Adjustment Factors for Heavy Vehicles at Signalized Intersections

The objective of this paper is to investigate the effects of heavy vehicles in through lanes and in left-turn lanes. Headway samples of 1,320 passenger cars and 722 heavy vehicles were measured in the field. Average headways were calculated for traffic flows of all passenger cars and all heavy vehicles in through lanes and left-turn lanes, respectively. The results show that different adjustment factors should be applied for through heavy vehicles and for left-turning heavy vehicles in calculating saturation flow rates. The adjustment factors for left turns should also be differentiated in accordance with the percent of heavy vehicles in traffic flow. Almost all estimated adjustment factors were found to be less than the corresponding figures suggested by the 1985 “Highway Capacity Manual.” A revised procedure is recommended for evaluating traffic conditions in Taipei. Similar studies may be desirable for cities in other parts of the world to verify the validity of the current procedures.