This paper provides a functional description of the SASPENCE system, which is developed within the so-called Sub-Project of the European Integrated Project PReVENT (PReVENTive and Active Safety Applications). SASPENCE (SAfe SPEed and Safe DistaNCE) is a support system which assists the driver through appropriate suggestions of the safe speed and safe distance to keep according to the actual driving context. In order to find the functional requirements that should be taken into account in the system development, the concepts of safe speed and safe distance must be thoroughly analyzed. As a result of applying a functional analysis methodology, the main functionalities of the system are identified and described in detail, based on the definition of user needs and expectations and showing how the SASPENCE application should operate in different situations.