Calibration of Capacity Parameters for Signalized Intersections in Indiana

Past research has indicated that the accuracy of traffic quality studies strongly depends on capacity parameters, particularly when demand approaches capacity. Minimal work has been done in the past on local values of the base saturation flow rate and lost times used in capacity analysis of signalized intersections. This paper discusses saturation flow studies conducted for Indiana. The results confirm that the default parameters recommended by the Highway Capacity Manual, such as the heavy vehicle equivalency factor and the protected left-turn adjustment factor, are adequate for Indiana. At the same time, the base saturation flow rate was found to vary strongly across locations, even when the geometry and traffic conditions were nearly identical, indicating that there are other factors not yet included in the Highway Capacity Manual. Regression analysis of the saturation flow rate variability identified the size of a town to be a good predictor. A model was developed and a set of base saturation flow rates were proposed, which depend on the number of lanes and the town population. The estimated impact of the size of the town estimated for Indiana is very close to the values obtained in the past for other states. The paper provides a convincing argument for adding to the Highway Capacity Manual a population adjustment factor for saturation flow rates.