Travel time and travel time reliability are important attributes of a trip. The current measures of reliability have in common that in general they all relate to the variability of travel times. However, travel time reliability does not only rely on variability but also on the stability of travel times. This paper clarifies the attributes of reliability and proposes a new analytical formula to express travel time unreliability in terms of these elements, in which the travel time (un)reliability is computed as the sum over the products of the consequences (variability or uncertainty) and corresponding probabilities of traffic breakdown (instability). In this Conceptual Travel Time Reliability (CTTR) model, the probability of breakdown on a section is categorized into spontaneous breakdown and induced breakdown, which are independent; the probability of breakdown of a route is formulated as the product of the probability of breakdown of adjacent sections along the route.