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Factors affecting driver speed choice along two-lane rural highway transition zones

ProQuest Dissertations and Theses, 2009
Dissertation
Author: Ivette Cruzado
Abstract:
The primary objective of this research was to develop speed prediction models to explain the relationship between the roadway features present along a two-lane rural highway transition zone and driver operating speeds. Two general model specifications were considered based on the available speed data. These included point speeds based on the "tracked" vehicles, and speed differentials between successive data collection points in a transition zone. In the point speed analysis, four repeated speed measurements were collected on each of the 2,859 drivers across 20 different sites. Longitudinal models were used to model these data and compared to the more traditional operating speed modeling approach, ordinary least squares (OLS) regression. Use of OLS regression violates the assumption of independent observations. The longitudinal models considered in this research were panel data models using both the fixed and random effects estimator, multilevel models, and generalized estimating equations (GEE). From the results of the analyses it was concluded that a three-level model in which speed observations were nested in drivers and drivers were nested in sites is more appropriate in explaining the influence of highway characteristics on driver speeds along two-lane rural highway transition zones. Key relationships between highway features and mean operating speeds in transition zones are as follows: (1) When compared to a posted speed limit of 55 mph, a speed limit of 45 mph is associated with a mean operating speed reduction of approximately 3.5 mph. A speed limit of 25 mph is associated with a mean operating speed that is approximately 10.5 mph lower than the baseline of 55 mph. Similarly, a posted speed limit of 35 or 40 mph is associated with a mean operating speed that is approximately 2.4 mph lower than the baseline of 55 mph. (2) Wider travel lanes and lateral clearance distances are associated with higher operating speeds along two-lane rural highway transition zones; a mean operating speed increase of 2.4 mph is expected per one-foot of lane width increase while a one-foot increase in lateral clearance is associated with a mean operating speed increase of 0.15 mph. (3) The presence of curb is associated with a mean speed reduction of approximately 4 mph while the analysis indicated that a mean speed reduction of 1 mph is associated with a one-unit increase in driveway density. (4) The presence of Intersection Ahead and School/Children warning signs were associated with 2 and 1 mph mean speed reductions, respectively, while the presence of a Curve Ahead warning sign was associated with a mean speed increase of almost 1 mph, when compared to the baseline of other warning sign types. (5) Finally, the presence of a horizontal curve was associated with a mean speed reduction of 1.5 mph; if the horizontal curve is combined with a warning sign, a mean speed reduction of almost 3 mph is expected when compared to the baseline of a tangent roadway section. A second data set was created in which the response variable was change in speed along the transition zone. By considering speed change as the response variable, only one data point per vehicle (driver) was available; however, a site cluster could still be considered in the model specification. Use of the speed differential as the dependent variable in a statistical model eliminated part of the repeated observation issue present in the point speed analysis. As such, two general modeling methods were considered. These included OLS regression and multilevel models in which speeds were nested in sites. The variables that were consistently associated with speed reductions across all models were changes in the posted speed limit, reduction in paved shoulder width (1 mph reduction per one-foot reduction in paved shoulder width), number of driveways (0.36 mph reduction per one-unit increase in driveway density), school/children related warning signs (8 mph mean speed reduction), length of transition zone (0.8 mph average speed reduction per 100 foot increase in transition zone length), and presence of horizontal curve that warrants a warning sign (3.2 mph mean speed reduction is expected with this type of horizontal curve). The presence of a Curve Ahead warning sign and tangent sections were consistently associated with a speed increase along transition zones across all models (3.2 mph average and 2 mph average, respectively). (Abstract shortened by UMI.)

