Below is a short video explaining the left-turn trap. Notice the vehicle wishing to make a left-turn during the permissive portion. When the signal changes to yellow, the driver will typically assume that the opposing through movement also has a yellow indication. With a lagging left-turn this may not be the case. The left-turning driver may have already entered the intersection waiting for a gap, where they may become trapped. Or they may proceed with the left-turn, causing a conflict with the opposing through vehicles that now have a green indication.