Do you mean flashing the headlights? I wrote this https://forum.studio-397.com/index.php?threads/headlight-controls.63904/
Indicators and flashing headlights supported from the game itself would be nice, especially because it's more of a endurance racing sim.
Turn signals/indicator lights can be confusing. If I turn on the left turn signal, do I want the chasing car to pass on the left or am I moving left to clear the road for the upcoming car? These can be determined in league races with strict rules, but pick-up races can/will end up with bruised egos and battered cars.