vi TABLE OF CONTENTS

LIST OF FIGURES ......................................................................................................... viii   LIST OF TABLES ............................................................................................................. ix   AKNOWLEDGEMENTS.................................................................................................. xi   CHAPTER 1 INTRODUCTION .............................................................................................................. 1   1.1 Background .............................................................................................................. 1   1.2 Statement of Problem ............................................................................................... 3   1.3 Importance of Research to Engineering ................................................................... 4   1.4 Research Objectives ................................................................................................. 5   1.5 Organization of Dissertation .................................................................................... 6   CHAPTER 2 LITERATURE REVIEW ................................................................................................... 7   2.1 High-Speed Rural Highways ................................................................................... 7   2.2 Low-Speed Urban Streets ...................................................................................... 25   2.3 Rural to Urban Transition Zone Highways ............................................................ 31   2.4 Summary ................................................................................................................ 36   CHAPTER 3 DESCRIPTION OF DATA .............................................................................................. 39   3.1 Site Selection ......................................................................................................... 39   3.2 Data Collection ...................................................................................................... 42   3.2.1 Speed Data ...................................................................................................... 42   3.2.2 Highway Characteristics ................................................................................ 50   3.3 Summary ................................................................................................................ 55   CHAPTER 4 ANALYSIS METHODOLOGY ....................................................................................... 57   4.1 Point Speed Analysis ............................................................................................. 59   4.1.1 Ordinary Least Squares .................................................................................. 59   4.1.2 Panel Data ...................................................................................................... 62   4.1.2 Multilevel Models............................................................................................ 68  

vii 4.1.3 Generalized Estimating Equations (GEE) ...................................................... 73   4.2 Speed Differential Analysis ................................................................................... 78   CHAPTER 5 DATA ANALYSIS RESULTS ........................................................................................ 80   5.1 Point Speed Analysis Results................................................................................. 80   5.1.1 Correlation Analyses ...................................................................................... 80   5.1.2 Ordinary Least Squares .................................................................................. 81   5.1.3 Panel Data Analysis Results ........................................................................... 87   5.1.4 Multilevel Model Analysis Results ................................................................ 101   5.1.5 Generalized Estimating Equations (GEE) Analysis Results ......................... 117   5.1.6 Point Speed Analyses Summary .................................................................... 123   5.2 Speed Differential Analysis Results .................................................................... 126   5.2.1 Correlation Analyses .................................................................................... 131   5.2.2 Centralization of Continuous Variables ....................................................... 133   5.2.3 One-Way ANOVA.......................................................................................... 134   5.2.4 Linear Regression Model and Variance Inflation Factors ........................... 135   5.2.5 Additional Remedial Measures and Linear Regression Assumptions .......... 138   5.2.6 Multilevel Model for Speed Differential ....................................................... 140   5.2.7 Speed Differential Analyses Summary .......................................................... 147   CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS ......................................................... 151   6.1 Conclusions .......................................................................................................... 151   6.2 Application and Relevance to Transportation Engineering ................................. 157   6.3 Recommendations ................................................................................................ 159   REFERENCES ............................................................................................................... 163  

viii LIST OF FIGURES

Figure 1 Evolution of Reduce Speed Ahead Sign ............................................................. 2   Figure 2 Study Sites Classification (Stamatiadis et al., 2004) ......................................... 34   Figure 3 Transition Zone Illustration ............................................................................... 39   Figure 4 Example of a Transition Zone with a Reduced Speed Ahead Sign ................... 40   Figure 5 Sensor Layout .................................................................................................... 43   Figure 6 Mean Speed Plot for each Data Collection Point at each Study Site ................ 46   Figure 7 Area Assigned at each Sensor Location ............................................................ 51   Figure 8 Flowchart of Model Development and Identification ....................................... 58   Figure 9 Panel Data Illustration ....................................................................................... 63   Figure 10 Three-Level Hierarchical Data Structure ........................................................ 64   Figure 11 Class Diagram for Multilevel Model Dataset .................................................. 71   Figure 12 Random Path Diagram for Unconditional Three-Level Model ....................... 72   Figure 13 Model hierarchy for Aggregate Data ............................................................... 97   Figure 14 Class Diagram for Alternative Hierarchy ...................................................... 109   Figure 15 Unit Diagram for the Alternative Data Hierarchy ......................................... 110   Figure 16 Histograms for Speed at Sensor 1 (Original and Centralized) ...................... 134   Figure 17 Scatterplot of Residuals versus Fitted Values ............................................... 139   Figure 18 Histogram of Residuals ................................................................................. 139   Figure 19 Residuals versus the Order of the Data ......................................................... 140  

