000 are done by the single-officer patrols and highway patrol. My alcohol buses do about 1.5 million breath tests a year. The highway patrol and general duties do the other three million.