I have never ridden on loose cranks without noticing, stopping, and tightening. You feel it in your legs waaaay before it's ready to come off. The same thing goes for just about any other part of your bike. Too-tight or too-loose hubs and BBs can be felt by wiggling the cranks/wheel (too loose) or spinning them (too tight.) The headset can be heard--it makes a terrible sound if it's loose when you put your bike down on the floor. I've never had any of these parts suddenly fly off(!!) without them giving me a looot of warning.