Evaluation of alignment indices in estimating tangent speeds on rural two-lane highways

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Parma, Kelly Daniel
Texas A&M University
civil engineering.; Major civil engineering.
thesis / dissertation description
The design-speed concept has been used in attempting to ensure design and operating-speed consistency of roads in the United States, However, this concept has no effective controls on maximum speeds (e.g., tangents), which reduces its effectiveness. In an attempt to ensure operating-speed consistency, a speed-profile model has been developed to supplement the design-speed concept. One of the components of the speed-profile model is an estimate of the desired speed of motorists on long tangents. A standard value is used in estimating the desired speed; however, this estimate does not consider geometric or environmental factors that may influence desired speed. This thesis tested the hypothesis that quantifying the general character of the roadway alignment through the use of alignment indices may provide a method of accurately estimating the speed motorists drive on long tangents of rural two-lane highways. By using previous alignment characteristics encountered by motorists, these indices attempt to quantify the impact the preceding roadway has on the speeds motorists drive. Other countries, specifically Germany and England, have been able to estimate the operating speeds of motorists on their roads using alignment indices, In this research, alignment indices were identified and developed to test their applicability in estimating the desired speed of motorists on long tangents. Speed and alignment data were collected from different geographical locations across the United States. The alignment indices were calculated and compared to observed 85th percentile speeds using graphs and statistical analyses. This research indicated that alignment indices were not significant predictors of the desired speeds of motorists on long tangents of rural two-lane highways. The mean of 85th percentile speeds for long tangents in this study, 96.8 kni/h (60. 1 mph), does not statistically differ from the mean of 85th percentile speeds, 97.9 km/h (60.8 mph), in a previous study on long tangents of rural two-lane highways. Therefore, it is recommended that 97.9 km/h (60.8 mph) continue to be used as the estimate of desired speed on long tangents of rural two-lane highways in the speed-profile model for roadways with posted speeds of 88.5 km/h (55 mph).