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ESTIMATION OF RELATIONSHIPS BETWEEN 85TH PERCENTILE SPEEDS, SPEED DEVIATIONS, ROADWAY AND ROADSIDE GEOMETRY AND TRAFFIC CONTROL IN FREEWAY WORK ZONES
ABSTRACT
The Manual on Uniform Traffic Control Devices defines a work zone as an area of highway with construction, maintenance or utility work activities. New federal work zone regulations require states to continually pursue improvement of work zone safety and mobility by analyzing work zone crash and operational data to enhance state processes and procedures. Work zone design guidance was identified as an area of needed improvement. Current guidance is heavily based on desirable speed-related outcomes, but knowledge related to actual speedrelated outcomes of design and traffic control decisions is limited. The objective of this research was to investigate relationships between speed behavior, roadway and roadside geometrics and traffic control in work zones. The research objective was accomplished through specification, estimation, evaluation and interpretation of a series of econometric models. Four speed-related performance measures were modeled: 85th percentile passenger car speed, 85th percentile truck speed, passenger car speed deviation and truck speed deviation. Data for model estimation were collected in Pennsylvania and Texas work zones. Three issues were addressed by the work zone speed models: contemporaneous correlation between equation disturbances, contemporaneous relationships between dependent variables and autocorrelation. A simultaneous equation model estimated with three-stage least squares was recommended. Effects of work design and traffic control features on speed were observed but small in magnitude. Additional data collection and modeling activities are recommended before direct implementation of speed findings into work zone design practice, including observation of larger samples with greater variability in geometric design elements and investigation of purely predictive modeling techniques. Selected results and conclusions have near-term value. No geometric or traffic control elements showed a direct effect on 85th percentile passenger car speeds. Passenger car speeds were controlled mostly by truck speeds, which were directly influenced by posted speed, work zone type, type of infrastructure and vertical alignment. Truck speed deviations were lower in work zones with a posted speed reduction of 10 or 15 mph than in work zones with no posted speed reduction. In addition, passenger car speed deviations were higher in work zones with a 70 mph posted speed limit compared to other posted speeds. Both of these findings contradict work zone posted speed guidance in the MUTCD, that a decrease in posted speed causes an increase in speed variance
and that posted speed reductions should be avoided or limited to 10 mph for this reason. In addition, passenger car speed deviations were lower in work zone areas with either a temporary concrete barrier or permanent roadside conditions compared to areas with drums, vertical panels or other similar roadside devices.
TABLE OF CONTENTS
Chapter 1 Introduction……………………………………………………………………………………………………. 1
1.1 History of Work Zone Policy………………………………………………………………………………. 2
1.2 Current Work Zone Design Policy and Practice…………………………………………………….. 3
1.3 The Role of Speed in Work Zone Design………………………………………………………………. 4
1.4 Research Objective……………………………………………………………………………………………. 6
1.5 Organization of Thesis………………………………………………………………………………………. 10
Chapter 2 Literature Review………………………………………………………………………………………….. 12
2.1 The Role of Speed in Work Zone Design Philosophy…………………………………………… 12
2.2 Operating Speed Effects of Work Zone Design and Traffic Control Features………….. 20
2.2.1 Advisory and Regulatory Speed Signing……………………………………………………. 22
2.2.2 Channelizing Devices………………………………………………………………………………. 22
2.2.3 Location in Work Zone…………………………………………………………………………… 23
2.2.4 Work Zone Type……………………………………………………………………………………… 24
2.2.5 Length of Two-Lane, Two-Way Sections in Median Crossover Work Zones…. 25
2.2.6 Median Crossover Design………………………………………………………………………… 26
2.2.7 Lane Width…………………………………………………………………………………………….. 26
2.2.8 Work Activity…………………………………………………………………………………………. 27
