Road signs aim at expressing a message to road users. This message is an order to perform an action about an object. For one given sign, both action and object are well determined because legislated. However, it is well-known that the road users do not respect systematically those laws. So, the issue of this contribution is why does the observed decision-making vary from the expected decision-making and why do not road signs fill their role in this process ? We reasoned that a first step to study this gap between expected and observed behaviors could be comparing expert and novice drivers (whose behaviors differ on the road). A second step would be to study the device itself by building an ontology of road signs. Finally, because understanding is context-dependent, we propose a formalism of representation based on context.