An ad hoc distributed simulation is a collection of autonomous on-line simulations brought together to model an operational system. They offer the potential of increased accuracy, responsiveness, and robustness compared to centralized approaches. They differ from conventional distributed simulations in that they are created bottom-up rather than top-down. They combine concepts from conventional distributed simulations and replicated trials, raising new issues in data management and synchronization. The ad hoc simulation approach is proposed as well as an optimistic synchronization algorithm. A prototype coupling in-vehicle transportation simulations is evaluated, yielding promising results. Experiences applying this concept to a commercial transportation simulator in an emergency scenario are described.