USE OF DEFAULT PARAMETERS FOR ESTIMATING SIGNALIZED INTERSECTION LEVEL OF SERVICE

The 1985 Highway Capacity Manual (HCM) operations method for estimating the level of service of signalized intersections can require a large amount of field data: turning movement volumes, lane geometry, signal timing, approach grades, percentage heavy vehicles, number of parking maneuvers per hour, number of buses stopping per hour, peak hour factors, number of conflicting pedestrians per hour, arrival types, and right turns on red. The effects on accuracy of replacing most of these required input data with the default values recommended in Table 9-3 of the HCM are tested. The average stopped delay was calculated for six signalized intersections starting with basic volume (flow rate), lane geometry, and signal timing data. The HCM-recommended default values for grades, heavy vehicles, and such were used in place of the rest of the required input data. The calculations were then repeated several times; each time one or more of the default values were replaced with field data. The resulting delay estimates were then compared with field measurements of delay. The test results indicated that users can obtain reliable estimates of intersection level of service and delay using only field-measured turning movements, lane geometry, and signal timing plus the HCM-recommended defaults for the rest of the required input data. Field measurements of peak hour factors, grades, percentage heavy vehicles, parking maneuvers, number of stopping buses, conflicting pedestrians, and arrival type improved the accuracy of the delay estimates, but the improvements were comparatively minor and did not change the estimated intersection level of service. Delay estimates for intersections with critical volume-capacity ratios of less than 85% of capacity were insensitive to additional data on peaking, arrival type, and saturation flow rates.