Kevin, what makes your practice "relatively safe"? How do you control the degree to which you reduce the flow of blood to the brain? How do you know that it's "enough" to cause them to pass out and no more? And how does someone learn such things, such that they can practice "safely"? Clearly trial and error is not the way.
Good question. Uke has a responsibility to tap. Nage also has a responsibility to recognize what is going on in the situation as well. You also have an instructor who should be watching and controlling the situation as well. I have been choked to passing out a few times. It was because I was too stubborn to admit I need to tap my opponent and thought I still had a chance!
The chokes don't happen immediately, you have 30 seconds or so to start feeling light headed and realize that things are getting dark!
Other than that, when a un-skilled student say gets a "air" choke...you have to tap before he applies it and let him know what is going on. Same with neck cranks. They can look like chokes, but are a different mechanism. Again, the instructor must be trained and know what is going on to control the situation.