Regardless of the law, I would not pass a vehicle on the left that is displaying a left hand signal. So, would I be in violation if I just slowed or stopped behind the left turn vehicle?
I do not know if there is a book they have to study in Mexico to get a driver's license. I know this seems weird to those of us coming from a country that has a different set of rules.
I think the reason they have this is so the front car wanting to make the left turn is not holding up traffic behind them - the goal being to keep traffic moving and not causing long lines of waiting vehicles.
When we first saw this in Mexico we did not know what was going on. Once we learned how it was supposed to be done we understood and starting doing it.
We learned about turning on your left turn signal to tell the vehicles behind when it was safe to pass - which is how the trucks and buses in Mexico usually do it. We found doing this in places like Baja where there are many gringos it did not work very well because no one seemed to know what we were doing. But in most parts of Mexico it is standard operating procedure.
It certainly does take some getting used to as on some roads you are never quite sure what that left turn signal really means. Then you start to pass and the Mexicans behind you start to pass you and here we are out there three across with passers being passed. Sometimes it is maddening. You just have to really pay attention to what everyone else is doing and hope you have it figured out correctly.