I'd love for you to find one study that shows signs deter crime in any way shape or form. 

Gun free zones kill. 

The NRA pushed back against them back in the 90's when the gun control lobby first started proposing them to feel-good politicians unwilling to think through the problems. 

Since then we've had slaughter after slaughter take place in these 'safe work zones' for criminals and psychos. 

Signs ARE effective, but only for the law-abiding. Just like all gun control laws, criminals could care less.