OPERATING SPEED MODELS FOR PASSENGER CARS AND TRUCKS ON HORIZONTAL CURVES WITH STEEP GRADES

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 in Washington, California, West Virginia, Maryland, and Pennsylvania.  All sites contained a tangent section with a steep grade (greater than 4 percent) that progresses 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 significant factors are horizontal curve radius, approach tangent percent grade, posted speed limit, and superelevation on the horizontal curve.  For passenger cars at the PC, the 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 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 cars and trucks.  

TABLE OF CONTENTS

List of Tables …………………………………………………………………………………………………….. iv. List of Figures ………………………………………………………………………………………………………v.

Chapter 1.  INTRODUCTION  ……………………………………………………………………………… 1

Chapter 2.  LITERATURE REVIEW……………………………………………………………………… 3

Speed Prediction Models for Trucks…………………………………………………………………….. 3

Speed Prediction Models for Multilane Highways…………………………………………………. 8

Summary………………………………………………………………………………………………………… 17

Chapter 3.  SITE SELECTION AND FIELD DATA COLLECTION PROCEDURES.. 20

Locations………………………………………………………………………………………………………… 20

Equipment and Procedures………………………………………………………………………………… 22

Chapter 4.  METHODOLOGY…………………………………………………………………………….. 25

OLS Approach…………………………………………………………………………………………………. 25

3SLS Approach……………………………………………………………………………………………….. 28

Chapter 5.  DATA………………………………………………………………………………………………. 31

Chapter 6.  ANALYSIS RESULTS………………………………………………………………………. 34

OLS Regression……………………………………………………………………………………………….. 34

 

 

 

 

 

Passenger Car Speed Models …………………………………………………………………………..45

Truck Speed Models ………………………………………………………………………………………..33

OLS Regression with Disaggregate Data ……………………………………………………………..61 Speed Models at Point 1 ………………………………………………………………………………….62

Speed Models at Point 2 ………………………………………………………………………………….65

3SLS Regression ……………………………………………………………………………………………….68 Speed Models at Point 1 ………………………………………………………………………………….69

Speed Models at Point 2 ………………………………………………………………………………….72

Chapter 7.  DISCUSSION …………………………………………………………………………………….75

OLS Model Discussion ………………………………………………………………………………………75

3SLS Model Discussion ……………………………………………………………………………………..85

Chapter 8.  CONCLUSIONS …………………………………………………………………………………93

Chapter 9.  RECOMMENDATIONS ……………………………………………………………………..96

 

REFERENCES ……………………………………………………………………………………………………98 APPENDIX A: Descriptive Statistics ……………………………………………………………………102

 

APPENDIX B: Speed Profile Plots ………………………………………………………………………108

Chapter 1.  INTRODUCTION

Design consistency is the matching of roadway features with the expectation of drivers.

It should be considered as part of highway design.  The American Association of State

Highway and Transportation Officials’ (AASHTO) A Policy on Geometric Design of Highways and Streets (herein referred to as the Green Book) uses the design speed concept to establish geometric design controls and criteria for roadway sections (AASHTO 2004).  The intent is to apply design criteria so that alignments meet driver expectations.  This would then allow vehicle speeds to be consistent through the extent of the roadway segment.  The design speed concept operates under the assumption that all vehicles will travel at or below the design speed (Krammes et al. 1994).  However, this assumption does not hold true for all roadways.  It has been suggested that operating speed profiles replace the design speed concept in the design of roadway alignments (Leisch and Leisch 1977).

Developing operating speed models is the first step toward determining the factors that are associated with driver speed choice.  A large body of operating speed modeling literature exists; however, only a few of these models are applicable to multilane highways.  One objective of this thesis will be to supplement the two-lane rural highway operating speed models with additional multilane highway models.

Research has shown that truck operating speeds are lower than passenger car speeds on open highways (Leisch and Leisch 1977).  This is exaggerated on vertical gradients, where truck operating speeds are determined primarily by the mechanical characteristics of the vehicle.  The speed differential between passenger cars and trucks causes inconsistent vehicle operations on vertical grades.  Thus, understanding the operating speeds of trucks is essential to the design consistency of roadway segments containing steep grades.  There is a need for further research on the development of operating speed profiles of large trucks (Donnell et al. 2001; Harwood et al. 2003).  There is also no research completed on the development of operating speed models of large trucks using only field data.  Therefore, a second objective of this thesis will be to develop operating speed models of large trucks on steep grades.  The truck operating speed models will be compared to the passenger car operating speed models to enhance the existing body of literature on the relationship between driver speed choice and geometric design features.

OPERATING SPEED MODELS FOR PASSENGER CARS AND TRUCKS ON HORIZONTAL CURVES WITH STEEP GRADES

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