Trade-off between travel time and safety: Modeling travelers’ heterogeneous preferences in traffic assignment

Empirical studies highlight the critical role of safety in route choice decisions, yet traditional traffic assignment models primarily emphasize travel time, often overlooking travelers’ heterogeneous safety preferences. This study bridges this research gap by providing a traffic assignment framework that models travelers’ trade-off between travel time and safety, considering their heterogeneous safety preferences. To capture these preferences, we introduce the value of safety (VoS), representing the additional travel time a traveler is willing to accept for improved safety. Unlike traditional approaches that categorize travelers into discrete classes, we model VoS as a continuous variable, allowing for a more refined representation of individual safety preferences. A bicriteria traffic assignment model incorporating this continuous VoS distribution is employed to determine route flows between origin–destination (OD) pairs based on both travel time and safety. Route safety is modeled in terms of risk, which is quantified by comparing routes between OD pairs based on various safety-related criteria. The proposed model is solved using a two-stage algorithm: the first stage evaluates route risk, while the second stage solves the bicriteria traffic assignment problem. Numerical experiments on both small-scale and real-world networks demonstrate how safety preferences influence route choices and subsequent flow patterns, showing that high safety-conscious travelers prioritize safer routes, even at the cost of longer travel times. The proposed framework serves as a foundation for safety-conscious planning, enabling planners to develop targeted safety improvement policies that are informed by traveler safety behaviors, rather than relying solely on historical crash data or past experiences.