In the context of route choice, inertial choice behaviour shows that drivers make choices that are satisfactory rather than optimal. Consequently, drivers may not necessarily respond to changes in the traffic system. Instead it is assumed that drivers only alter their choice when the utility difference between alternatives becomes larger than some individualsituation- specific threshold. Route choice data from a real-world driving experiment was used to study inertial behaviour and to estimate inertia thresholds. The data analysis consisted of two parts. One defined as expected travel time savings relative to choice alternatives and another defined as experienced travel time savings on the current route. With regard to the former it was found that on average roughly one-quarter of the choices were inertial choices. Small travel time differences and dominant non-travel time route attributes had a positive effect on the frequency of inertial choices. Based on lost travel time (compared to shortest time) for routes with an average travel time of 8 to 16 minutes, inertia thresholds up to 2.90 minutes or 33% of the average travel time were found, while thresholds up to 1.22 minutes or 13% were most common. Considerable differences between OD-pairs and routes indicated that thresholds are probabilistic and dependent on the choice set. Considering experienced savings, it was found that on average roughly half of the choices concerned inertial behaviour. Inertia thresholds up to 3.77 minutes or 47% of the average travel time were found, while thresholds up to 0.97 minutes or 11% were more common. Switching behaviour could not be explained by experienced savings as participants were much less responsive to experienced loss on the current route as opposed to expected loss relative to the choice alternative.