Ontology-based approaches to situation awareness have gained increasing popularity in recent years. However, most current approaches face two inherent problems. First, they lack sufficient support for assessing evolutions of situations, which is crucial for informing (human) agents about emerging instances of interesting situation types. Second, they are confronted with the problem of recognizing situations that are just similar to a situation type an agent is interested in. Our approach contributed in this paper is based on conceptual neighborhoods of relations which we generalize to conceptual neighborhoods of situations. These conceptual neighborhoods turn out to be the basis for addressing both problems, the assessment of evolving as well as similar situations. The applicability of our approach is demonstrated by an in-depth case study in the domain of road traffic management.