Abstract: As road characteristics of long steep downgrades (LSD) hasn't been considered in existing alignment consistency assessment methods, a new method of highway alignment consistency assessment for LSD is presented using operating speed of trucks. With operating speed data of trucks collected at ninety-five sites selected from ten typical LSD in mountain freeways, the relationship between operating speed and the cumulative length, the average grade and the curvature of the downgrade was analyzed. Then the operating speed prediction model for trucks was developed. The distance and the average grade to the beginning of the downgrade were main geometric indicators considered in the model. And the traffic safety assessment technique of the alignment of LSD was put forward. It is proved that the operating characteristics of trucks on LSD have been reflected in the proposed model. So it provides theoretical support for the safety design and the consistency analysis of the alignment of LSD in mountain freeways.