ix LIST OF TABLES

Table 1 Models Developed by Polus, et al. (2000) for Several Radius and Tangent Combinations .................................................................................................................... 12   Table 2 Speed Prediction Models (Schurr, et al., 2002) .................................................. 14   Table 3 85 th Percentile Speed Prediction Models (Lamm, et al., 2002) ........................... 16   Table 4 85 th Percentile Speed Reduction Models Due to Introduction of a Horizontal Curve (McFadden and Elefteriadou, 2000) ...................................................................... 20   Table 5 Two-level Model developed by Park and Saccomanno (2005) .......................... 22   Table 6 Coefficients of the Mixed Models with Fixed Effects by Sensor Location (Poe and Mason, 2000).............................................................................................................. 26   Table 7 Description of Study Sites .................................................................................. 41   Table 8 Sample Sizes for Different Levels of Confidence .............................................. 44   Table 9 Mean Speed and Speed Deviation at each Study Site ........................................ 45   Table 10 85 th Percentile Speeds ....................................................................................... 48   Table 11 Summary Statistics for Quantitative Highway Features ................................... 52   Table 12 Summary Statistics for Indicator Variables for Change in Roadway Alignment ........................................................................................................................................... 53   Table 13 Summary Statistics for Indicator Variables for Speed Limit, Total Number of Driveways, Warning Signs, and Centerline ...................................................................... 54   Table 14 Summary Statistics for Indicator Variables for Lateral Clearance, Guiderail, Curb, Building, and Regulatory Signs .............................................................................. 55   Table 15 Linear Regression Model Results ..................................................................... 82   Table 16 Prais-Winsten Speed Prediction Model ............................................................ 85   Table 17 Fixed-Effects Panel Data Model ....................................................................... 88   Table 18 Fixed-Effects and Random-Effects Comparison .............................................. 91   Table 19 Fixed-Effects Panel Data Models with and without Speed Limit .................... 94   Table 20 Correlation Values with Response Variable Mean Speed ................................ 98   Table 21 Fixed-Effects Panel Data Models for Aggregate and Disaggregate Data ........ 99   Table 22 Measures of Fit for the Aggregate and Disaggregate Fixed-effects Panel Data Models............................................................................................................................. 101  

x Table 23 Comparison between Two-Level and Panel Data Models .............................. 102   Table 24 Maximum Likelihood Estimates for Multilevel Unconditional Models Fitted ......................................................................................................................................... 106   Table 25 Comparison between Three-level, Two-level and Fixed-Effects Panel Data Models............................................................................................................................. 108   Table 26 Two- and Three-Level Variance Components Models for the Alternative Hierarchy ......................................................................................................................... 111   Table 27 Comparison between Three-level Models Hierarchies ................................... 113   Table 28 Three-Level Models with Previous Speed for Alternative Hierarchy ............ 115   Table 29 Variable Coefficients for each of the GEE Models According to Working Correlation Structures ..................................................................................................... 118   Table 30 Comparison Between all Model Selected as Appropriate .............................. 124   Table 31 Speed Differential along Transition Zone Descriptive Statistics .................... 127   Table 32 Descriptive Statistics for Continuous and Indicator Variables ....................... 129   Table 33 Correlations between Potential Explanatory Variables and Response Variable ......................................................................................................................................... 132   Table 34 Speed Differential OLS Results ...................................................................... 136   Table 35 Comparison between Two-Level and OLS Models ....................................... 141   Table 36 Two-Level Model for Speed Differential ....................................................... 142   Table 37 Random Intercept and Random Coefficient Models for Two-Level Speed Differential Prediction Model ......................................................................................... 145   Table 38 Speed Differential Models Comparison ......................................................... 149  

