Research has shown that area population, the number of lanes in the lane group, and speed limit are correlated with saturation flow rate. However, the procedures in Chapter 16 of the 2000 "Highway Capacity Manual" (HCM) do not recognize these influences. The objective of this paper is to report the findings and conclusions from a research project that examined the influence of these and other factors on the saturation flow rate of the through lane group at several signalized intersections. An examination of several thousand headway observations indicates that saturation flow rate increases in a predictable way with an increase in population, number of lanes, or speed limit. Several saturation flow rate adjustment factors are calibrated on the basis of field data and are recommended for inclusion in the next edition of the HCM.