New Insights on Evaluations of Design Consistency for Two-Lane Highways

Abstract

Operating speed prediction models along horizontal curves of two-lane rural highways have been the topic of a significant body of literature. Operating speed models for two-lane rural highway tangents and for horizontal curves on low-speed urban streets also exist but are less developed. These models, estimated primarily by means of ordinary least squares regression, generally use aggregate-level data to estimate the effect of geometric design variables on the mean or 85th percentile operating speed but do not explicitly consider measures of speed dispersion (e.g., standard deviation). Most existing models consider speeds of free-flow passenger vehicles; traffic flow effects on speed are generally not quantified. Finally, published literature shows that researchers have not converged on a consistent modeling practice about the use of posted speed limit as an explanatory variable in operating speed models. Collectively, these issues may lead to design consistency assessments that are not truly reflective of the operating conditions along a two-lane highway. These issues are explored in this paper with the use of operating speed data collected at multiple points along eight two-lane highway segments. A simultaneous equations approach is used to estimate a three-equation model of the posted speed limit, mean speed, and speed deviation. The predicted 85th percentile speed is compared with the observed 85th percentile speed along a high-speed, two-lane rural highway. Predicted 85th percentile operating speeds along the same high-speed highway determined with the Interactive Highway Safety Design Model design consistency module are also compared with observed 85th percentile speeds as a basis for evaluating the simultaneous equations approach. Finally, the paper proposes an alternative approach to evaluating design consistency that uses inferred design speed.

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

“…Previous researches (Krammes et al 1995;Hassan et al 2001;Ng, Sayed 2004;Cafiso et al 2004;Park, Saccomanno 2006;Mattar-Habib et al 2008;Cafiso, Cava 2009;Himes et al 2011) have shown that one of the main reasons for accident occurrence is the geometric design inconsistency defined as the degree to which highway systems are designed to avoid critical driving manoeuvres and ensure safe traffic operation. A consistent road design ensures harmonized driving speed, in accordance with drivers' expectations and without sur- prising events.…”

Development and Calibration of a Global Geometric Design Consistency Model for Two-Lane Rural Highways, Based on the Use of Continuous Operating Speed Profiles

“…The operating speed dispersion is lower in this case (but higher than for horizontal curves). Himes et al (2011) found that the geometric design features on two-lane roads were not correlated with the posted speed limit. Hence, the speed limit should not be ignored as a predictor of operating speed.…”

“…Instead, using density and V/C ratio offered a richer description. Zhou and Sisiopiku (1997) noticed that hourly crash rates presented a 'U' shape, decreasing while the V/C ratio increased.Himes, Donnell and Porter (2011) examined the influence of the hourly traffic volume on the mean speed and its dispersion. They examined 79 sites of 8 roads in Pennsylvania and Virginia, finding that the hourly traffic volume was strongly correlated to the speed dispersion.…”

“…Himes, Donnell and Porter (2011) examined the influence of the hourly traffic volume on the mean speed and its dispersion. They examined 79 sites of 8 roads in Pennsylvania and Virginia, finding that the hourly traffic volume was strongly correlated to the speed dispersion.…”

“…0 : horizontal distance between point of vertical intersection and point of horizontal intersection (m). Himes et al (2011) developed a comprehensive research about the operating speed variability. The most important contribution was that they considered many exogenous and endogenous parameters into a 3-equation systems, performing a DEVELOPMENT AND CALIBRATION OF A GLOBAL GEOMETRIC DESIGN CONSISTENCY MODEL FOR TWO-LANE RURAL HIGHWAYS, BASED ON THE USE OF CONTINUOUS OPERATING SPEED PROFILES 3OLS regression.…”