I believe TapFly sets the direction not the distance. In the case you describe it would hit the billboard except if the collision avoidance is active and works. I wouldn't count on it... I view collision avoidance as a great benefit to save you from accidents but not trustworthy enough to use it regularly like a flight mode. Eventually you would pay the price.
When I use Tapfly, I just send the Mavic to it's destination and just before it gets there, I just press the pause button on the controller or the stop on the go app. It should brake to a hover waiting for your next input of control. You just got to give it some braking distance and have a good eye for distance as well. But like John Shaw said, do not rely on the OA for stopping it, especially in low light conditions.