Speed Factors on Two-Lane Rural Highways in Free-Flow Conditions

Abstract

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.

Operating Speed Prediction Model as a Tool for Consistency Based Geometric Design of Four-Lane Divided Highways

“…Vehicles are safe, when the preferred speed in a horizontal curve is equal to or less than its safe speed. Now, curves with radius 500 m or less are considered as sharp (Figueroa Medina, Tarko 2005). Sharp horizontal curves affect the preferred speed (Fitzpatrick et al 2000a;Jacob, Anjaneyulu 2013;Russo et al 2016).…”

“…Researchers have found that V 85 increases with increase in curve radius; however, remains unchanged for flatter curve (Abdul-Mawjoud, Sofia 2008;Fitzpatrick et al 2000b;Collins et al 1999). Flatter curves have radius of 1700 ft (i.e., 518 m) or more (Figueroa Medina, Tarko 2005). Russo et al (2016) reported that curves having radius more than 500 m do not influence driving performance in horizontal alignment.…”

Modelling the Operating Speed in Segments of Two-Lane Highways from Probe Vehicle Data: A Stochastic Frontier Approach

“…Medina and Tarko [32] employed an ordinary least squares method to estimate a customized regression equation for any intended percentile. The resulting models represent a linear combinations of factors associated with the mean and the standard deviation of the speed distribution.…”

Detection and analysis of hazardous locations on roads: a case study of the Croatian motorway A1

“…Inclusion of operating speed in the design phase and the evaluation of the road alignment consistency is in the focus of many significant researches in recent years (Gibreel et al 1999(Gibreel et al , 2001Ottesen, Krammes 2000;Hassan 2004;Fitzpatrick et al 2005;Marchionna, Perco 2008;Perco 2008;Medina, Tarko 2005;Cafiso, Cerni 2012;Wang et al 2013).…”