2.3 Summary of Work Zone Speed Literature…………………………………………………………… 30
Chapter 3 Data and Methodology…………………………………………………………………………………… 33
3.1 Scope………………………………………………………………………………………………………………. 33
3.1.1 Work Zone Strategy and Facility Type………………………………………………………. 34
3.1.2 Vehicle Type………………………………………………………………………………………….. 39
3.1.3 Operating and Environmental Conditions…………………………………………………… 40
3.2 Work Zone and Infrastructure Data…………………………………………………………………….. 41
3.3 Selection of Data Collection Equipment……………………………………………………………… 45
3.4 Speed Data………………………………………………………………………………………………………. 49
3.5 Characterization of Data and Identification of Modeling Techniques……………………… 52
3.6 Modeling Philosophy………………………………………………………………………………………… 52
3.7 Model Estimation……………………………………………………………………………………………… 54
Chapter 4 Ordinary Least Squares and Seemingly Unrelated Regression Models…………………. 58
4.1 Ordinary Least Squares Regression Models…………………………………………………………. 58
4.2 Model Specification and Estimation with OLS…………………………………………………….. 60
4.3 Seemingly Unrelated Regression Estimation……………………………………………………….. 73
4.4 Investigation of Autocorrelation in OLS and SUR Models…………………………………….. 80
4.5 Summary of Statistical Findings…………………………………………………………………………. 94
Chapter 5 Simultaneous Equation Models……………………………………………………………………… 102
5.1 Estimation of Simultaneous Equation Models…………………………………………………… 102
5.1.1 Identification…………………………………………………………………………………………. 104
5.2 Two-Stage Least Squares Estimation………………………………………………………………… 106
5.3 Three-Stage Least Squares Estimation………………………………………………………………. 116
5.4 Ordinary Least Squares Estimation…………………………………………………………………… 124
5.5 Investigation of Autocorrelation in the Simultaneous Equation Models………………… 129
5.6 Summary of Statistical Findings……………………………………………………………………….. 144
Chapter 6 Recommended Work Zone Speed Model……………………………………………………….. 153
6.1 Evaluation of Alternative Models and Final Model Recommendation…………………… 153
6.2 Interpretation of Model Parameters…………………………………………………………………… 165
Chapter 7 Summary, Conclusions and Recommendations……………………………………………….. 175
7.1 Model Specification and Estimation-Related Conclusions………………………………….. 176
7.2 Modeled Effects of Work Zone Design and Traffic Control Features on Speed177 ……..
7.3 Recommendations for Future Work………………………………………………………………….. 180
Bibliography……………………………………………………………………………………………………………… 182
Chapter 1 Introduction
The objective of this research is to investigate relationships between speed behavior, roadway and roadside geometrics and traffic control in work zones. The Manual on Uniform Traffic Control Devices (MUTCD) defines a work zone as an area of highway with construction, maintenance or utility work activities [1]. In addition to providing for the movement of traffic, work zones accommodate mechanized and labor-intensive construction, rehabilitation and maintenance activities. These activities and normal transportation functions (i.e. safety, efficiency and access) are often at cross-purposes. Reduced cross sections, increased curvature and other temporary design and traffic control features may be present, resulting in deviations from pre- or post work zone operations. Effects may be observed on both local and system-wide scales.
Work zones are present during highway improvement projects including new route construction, relocation, reconstruction and restoration, rehabilitation and resurfacing (3R). Work zone environments currently are and will continue to remain prevalent to motorists as illustrated by the following statistics:
- Approximately 12 billion vehicle-miles of travel were exposed to active work zones during 2001; approximately 61 billion vehicle-miles were exposed to inactive work zones [2].
- Between 1985 and 2005, vehicle-miles traveled (VMT) increased by approximately 70 percent [3], [4].
- An estimated 7,141 work zones were present on 163,734 miles of the National Highway System (NHS) during the peak summer roadwork season of 2003. With an average length of 6.1 miles, these work zones occupied an estimated 44,200 miles of the NHS (27 percent of total NHS mileage) [5].
- Obligations for Federal-aid highway and highway safety construction programs in the
Safe Accountable Flexible Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) increase from $34.4 billion in 2005 to $41.2 billion by 2009 [6].
