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Final Report for NCHRP Report 581: Design of Construction Work Zones on High-Speed Highways Copyright National Academy of Sciences. All rights reserved. 1 SUMMARY Driving through construction work zones is an increasingly common part of the transportation experience. The combination of two long-term trends has increased the expanse and importance of work zones: (1) functional obsolesce and physical deterioration of aging highways; (2) increasingly intense use (i.e., ongoing growth in daily traffic volumes and loadings) of these same facilities. To varying degrees, ongoing reconstruction activities require alteration of traffic patterns and the introduction of new features to the driving environment. Whereas permanent roads are designed solely to facilitate safe and efficient traffic movement, roadways in work zones must also accommodate mechanized and labor-intensive construction activity. The transportation and construction functions are often at cross-purposes, with previous research indicating that crash rates in work zones are generally higher than those for the same site during normal operations. Until fairly recently, work zone design generally consisted of developing temporary traffic control plans. While temporary traffic control is critical to work zone safety and operational efficiency, the work zone challenge cannot be met with temporary traffic control alone. Transportation management and the design of supporting infrastructure for work zones are also necessary to mitigate the potential negative impacts of work zones. Numerous existing publications provide insight and guidance on various aspects of designing construction work zones on high-speed highways. On a national basis, three publications were found to be widely referenced. The Manual on Uniform Traffic Control Devices (MUTCD) is a national standard used to develop temporary traffic control plans. The Roadside Design Guide is published by the American Association of State Highway and Transportation Officials (AASHTO) and devotes a chapter to Traffic Barriers, Traffic Control Devices, and Other Safety Features for Work Zones. This chapter provides considerable information on hardware details and the functional performance requirements (e.g., crashworthiness). A Policy on Geometric Design of Highways and Streets (Green Book), also published by AASHTO, provides limited guidance on work zone design and recommends providing geometrics and traffic control devices that are as nearly comparable to those for normal operating situations as practical, while providing room for the contractor to work effectively. Taken together, these publications provide a wealth of useful information but do not address many routine work zone design decisions. Identifying and addressing these knowledge gaps were central themes of this research. The subject of speed is inextricably connected to work zones. There is a widely held perception that speed is one of the most significant factors in road crashes. This perception is especially strong with regard to work zones. Speed reduction measures are a prominent topic in work zone practice and published research. Perceived speed-safety linkages stem, in part, from relationships between vehicle speed and operator capability. For permanent roadways, there is evidence indicating that crash probability is related to
Final Report for NCHRP Report 581: Design of Construction Work Zones on High-Speed Highways Copyright National Academy of Sciences. All rights reserved. 2 speed deviation above and below the mean speed. Crash rates are lowest for vehicles traveling near the mean speed. Based on information summarized in the two previous paragraphs, this research was directed toward the development of two products: (1) Design Decision Guidance for Construction Work Zones on High-Speed Highways, and (2) Work Zone Speed Prediction Model. These objectives were accomplished. As survey of state DOTs was conducted as part of the research. The survey yielded information in two areas: current state of work zone design practices and priority subjects for improving work zone design. Both sets of input were valuable. Existing state DOT practice were a key input to developing the design guidance. The priorities expressed in the survey were used to identify topics worthy of rigorous effort. The Design Decision Guidance (design guidance) was prepared as a stand-alone, hard copy appendix to the final report. The design guidance provides information not otherwise available in nationally-referenced publications. It is intended for consideration by transportation agencies in developing policies and procedures related to work zone safety and mobility. The design guidance is also intended for use by transportation agency and consultant personnel involved in the planning, design and review of construction work zones on high-speed highways. It is not recommended for adoption as a standard. The design guidance is written in a manner (i.e., terminology, format and conventions) similar to the AASHTO Green Book and consists of the following chapters: 1. Introduction; 2. Design Controls and Principles; 3. Conceptual Design and Planning of Work Zones; 4. Roadway Design; 5. Roadside Design and Barrier Placement; 6. Ancillary Design Information. The information was developed using various methods and information sources including previous research, design guidance for permanent roads, state DOT work zone guidance publications and focused studies. In a number of areas (e.g., sight distance, cross section features, superelevation distribution method), there is a reasonable basis for designing construction work zones with guidelines different than the criteria typically used for permanent roads. A critical distinction between construction work zones and permanent roads is exposure. Construction work zones have finite service lives, which significantly reduces total exposure in comparison to permanent features. Since exposure is a key predictor of safety performance, it is explicitly and implicitly considered in design decisions.
