I was in Cancun or Puerto Vallarta, and the red light came up every 8th or 9th person, completely random, but it was so long ago that I can't remember what airport it was.
I wanna say the Tijuana car crossing is 1~20 or so cars, but that crossing gets so packed, it could be 1~50 or even more. I never stood out and counted, but also never waited in line that long