Seriously? Is it really necessary to know every single time a car is coming up from behind? If we begin with the premise that all the riders in the group are competent to ride as far to the right as practicable and can keep a straight line, what's to be gained by announcing every single car that passes? If the riders aren't competent in those skills, maybe the answer is to improve those skills rather than hearing about every single car that happens to come by.

I'm not saying there aren't some circumstances where it helps to know what's behind, but I find it really isn't necessary to know about every single car that might be on the road. After all, we can all recognize when we are riding on roads open to traffic and can safely assume the presence of cars at some time.