The issue is not so much about keeping us safe from
criminals as it is keeping us safe from
stupid people.
Someone intent on going into a place and shooting a lot of people probably consider any place, whether it is a Gun Free area or not, to be filled with unarmed people, since just about everyplace is. The only places he might avoid are the ones where he knows everyone is armed, like a shooting range or a police station. Where almost everyone is expected to be armed.
What is more concerning to me is someone who is armed for his own protection who suddenly gets mad, or is drunk, or is having a bad day, and on the spur of the moment, without thinking, decides to shoot someone. This is a scenario which I believe
is far, far more likely than a mass shooter deciding to show up.
After all, how many people here have seen someone someone suddenly get mad and turn violent? Now how many have seen a mass shooting? (And I don't mean on TV.)

When it comes to probability, a good, law-abiding citizen suddenly getting mad and doing something stupid is a far more likely occurrence than a homicidal idiot showing up to kill people. Which means that a gun-free zone is safer than one that is not. Because stupid people are more likely to obey the rule than not, since they never had any intention of murdering anyone with their gun--until they do.
