That instruction leaves ambiguous the definition of "safe". In British driving what's safe is to take your turn to go, immediately and without hesitation. Americans in such situations hesitate, to make absolutely certain nobody else is going to do anything unexpected, waiting for the other car to wave them on, even if it's their turn and not the other car's turn. In British driving such "safety-first" behavior would be unexpected, clogging of traffic, and dangerous - hence, completely unsafe.