Exposure to work zones will increase as vehicle-miles of travel and funding levels for highway improvement projects increase. Recognition of these trends is reflected in recent evolutions of work zone policies. A brief progression through the history of work zone policy leads to the recognition of an immediate need for this research.
1.1 History of Work Zone Policy
Prior to the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA), the results of several studies had indicated a higher level of crash risk in work zones than in permanent highway conditions (e.g., [7] and [8]). Section 1051 of ISTEA authorized development of a work zone safety program [9]:
“The Secretary shall develop and implement a work zone safety program which will improve work zone safety at highway construction sites by enhancing the quality and effectiveness of traffic control devices, safety appurtenances, traffic control plans, and bidding practices for traffic control devices and services.”
The Federal Highway Administration (FHWA) developed and implemented a work zone safety program by publishing a notice in the Federal Register on October 24, 1995 (60 FR
54562) [10]. This notice officially established the National Highway Work Zone Safety Program
(NHWZSP). The objective of the NHWZSP is to “enhance safety and operational efficiency of highway work zones for highway users – motorists, pedestrians, motorcyclists, bicyclists…and highway workers” [11]. The program consists of four components: standardization, compliance, evaluation and innovation. Under each component, future FHWA activities that aid NHWZSP implementation were recommended.
A relevant activity falling under the standardization component was the need to update the FHWA regulation on work zone safety and mobility (23 CFR 630, Subpart J, Traffic Safety in Highway and Street Work Zones [12]). The final updated rule was published in the Federal
Register (69 FR 54569) on September 9, 2004 with an effective date of October 12, 2007 [13].
A large portion of the old regulation addressed the preparation of a temporary traffic control (TTC) plan that is in agreement with the principles and standards of the MUTCD for routing traffic through a work zone. The new regulation discusses comprehensive evaluation and management of broad (e.g. system-wide as well as local) safety and mobility work zone impacts. These impacts will be considered during all phases of project development (as opposed to just
construction) and impact-management strategies (e.g. design, technology, contracting) will continue to be developed, improved and evaluated.
The final rule requires States to “implement a policy for the systematic consideration and management of work zone impacts on all Federal-aid highway projects” [13]. The policies developed by the States may take the form of processes, procedures and guidance. “States shall continually pursue improvement of work zone safety and mobility by analyzing work zone crash and operational data…to improve State processes and procedures” [13]. Work zone design procedures are an area of needed improvement, made evident by a review of current design policy and practice and identified in Guidance for Implementation of the AASHTO Strategic Highway Safety Plan [14].
1.2 Current Work Zone Design Policy and Practice
At a national level, design guidance for work zones is currently covered by the MUTCD
[1], published by the FHWA, and A Policy on Geometric Design of Highways and Streets
(Green Book) [15] and the Roadside Design Guide [16], both published by the American
Association of State Highway and Transportation Officials (AASHTO). A summary of publication scope and their application to work zone design is provided in Table 1-1. The table illustrates that national guidance related to temporary traffic control devices and crashworthiness of work zone roadside hardware is extensive. Guidance related to work zone geometrics is limited.
Work zone design guidance has been developed in-house by state departments of transportation (DOTs). Of 32 states responding to a recent survey, 25 indicated having a publication related to work zone design [17]. The extent of state guidance varied from standard drawings of TTC plans to detailed guidance in areas such as geometric design, drainage, roadside safety and use of traffic barriers and interchange auxiliary lanes [17].
Published research documents also include work zone design recommendations [18],
[19], [20], [21]) (see 2.1). The most recent were part of a research effort sponsored by the National Cooperative Highway Research Program (NCHRP) to develop design-decision guidance for construction work zones on high-speed highways. Results were published in two media [17], [21]. An important commonality between national guidance, state DOT-developed guidance and research recommendations is the prominent role of speed in work zone design and traffic control decisions.
