The Field of Safe Motion: Operationalizing Affordances in the Field of Safe Travel Using Reachability Analysis

The Field of Safe Motion (FSM) brings a 90-year-old theoretical concept into the computational era. Originally proposed in the 1930s, the Field of Safe Travel described the sensory and action space available to a driver, but remained purely conceptual. Now, researchers have operationalized it using reachability analysis—a robotics technique that computes all possible future states of a system given kinematic constraints. The FSM model checks at every moment whether a driver (human or autonomous) can still reach a safe state (i.e., an escape route) given the foreseeable actions of other road users. The approach relies on a small set of interpretable kinematic assumptions, making it easy to reason about and audit.

The key innovation is bounding uncertainty about where other road users might go next, using worst-case reachable sets. This allows the FSM to work as a real-time safety monitor that doesn't require probabilistic predictions or black-box models. The authors demonstrate the FSM across several driving scenarios, highlighting its strengths in capturing affordances (what actions are possible) and its limitations in complex or highly interactive situations. For robotics and autonomous driving engineers, this offers a transparent safety layer that can complement data-driven methods. The paper is a step toward formal, interpretable safety guarantees in motion planning.