The mean free-flow speed and its variability across drivers are considered important safety factors. Despite a large body of research on operating speeds, there is still much to learn about the factors of free-flow speeds, especially on tangent segments of two-lane rural highways. The roadway factors of speed dispersion across drivers are largely unknown. Also, the use of the entire free-flow speed distribution suggested by other authors has not yet been addressed. Consequently, the existing models are not aimed to evaluate the speed variability at a site. This paper presents free-flow speed models that identify factors of mean speed and speed dispersion on tangent segments and horizontal curves of two-lane rural highways. Ten highway variables, six of them functioning as both mean speed and speed dispersion factors, were identified as speed factors on tangent segments. Four highway and curve variables, two of them functioning as both mean speed and speed dispersion factors, were identified as speed factors on horizontal curves. The developed free-flow speed models have the same prediction capabilities as traditional ordinary-least-squares models developed for specific percentile speeds. The advantages of the developed models include predicting any user-specified percentile, involving more highway characteristics as speed factors than traditional regression models, and separating the impacts on mean speed from the impacts on speed dispersion.

The mean free-flow speed and its variability across drivers are considered important safety factors. Despite a large body of past research on operating speeds, there is still much to learn about the factors of free-flow speeds, especially on tangent segments of two-lane rural highways. The roadway factors of speed dispersion across drivers are largely unknown. Also, the use of the entire free-flow speed distribution suggested by other authors has not been yet addressed. Consequently, the existing models are not aimed to evaluate the speed variability at a site. This paper presents free-flow speed models that identify factors of mean speed and speed dispersion on tangent segments and horizontal curves of two-lane rural highways. Ten different highway variables, six of them functioning as both mean speed and speed dispersion factors, were identified as speed factors on tangent segments. Four different highway and curve variables, two of them functioning as both mean speed and speed dispersion factors, were identified as speed factors on horizontal curves. The developed free-flow speed models have the same prediction capabilities as traditional ordinary least squares models developed for specific percentile speeds. The advantages of the developed models include predicting any user-specified percentile, involving more highway characteristics as speed factors than traditional regression models, and separating the impacts on mean speed from the impacts on speed dispersion.

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IntroductionThe INDOT Design Manual recommends that a design speed is selected based on the functional classification, urban vs. rural environment, terrain, traffic volumes and project scope of work. The design speed should be equal or greater than the legal or anticipated speed limit. According to AASHTO, a design speed should be consistent with the speed a driver is likely to expect on the highway.By using design speeds, highways are designed in a conservative manner to facilitate the safe motion of vehicles even in adverse but reasonable conditions. Consequently, the 85 th percentile of actual freeflow speeds may exceed the design speed. This situation does not have to cause excessive hazard because the majority of drivers adequately perceive the risk.Some Indiana road sections designed and built in the past do not meet the current design standards. INDOT makes a continuous effort to modernize these sections. Due to prohibitive costs, reduced design speeds and design exceptions are considered. Guidance is needed to help reduce the discrepancy between the economically justifiable design solutions and the design standards expected by the motorists. Predicting the 85 th percentile speed on modernized sections would help designers in finding solutions that meet both the motorists' expectations and the current design standards to the possible extent.The objective of the research was the development of a tool for predicting the actual speeds on modernized twoand four-lane roads in Indiana. FindingsThe mean free-flow speed and its variability across drivers are considered important safety factors. The existing speed-predicting models combine the mean speed impacts with the speed dispersion impacts, which make identification of the speed factors and interpretation of the results difficult. Furthermore, the existing models are specialized to selected percentiles and are not able to estimate the entire range of the speed variability at a site. This report presents an advanced method of modeling free-flow speeds that overcomes the limitations of the existing models.This has been accomplished by representing the percentile speed as a linear combination of the mean and the standard deviation.Free-flow speeds and highway geometry characteristics collected on two-lane rural highways and four-lane suburban and rural highways were used to develop the models. The crash experience on the studied highways was considered to eliminate segments where a high number of crashes indicated that the driver perception of the risk might be incorrect. The models demonstrated their efficiency in identifying relationships between speed and diverse road geometry characteristics, e.g. crosssection dimensions, horizontal curve elements, intersection and driveway densities and median type. The developed speed models were included in a prototype software tool to help highway designers implement the models. The tool generates a profile of the mean speed and any specified percentile speed for the entire project length based on the preliminary highwa...