In the area I drive there is a section of road where the speed limit is 35 or 40 where it comes from one lane and opens up into two lanes. When I go to take the outside/slow/right lane if LDW is enabled a voice prompt will go off. I always make sure to turn mine off, but last time i updated to 1.53 it turned it back on and I forgot to turn it off. I have not tested it on the highway, but as a driver I know to stay in my own lane so I turn it off.
The only real use i see of it is if you are a commercial driver who puts long highway hours in to highway driving and might push yourself and fall asleep at the wheel where a loud warning might wake you, but as a regular driver it seems rather useless.