The goal of this work is to explore the limitations of en-route platoon formation for heavy-duty vehicles. Platooning is known to reduce vehicle fuel consumption by decreasing the aerodynamic drag acting on a set of vehicles, but real-world factors such as the interference of traffic and differing route destinations are a restriction on long duration platoons with tight inter-vehicle spacing. One solution is to relax the assumption that platoons are fully autonomous sets of vehicles coordinated at a depot and instead investigate potential fuel savings by two or more vehicles joining for a portion of a shared route. This work examines a leader and a follower vehicle separated by a set distance on a shared route. The first investigation outlines net energy expenditure for a wide array of platoon formation scenarios. The resulting equation defines the maximum separation distance between vehicles beyond which platoon formation consumes more energy than platooning saves. Working from this result, a vehicle model is used to simulate the same set of platoon formation scenarios on a simplified route without the influence of traffic or road grade. The results demonstrate that physics-based energy equations are an effective predictor of the boundaries of en-route platoon formation. The energy investigation is expanded to vehicle operational cost by considering time constraints in an optimization formulation and converting energy and time costs to monetary equivalents.