xi AKNOWLEDGEMENTS

I am very grateful to my advisor, Dr. Eric T. Donnell; I do not believe I could have finished this dissertation without his help. I am also grateful for the insight of my dissertation committee members: Dr. Paul P. Jovanis, Dr. Venky Shankar, and Dr. Steven F. Arnold. I would like to thank all my friends who supported me and helped me with data collection. I would also like to thank the employees at PennDOT as well as the personnel at PTI’s Test Track: Rick, Mike (a.k.a. Casanova), and Rae; thanks for making me laugh. Thanks to my friends at PTI’s Team Lab, especially to Scott Himes and Vishesh Karwa, for letting me interrupt their work so we could brainstorm about my research. I am very grateful to Miss Terry Reed, who taught me everything I needed to know about networking and etiquette. Thanks to my best friend, Ingrid Guadalupe, for believing in me. I am most grateful to a certain group of friends, whose help, love, and support during the bad days were vital to arrive to the finish line: Mildred Rodríguez, Marta Ventura, Maria Schmidt, and Nancy Vanessa Vicente. Lastly, I would like to dedicate this dissertation to my mother, Eileen I. Vélez de Cruzado, the most important person in my life.

1 CHAPTER 1 INTRODUCTION

Rural highways provide connections between developed areas, both residential and commercial. Safety issues may arise when traveling from a high-speed undeveloped to a low-speed developed environment. The roadway section between the high- and low- speed environments is referred to as a transition zone. In some cases, transition zone design may be accompanied by changes in roadway features; however, it is hypothesized that drivers fail to adjust their speeds accordingly. In other instances, drivers are only informed of the required speed changes by traffic signs with no corresponding changes in the roadway geometry. There are currently no geometric design guidelines for transition zones on two-lane rural highways. As such, the objective of this research is to collect operating speed, geometric design, roadside, and land use data along two-lane rural highway transition zones in Pennsylvania. Operating speed models are then estimated in order to obtain information about which roadway, roadside, and land use features are associated with changes in speed along transition zones.

1.1 Background In 2004, there were more than 4.0 million miles of publicly-owned highways in the United States (U. S.), 77 percent of which are rural roadways (FHWA, 2004). Two-lane rural highways must balance mobility and access, especially when passing through remote or sparsely developed areas. For the purposes of this research, a “transition zone” is defined as the section of a two-lane rural highway where the regulatory speed changes as the roadway passes through a developed area, either commercial or residential. Speed limits along high-speed two-lane rural highways typically exceed 40 mph. When passing through a developed area, the posted speed on two-lane rural highways is often reduced. The posted speed limit change is often accompanied by an increase in access density or pedestrian activity in the low-speed section of the two-lane rural highway. Traffic signs are sometimes the only way of communicating to drivers concerning the required change in vehicle operating speeds in transition zones.

2 The Manual on Uniform and Traffic Control Devices (MUTCD, 2003) contains guidelines on the size, shape, color, and placement of traffic signs. The “Speed Limit Sign” informs drivers about the limit established by law, ordinance, or regulation, and is thus classified as a regulatory sign. The “Reduced Speed Ahead Sign” informs drivers of an upcoming speed limit change; it is classified as a warning sign. Prior to passage of the 2003 edition of the MUTCD, the “Reduce Speed Ahead Sign” was classified as a regulatory sign. Figure 1 shows the evolution of the Reduced Speed Ahead sign, from the 2000 MUTCD edition, R2-5 series, to the 2003 edition, W3-5 series. The pre-2003 speed-zone signs are frequently seen along rural roads in central Pennsylvania.

Figure 1 Evolution of Reduce Speed Ahead Sign

Since speed changes should not be abrupt, drivers are warned of speed changes in advance. The Pennsylvania Department of Transportation’s (PennDOT) Publication 212 “Official Traffic Control Devices” (2006) indicates that a “Reduced Speed Ahead” or “Speed Reduction” sign must be installed between 500 and 1,000 feet in advance of a speed reduction unless the speed reduction is 10 miles per hour or less.