Table 1-1
Table 1-1: Scope of current work zone design guidance in national publications
| Publication | Scope of Publication | Construction Work Zone Design Coverage |
| MUTCD | Traffic control devices | • Typical applications of temporary traffic control devices for work zones that consider the needs and control for all road users
• Limited guidance on work zone geometrics and roadside features |
| Green Book | Geometric guidance for all types of roads | • Very limited (2 pages) coverage regarding work zone geometrics
• No dimensional guidance or quantitative methods are included |
| Roadside
Design Guide |
Roadside design practice and principles | • Comprehensive guidance regarding physical characteristics and crashworthiness of work zone traffic control and other roadside devices and barriers
• Limited dimensional and quantitative design guidance with respect to clear zones, slopes, horizontal clearance, and temporary barrier use |
1.3 The Role of Speed in Work Zone Design
Speed is a primary input into past and current geometric and roadside design processes for permanent facilities. It is an important performance measure used to assess the quality of highway operation. Ideally, the speed that drivers travel on a facility should match the intended purpose of that facility and be harmonious with the surrounding environment. This is not always the case. Most recent research and opinion recognize that driver speed is a “complex issue involving engineering, driving behavior, education and enforcement” [22].
Speed is also prominent in current work zone design policies and practice. It is an input to several decisions related to TTC covered by the MUTCD (see Table 1-2). In addition, the MUTCD recommends an overall design philosophy of maintaining upstream or pre-work zone speeds if practical and minimizing magnitudes of speed reductions if necessary. The following excerpts illustrate this philosophy [1]:
“The basic safety principles governing the design of permanent roadways and roadsides should also govern the design of TTC zones. The goal should be to route road users through such zones using roadway geometrics, roadside features, and TTC devices as nearly as possible comparable to those for normal highway situations.”
“Reduced speed limits should be used only in the specific portion of the TTC zone where conditions or restrictive features are present.”
“A TTC plan should be designed so that vehicles can reasonably safely travel through the TTC zone with a speed limit reduction of no more than 10 mph.”
“A reduction of more than 10 mph in the speed limit should be used only when required by restrictive features in the TTC zone. Where restrictive features justify a speed reduction of more than 10 mph, additional driver notification should be provided. The speed limit should be stepped down in advance of the location requiring the lowest speed, and additional TTC warning devices should be used.”
Limiting speed reductions to 10 mph is based on desirable speed variance effects:
“Smaller reductions in the speed limit of up to 10 mph cause smaller changes in speed variance and lessen the potential for increased crashes. A reduction in the regulatory speed limit of only up to 10 mph from the normal speed limit has been shown to be more effective.”
Table 1-2
Table 1-2: Summary of speed-related temporary traffic control decisions in the MUTCD
| Traffic control decision | Speed measure used as decision input |
| Location of first advanced warning sign | Speed limit |
| Distance between advance warning signs | Speed category of roadway (e.g. low-speed, highspeed) |
| Stopping sight distance | Posted speed, off-peak 85th percentile speed prior to work or anticipated operating speed |
| Length of taper | Posted speed, off-peak 85th percentile speed prior to work or anticipated operating speed |
| Distance between taper devices | Speed limit |
| Use of temporary traffic barriers | Speed of traffic |
The speed-related design philosophy endorsed by the MUTCD is consistent with state DOT practice and recommended design procedures in research literature [17], [21], [19].
1.4 Research Objective
The speed-related design philosophy discussed in 1.3 is logical, but difficult to apply given the current state of work zone speed-related knowledge. Several observations support this general conclusion:
- Although current work zone design guidance is heavily based on desirable speedrelated outcomes (e.g. maintaining certain operating speeds, minimizing speed variance), knowledge related to actual speed-related outcomes of design decisions is limited (see 2). This includes speed variance effects of posted speed reductions, which is the basis for current work zone design policy and practice.
- Speed-related work zone design decisions use a variety of speed inputs interchangeably (e.g. speed limit, speed category, 85th percentile speed and design speed). Relationships between these measures are not consistent for permanent roadways or work zones.
