A method for detecting a rough road surface upon which a vehicle is travelling, such detection being utilized by a crash sensing system for improving its immunity to inadvertent deployment during minor crash incidents. The onset of a crash event is sensed by an on-board accelerometer when the detected vehicle deceleration exceeds a predetermined deceleration threshold value. A rough road measurement is calculated as a function of the vehicle deceleration data and compared to a threshold boundary curve. A value of the rough road measurement exceeding the boundary curve is indicative of a rough road event.