Many Mexicans will and do interpret the left hand turn signal from the vehicle in front to mean that they have a clear stretch in witch to overtake the vehicle in front, so they pull on to the left lane and proceed to pass, under no circumstances should you ever turn left with traffic following, pull to the right and let all vehicles pass, when both lanes are devoid of any traffic, cars, trucks, passenger buses, motorcycles, natives on bicycles, etc then do the left turn.
Many cargo trucks on the libres two lane roads will signal with the left turn to let the vehicle following that the left lane is free to overtake, there is where the confusion stands, same as flashing lights that mean move over I am overtaking you, some times at night the truckers will turn their driving lights off so that the vehicle behind can see if there is someone coming, specially on long curves since you will see the glow of the on coming vehicle lights, specially on the phone wires, there is a light refection.
Just be careful and do not take anything for granted.
navegator