Operating speed models for passenger cars and trucks on horizontal curves with steep grades

Open Access
Author:
Morris, Cody M
Graduate Program:
Civil Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 20, 2012
Committee Members:
  • Eric Todd Donnell, Thesis Advisor
Keywords:
  • speed
  • passenger cars
  • trucks
  • multilane highway
  • mathematical models
  • regression analysis
Abstract:
Past research suggests that operating speed profiles replace the design speed concept as the primary instrument when designing highways. This thesis builds upon this research by investigating the effect of alignment geometry and highway characterisitcs on passenger car and truck operating speeds. Continuous speed data were collected from 19 different multilane highway segments located in Washington, California, West Virginia, Maryland, and Pennsylvania. All sites contained a tangent section with a steep grade (greater than 4 percent) that progressed into a sharp horizontal curve. Mean speed prediction models were developed using an ordinary least squares (OLS) modeling approach. Separate models were developed for the approach tangent and the point of curvature (PC) of the horizontal curve. For passenger cars on the approach tangent, the statistically significant factors that were found to be associated with mean speeds include the horizontal curve radius, approach tangent percent grade, posted speed limit, and superelevation on the horizontal curve. For passenger cars at the PC, the statistically significant factors are horizontal curve radius, approach tangent percent grade, posted speed limit, the presence of an advisory speed sign, lane width, and superelevation on the horizontal curve. For trucks on both the approach tangent and at the PC, the statistically significant factors are horizontal curve radius, approach tangent percent grade, posted speed limit for trucks, the presence of an advisory speed sign, and lane width. A three-stage least squares (3SLS) modeling approach was used to investigate the possible endogeneity of passenger car speed and truck speed in the system of equations and to account for the contemporaneous correlation between the disturbances across the equations. The results indicate that endogeneity may exist between passenger car and truck speeds. Thus, it is recommended that future multilane highway speed models consider using simultaneous equations to account for the endogenous relationship between passenger car and truck speed measurements.