3 1.2 Statement of Problem Rural highways do not serve a vast majority of trips; they often serve traffic volumes less than 100 vehicles per day (McShane, 1998). However, fatal crashes are over-represented on rural highways in the U. S.; it has been estimated that approximately 60 percent of the more than 40,000 annual vehicle-related fatal accidents occurring in the U.S. take place on rural highways (FHWA, 2008). Evans (1991) compared these fatalities by type and functional classification of roads. His research indicated that if all rural and urban non- Interstates had the same fatality rate as the Interstate system, then a 50 percent reduction in fatalities could be achieved. Evans concluded that these statistics demonstrate the influence that roadway characteristics have on traffic safety. Therefore, it has been recommended that highways should be designed in a consistent manner to ensure that driver expectancy is not violated. The Fatal Accident Reporting System (FARS) indicates that nearly 15 percent of fatal crashes in 2005 were attributed to drivers traveling in excess of the posted speed limit (FARS, 2005). The American Association of State Highway and Transportation Officials’ (AASHTO) Policy on Geometric Design of Highways and Streets (2004), commonly referred to as the Green Book, contains a collection of design controls and criteria for all functional classes of highways and streets. The Green Book design criteria intend to provide consistency among design practices nationwide. Design speed is one of the primary design controls that influence highway design. The design speed is defined as “a selected speed used to determine the various geometric design features of the roadway (AASHTO, 2004).” In highway design, it is desirable to use only a single design speed along a corridor with the anticipation that uniform, consistent operating speeds will result. In the case of transition zones, however, a change in operating speed is required to be in compliance with the associated regulatory speed change, sometimes resulting in speed discord or inconsistencies, particularly in the low- speed operating environment. At the same time, the change in driving environment along transition zones may be accompanied by a change in the roadway or roadside design features. For example, the undeveloped rural area with a clear roadside at the high-speed end of a transition zone may suddenly transform into a developed area with sidewalks, curbs, and a high density of driveways at the low-speed end of a transition zone. While

4 design guidelines are available for both the high- and low-speed environments at either end of a transition zone, there are neither existing guidelines that provide designers with guidelines to link these environments nor are there design guidelines that have been shown to effectively reduce speeds in transition zones. Safety concerns can arise when drivers fail to appropriately adjust their speeds in transition zones. Since the driving environment changes from high-to-low speed, roadway design features along transition zones represent a challenge to the engineering profession. Furthermore, the low-speed environment presents possible safety concerns due to the presence of pedestrian activity and the increase in turning traffic (TRB, 2007). A recent study sponsored by PennDOT explored the effectiveness of dynamic speed display signs (DSDS) in reducing vehicle operating speeds along 12 two-lane rural highway transition zone sites in central Pennsylvania (Donnell and Cruzado, 2007). The DSDS devices were located 500 feet after the end of the transition zone and speed data were collected before, during, and after implementation of the DSDS. The before data indicated that drivers fail to adjust their speeds along the transition zone; mean operating speeds were 1.4 to 13.9 mph higher than the speed limit at the DSDS location while 85 th

percentile speeds were 7 to 20 mph higher than the posted speed limit. During DSDS implementation, both mean speeds and 85 th percentile speeds next to the DSDS were lower by an average of 6 and 7 mph, respectively. However, after the DSDS was removed, speeds increased to levels similar to the before data collection period suggesting that DSDS were only effective in reducing speeds along transition zones while in place and activated. Several geometric variables can influence driver behavior as reflected in past research studies (Yagar and Van Aerde, 1983; Poe and Mason, 2000). Therefore, identifying which geometric design elements are associated with operating speeds along transition zones can be the first step in the development of transition zones design guidelines.

1.3 Importance of Research to Engineering The Transportation Research Board’s Committee on Geometric Design (AFB10) and Operational Effects of Geometrics (AHB65) published a strategic research needs

5 document to outline a program to advance geometric design into the 21 st century (TRB, 2007). One of the 22 high-priority research needs identified in this long-range plan was to develop design guidelines for high-to-low speed transition zones. The objective of such a research project is to develop treatments and procedures to design high-to-low speed transitions in rural areas. It was recommended that changes in the alignment, vertical profile, and roadway and roadside cross-section be considered as methods to slow vehicle speeds in transition zones. A first step in this process is to estimate speed prediction models along rural highway transition zones to determine the roadway, roadside, and land use characteristics that are associated with driver operating speeds in these areas.

