i read it was 5 mins notice.... better hope you're not taking a shit or sleeping in a basement or wheelchair bound. also, U.S. dropped leaflets in WWII but again it really didn't save many lives, as we're seeing here.
collateral damage from shrapnal to people and property would be immense as well, blocks and miles away.
lastly, Niccolo Machiavelli once said in "The Prince:" never take a mans home, my prince, for he'll sooner forget the execution of his father than the destruction of his house.
IOW if you're going to lose everything, all your possessions, some people won't bother to evacuate. and those that do will hate you forever (especially if not all their family members made it). people don't forget things like that, and that was what Machiavelli was trying to tell his prince.