How is Time-to-Collision (TTC) calculated and used in collision avoidance systems?

TTC estimates when a collision will occur if current motion continues unchanged. Basic calculation: TTC = Distance / Relative velocity, for objects on collision course. Refinements include: Accounting for acceleration (deceleration of lead vehicle), lateral motion (crossing paths), object dimensions (not point objects), and uncertainty bounds. TTC is used to: Trigger warnings (typically 2-3 seconds), activate brake pre-fill (<2 seconds), initiate partial braking (~1.5 seconds), and engage full emergency braking (<1 second). Thresholds vary based on driver state, road conditions, and confidence in object detection. Headway time (distance/speed) complements TTC for following distance management.