How Mandatory Are "Mandatory" Lane Changes?

Lane changing, recognised as one of the most intricate manoeuvres in road traffic, has attracted extensive scholarly interest. To date, the concept of lane change has been categorised into two distinct classifications, namely mandatory and discretionary. Mandatory lane changes (MLCs) are often regarded as absolute, implying that the possibility of not executing the lane change is frequently disregarded. This paper questions this widely accepted proposition by evaluating the costs of neglecting an MLC. Specifically, we examine the costs associated with not making MLCs for exiting freeways, effectively quantifying the cost of missing such exits. The core of this study involves a dual approach comprising an analytical model for the costs of missing exits alongside an empirical analysis of two GPS datasets from the Minneapolis - St. Paul metropolitan area. The performance of the analytical model is validated by cross-referencing it against exitmissing costs from the top 50 metropolitan areas in the US. Regarding the empirical study, it was found that while both time and distance costs are associated with missing exits, the magnitudes of these costs are not substantial. The results obtained in this study offer novel insights into the nature of MLC, and we argue that future models should consist of discretionary (DLC), mandatory (MLC), and expedient (ELC) lane changes. Moreover, the analytical model developed in this study can be integrated into the trade-off function of an ELC model, enabling drivers to bypass their intended exit when needed.