In this paper we examine the question of whether a vehicle that is following the “rules of the road” can always be regarded as operating safely, especially in an environment where human actors are operating. We do this in two ways: first we pose the question, “what is a ‘lane’?” to highlight the problem of defining system behaviours in terms a human would understand; we then look at a pair of well-formed rules and assess the possible consequences of applying those rules in a traffic environment with a mixture of human-driven and autonomous vehicles (AVs). Overall the paper highlights the multitude of problems associated with defining how AVs should behave in a mixed human AV environment.