Eons of years ago, I remember when cars used to let the driver behind them know it was safe to pass.... then that went away...then it came to flashing someone it was safe to pull back in front of you. It seems that has gone away, too. Where does the responsibility lie if you flashed your lights, the truck or car pulls back in and then some jerk cuts in front of you and the vehicle you signaled swipes them? I gave up doing that. I just slow down, the driver can make his(her) decision when to change lanes for whatever reason.