[smsm42]

How would it make them any more useful? It's not like hanging out parking garages and breathing in exhaust are a favorite pastimes for Californians now (though this impression can easily be made if one considers the results of California lawmaking, I admit). People know exhaust gases are not a healthy thing, and there's no reason to hang out in parking garage anyway. However, since being in parking garages, when parking, is unavoidable, putting useless warnings there desensitizes people to all sort of such warnings, and makes them ignore any kind - including those kinds that might have been useful (such as in a facility where actual dangerous chemicals are present) - but now, nobody would even notice those warnings anymore, just as nobody reads what's on Windows warnings, people just click 'OK', whatever it is.