A Flashing Yellow Arrow (FYA) can be used to install lead/lag phasing without a left-turn trap. It is also required, in certain situations, by the 2009 Manual on Uniform Traffic Control Devices (MUTCD). So how does it work? And how can it be modeled?
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.
To select the phasing described in the video above, select Turn Type D.P+P (Dallas Permitted plus Protected). This will assign the permissive (flashing yellow arrow) portion to the opposing through phase instead of the adjacent through phase. After selecting this Turn Type, an additional setting for Permitted Flashing Yellow will become available. Checking this box will simulate the permitted phase with a flashing yellow arrow instead of a green ball.