The aim of this research was to test whether there is an inherent difficulty in understanding prohibition signs rather than obligation signs. In the experiment conducted, participants decided whether simple car movements presented on a computer screen were allowed or not according to either obligation or prohibition traffic signs. The information provided by obligation and prohibition signs at a T-junction can be understood as messages in the form: A “mandatory left turn” sign indicates that turning right is not allowed, the same as a “no right turn” sign. Both statements mark each of the relevant roads as “allowed” and “not-allowed” in exactly the same way. However, reasoning studies suggest dramatic differences in behaviour. Previous research showed a general advantage for obligation signs. In this study, the number of alternative roads indicated by these two kinds of traffic sign was controlled using different crossroad junctions. In those particular conditions, our results showed that there is no overall advantage for either obligation or prohibition signs. It depends on the manoeuvre performed by the vehicle. Obligation signs produce faster responses when the manoeuvre is allowed, whereas prohibition signs show faster reaction times when the manoeuvre is not allowed. Those results obtained with diagrammatic information are consistent with some cognitive theories, such as the mental model theory about reasoning with deontic propositions.