1.4 Research Objectives Design guidelines are currently not available for the design of transition zones on two- lane rural highways. The development of design criteria for transition zones may produce more uniformity in the roadway and roadside features encountered by motorists along these highway segments. Past research studies have indicated that geometric design, roadside, and land use features influence driver speed choice (Yagar and Van Aerde, 1983; Poe and Mason, 2000; Figueroa and Tarko, 2005), thus changes in these features may influence vehicle operating speeds when high-speed rural highways pass through rural communities. By identifying the highway features that are associated with speed reductions along transition zones, a contribution can be made to the development of design guidelines for high- to low-speed highway sections. As such, the scope of this research is to identify the roadway, roadside, and land use characteristics that are associated with reductions in operating speeds along two-lane rural highway transition zones. Point speed and speed differential models are estimated using a variety of longitudinal and hierarchical modeling methods. In past operating speed modeling literature, most models have been developed using ordinary least squares regression. Although linear regression models were specified in this research, other analysis methods were also explored and compared in an effort to determine if these alternative methods provide advantages over conventional operating speed modeling methods. The specification of alternative speed prediction

6 models may be helpful in overcoming the limitations of the ordinary least squares regression model in modeling vehicle operating speeds in transition zones.

1.5 Organization of Dissertation This dissertation is divided into five subsequent chapters. The second chapter discusses previous research studies that are related to the present study and have helped shape the proposed research. Specifically, those studies that have estimated speed prediction models as a function of the roadway environment are critically synthesized for both high- speed, two-lane rural highways and low-speed urban streets. The third chapter describes the site selection process and data collection methods. The fourth chapter discusses the analysis methods used in this research. The results of the analyses and the conclusions from this research are discussed in the fifth and the sixth chapters, respectively.

7 CHAPTER 2 LITERATURE REVIEW

Rural highways do not serve a vast majority of vehicle trips and often have traffic volumes less than 100 vehicles per day (McShane, 1998). However, approximately 77 percent of publicly-owned highways in the U.S. are classified as rural (FHWA, 2004). More than 50 percent of fatal crashes in the U.S. occur on two-lane rural highways (NHTSA, 2006). Because fatal crashes are overrepresented on two-lane rural roads in the U.S., these roadway types were considered the highest priority research need by the Transportation Research Board’s Committee on Geometric Design (Choueiri, et al., 1994). To address this need, the first version of the Federal Highway Administration’s (FHWA) Interactive Highway Safety Design Model (IHSDM) contains safety prediction and design consistency modules that can be used to assess the safety and operational performance of current and planned two-lane rural highways (Krammes and Hayden, 2003). Published literature related to speed prediction along rural highway transition zones between high- and low-speed operating environments is limited. As such, this literature review focuses primarily on speed prediction models that were developed exclusively for both high- and low-speed operating environments. High-speed roadways are considered those with a design speed of 50 mph or greater while low-speed roadways are considered those with a design speed of 45 mph or less (AASHTO, 2004). Much of the high-speed operating speed literature is focused on two-lane rural highways and some of this literature serves as the basis for the IHSDM design consistency module. Most of the low-speed operating speed literature relates to low-speed urban streets. In all cases, speed prediction literature that contains roadway, roadside, and land use characteristics are synthesized in this section of the dissertation.

2.1 High-Speed Rural Highways Design speed is a fundamental criterion in roadway design as it is used to establish the geometric design features of a highway (AASHTO, 2004). The design speed concept is intended to ensure geometric design consistency. Several operating speed studies have

8 been published on two-lane rural highways that specifically address the relationship between the design speed and operating speed that result from the design process. Operating speeds should be in harmony with the roadway’s design speed; discrepancies between design and operating speeds are evidence of a lack of design consistency. Differences between design and operating speeds led McLean (1979) to develop an alternative concept to the design speed. His research indicated that roadways with design speeds of 70 mph (110 km/hr) or greater had operating speeds that were in accordance with the design speed concept (i.e. operating speeds were uniform and lower than the design speed). McLean showed that operating speeds along horizontal curves on roadways with posted speed limits between 55 and 70 mph (90 and 110 km/hr) were lower than the design speed. On roadways with posted speed limits below 55 mph (90 km/hr), operating speeds exceeded the design speed on horizontal curves. McLean introduced a new concept which indicated that desired operating speeds can be related to the roadway’s terrain classification and alignment. McLean’s study considered speed data from 230 sites on two-lane rural highways in Australia, collected on both horizontal curves and the upstream approach tangent. The term “desired speed” was used to identify the speed under free-flow conditions when drivers are not constrained by alignment features, represented by the speed along tangent sections. The data collected indicated that this desired speed was influenced by road function, trip purpose and length, proximity to urban centers, overall design speed, and terrain type. For horizontal curves with design speeds of 60 mph (100 km/hr) and above, results showed that 85 th percentile speeds tend to be less than the design speed of a horizontal curve; however, the reverse is true along horizontal curves with lower design speeds. It was determined that available sight distance was correlated with 85 th percentile operating speeds, but explained less than one percent of the variability in a statistical model. As such, it was not included in the model specified below: 4 2 3 10 1 5.810 1 26.3464.08.53)85( × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ +× ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −+= RR VV FC (1)