- Existing and proposed design and traffic control practices are based on achieving desirable speed magnitudes while minimizing speed variance. These objectives may be complimentary or conflicting depending on the design or traffic control decision.
- Recommendations based on research results for posted speed reductions in work zones have been applied to other speed measures (e.g. design speed, target speed, anticipated operating speed). These measures may or may not be surrogates for actual operating speeds.
- Inconsistencies between pre-work zone operating speeds, desired operating speeds, posted speed and actual operating speeds lead to reactive implementation and unanticipated expenditures for work zone speed management strategies (e.g. police presence, intelligent transportation systems) when actual speeds are higher than intended speeds or after the occurrence of one or more severe crashes.
The objective of this research is to investigate relationships between speed behavior, roadway geometrics and traffic control in work zones. Current and recommended work zone design processes would benefit from an understanding of the speed-related outcomes of design and traffic control decisions. The research need is illustrated by the bulleted observations above and is consistent with specific requirements in the final rule on Work Zone Safety and Mobility (i.e. analyzing “operational data…to improve State processes and procedures” [13]). The location of this research in an overall scheme to improve work zone design practice is proposed in Figure 1-1.
The research objective is accomplished through specification and estimation of a series of econometric models. The models explain variation in speed-related performance measures based on work zone design and traffic control features as well as other speed-related measures (see 3.7). Current work zone design guidance is based on desirable outcomes related to speed magnitude and speed variance. Eighty-fifth (85th) percentile free-flow speed is the most commonly referenced measure of operating speed used in design and traffic control decision processes. Standard deviation of speed (referred to as speed deviation in the remainder of the document) is directly related to speed variance and has the same measurement units as operating speed (e.g. mph). Passenger cars and trucks (see 3.1.2 for definitions) have inherently different physical dimensions and performance capabilities. Work zones often introduce restrictive geometry including reduced sight distance, narrower cross sections and increased curvature.
Different speed behavior of passenger cars and trucks is expected.
Given the preceding discussion, four speed-related performance measures are modeled in this research:
- 85th percentile passenger car speed;
- 85th percentile truck speed; Passenger car speed deviation; and
- Truck speed deviation.
No other work zone speed models of this type exist. These first modeling steps are intended to discover possible associations between work zone variables and vehicle speeds. In the absence of other information, they may also be used for forecasting or policy development.
Results and conclusions will be presented in a way amendable to all three applications.
Figure 1-1
Figure 1-1: Location of research in proposed process to improve speed-related work zone design guidance
1.5 Organization of Thesis
The thesis consists of seven chapters:
- A general introduction is provided in Chapter 1. The need for this research is demonstrated through a review of past and current work zone policy and design practice. The chapter concludes with a specific research objective and the place of this research in an overall process to improve work zone design guidance.
- Published work related to the research objective is summarized in Chapter 2. Specifically, the focus is on findings related to the role of speed in work zone design and effects of work zone design and traffic control features on operating speeds. Techniques used to model relationships between speed and roadway features on permanent roadways are also identified.
- The research scope, data collection and general modeling philosophy is defined in Chapter 3. Data collection scope and methods, definitions and descriptive statistics of variables, characterization of data, identification of modeling techniques and general modeling philosophy are addressed.
- Model specifications and estimations with only exogenous explanatory variables (e.g. work zone geometry and traffic control) are addressed in Chapter 4. Estimator details, relevant hypothesis testing and estimation results are provided for ordinary least squares regression, seemingly unrelated regression and first-order autoregressive models. The chapter concludes with a summary of statistical findings.
- Specifications and estimation results for simultaneous equation models are summarized in Chapter 5. Estimator details, relevant hypothesis testing and estimation results are provided for two-stage least squares, three-stage least squares and ordinary least squares regression. Simultaneous equation models that account for autocorrelation are also included. The chapter concludes with a summary of statistical findings.
- An evaluation of alternative model structures and model recommendations are included in Chapter 6. Detailed interpretations of model parameters are also provided.
11 • A summary of the research effort, conclusions of this research and recommendations for future work are addressed in Chapter 7.
