The study of road projects and traffic facilities in general has always been carried out according to a perfectly defined cutting out. This cutting out gives rise to the successive study of the three aspects of the plan: plan view, longitudinal profile and cross-profile. These three notions, strictly connected are of easy access; it becomes more difficult when one wants to handle them simultaneously.

It is nevertheless essential to have a global overview on specific problems that cannot be processed by the successive study of three traditional plans. Visibility, as well as the integration in the landscape for example, is typical aspect characterized by the integration of the three dimensions (plan view, longitudinal and cross profile). They are on top of the fundamental characteristics of any road plan. Viability, safety and quality of a plan will depend on it. It is therefore essential to master perfectly these questions.