Full document contains 181 pages
Abstract: The primary objective of this research was to develop speed prediction models to explain the relationship between the roadway features present along a two-lane rural highway transition zone and driver operating speeds. Two general model specifications were considered based on the available speed data. These included point speeds based on the "tracked" vehicles, and speed differentials between successive data collection points in a transition zone. In the point speed analysis, four repeated speed measurements were collected on each of the 2,859 drivers across 20 different sites. Longitudinal models were used to model these data and compared to the more traditional operating speed modeling approach, ordinary least squares (OLS) regression. Use of OLS regression violates the assumption of independent observations. The longitudinal models considered in this research were panel data models using both the fixed and random effects estimator, multilevel models, and generalized estimating equations (GEE). From the results of the analyses it was concluded that a three-level model in which speed observations were nested in drivers and drivers were nested in sites is more appropriate in explaining the influence of highway characteristics on driver speeds along two-lane rural highway transition zones. Key relationships between highway features and mean operating speeds in transition zones are as follows: (1) When compared to a posted speed limit of 55 mph, a speed limit of 45 mph is associated with a mean operating speed reduction of approximately 3.5 mph. A speed limit of 25 mph is associated with a mean operating speed that is approximately 10.5 mph lower than the baseline of 55 mph. Similarly, a posted speed limit of 35 or 40 mph is associated with a mean operating speed that is approximately 2.4 mph lower than the baseline of 55 mph. (2) Wider travel lanes and lateral clearance distances are associated with higher operating speeds along two-lane rural highway transition zones; a mean operating speed increase of 2.4 mph is expected per one-foot of lane width increase while a one-foot increase in lateral clearance is associated with a mean operating speed increase of 0.15 mph. (3) The presence of curb is associated with a mean speed reduction of approximately 4 mph while the analysis indicated that a mean speed reduction of 1 mph is associated with a one-unit increase in driveway density. (4) The presence of Intersection Ahead and School/Children warning signs were associated with 2 and 1 mph mean speed reductions, respectively, while the presence of a Curve Ahead warning sign was associated with a mean speed increase of almost 1 mph, when compared to the baseline of other warning sign types. (5) Finally, the presence of a horizontal curve was associated with a mean speed reduction of 1.5 mph; if the horizontal curve is combined with a warning sign, a mean speed reduction of almost 3 mph is expected when compared to the baseline of a tangent roadway section. A second data set was created in which the response variable was change in speed along the transition zone. By considering speed change as the response variable, only one data point per vehicle (driver) was available; however, a site cluster could still be considered in the model specification. Use of the speed differential as the dependent variable in a statistical model eliminated part of the repeated observation issue present in the point speed analysis. As such, two general modeling methods were considered. These included OLS regression and multilevel models in which speeds were nested in sites. The variables that were consistently associated with speed reductions across all models were changes in the posted speed limit, reduction in paved shoulder width (1 mph reduction per one-foot reduction in paved shoulder width), number of driveways (0.36 mph reduction per one-unit increase in driveway density), school/children related warning signs (8 mph mean speed reduction), length of transition zone (0.8 mph average speed reduction per 100 foot increase in transition zone length), and presence of horizontal curve that warrants a warning sign (3.2 mph mean speed reduction is expected with this type of horizontal curve). The presence of a Curve Ahead warning sign and tangent sections were consistently associated with a speed increase along transition zones across all models (3.2 mph average and 2 mph average, respectively). (Abstract shortened by UMI.)