Final Report for NCHRP Report 581: Design of Construction Work Zones on High-Speed Highways Copyright National Academy of Sciences. All rights reserved. 3 The development of design aids for the placement of temporary concrete barriers in construction work zones was a research emphasis area. Better guidance in this area was the top priority of state DOT survey respondents. Historically, work zone barrier placement decisions were made through either subjective judgment or application of the clear zone convention. Where the clear zone approach is used, practice and policies vary substantially on the design clear zone dimension (width) used for work zones. To provide an alternative method of assessing barrier placement in construction work zones, this research developed estimated benefit-cost ratios for a series of common work zone scenarios. Several analysis methods were considered. The Roadside Safety Analysis Program (RSAP) was selected as the best available analysis tool even though the RSAP documentation does not indicate the program is intended for application to work zones. In consultation with the RSAP development team, selective departures and alterations were made from the default RSAP procedures to more closely represent the distinctive characteristics of work zones and to compensate for program errors. The results are graphic plots of the estimated benefit-cost ratios for each scenario, with speed limit and exposure as variables. A narrative description, plan and cross-section view is provided for each scenario. The Work Zone Speed Prediction Model estimates free flow vehicle speeds through two types of construction work zones on four lane freeways: single lane closures and median crossovers. The user interface is a Microsoft Excel spreadsheet, selected because of its wide availability. Input values are entered into cells by the user and results are displayed in labeled cells. The model uses an Artificial Neural Network which (as outlined in section 4.2.1) is a mathematical system based on the biological nervous system. For this particular application, the user provides two inputs (speed approaching work zone, type of work zone) that apply to the entire work zone and 14 inputs (primarily geometric and traffic control variables) for each representative location along the work zone. The model works in either the metric or US Customary system of units, as selected by the user. To develop the model, free flow speeds were collected at 17 work zones, 10 in Pennsylvania and 7 and Texas. Spot speeds were collected at 119 locations at the 17 sites (including a location upstream of the work zone). Approximately 200 observations were made at each location for a total of about 24,000 speed observations. Using the model for conditions (e.g., curve radii, lane widths, posted speeds) that are outside the range of observed conditions is discouraged. The user interface provides guidance notes on appropriate ranges for various inputs and warnings when values are entered outside the range. For each work zone, three sets of speed models are created: all vehicles, cars only, and trucks only. Each is displayed on a separate Excel worksheet. For each of the three models, the 15th, 50th and 85th percentile speeds are predicted at each location where inputs are provided. A graphic plot (speed profile) is also generated. The model is provided on a compact disc (CD) that is included with the final report. A User Manual was prepared is also included in electronic format on the same CD.
Final Report for NCHRP Report 581: Design of Construction Work Zones on High-Speed Highways Copyright National Academy of Sciences. All rights reserved. 4 The research report consists of five chapters. The Introduction (Chapter 1) provides background information related to work zones and specifically work zone design, including relevant guidance publications. The research scope and priorities are also identified. Chapter 2 is the Work Zone Literature Review and summarizes most of the recent relevant research publications. Numerous studies have been conducted, sometimes with conflicting findings. The methodologies and data for individual studies were evaluated. From this overview, several general conclusions were reached. For example, although results among studies vary, the preponderance of evidence indicates that the imposition of a work zone on a roadway is likely to diminish safety. There were many research findings that were neither refuted nor affirmed by other studies. As the research advanced, specific reported findings were considered in view of how the underlying study was conducted and the extent to which it was collaborated. Research findings were sought that established relationships between design decisions and performance results (e.g., safety, traffic flow). A very limited number of findings in this category were identified. A number of seemingly-reliable work zone safety characteristics, such as the distribution of work zone crash by type and severity, were identified and reported. Chapter 3, Current Work Zone Design Guidance, is an inventory of potentially- relevant publications, which are divided into national and state categories. The publications in the former category are the AASHTO Green Book, AASHTO Roadside Design Guide, Highway Capacity Manual and MUTCD. The latter group were the publications and documents obtained from state DOTs through a survey and agency Web sites. Through the review of these documents, the research team was able to identify gaps in current guidance. Additionally, a review of state documents led to conclusions on areas of needed coverage, the range of practice in specific areas and unique and innovative approaches. Chapter 4, Methodology and Findings, is a detailed summary of how the two research products were developed. The bases for the Design Decision Guidance are outlined including a detailed discussion on the application of RSAP to work zone scenarios. The data collection and development efforts associated with Work Zone Speed Prediction Model are also summarized. Chapter 5 reports on Conclusions and Recommendations. Several conclusions identify information gaps that, if filled, could be used to develop work zone design guidance with more direct safety linkages. A key recommendation is for agencies to adopt the Design Decision Guidance. Other recommendations provide specific suggestions on how to improve the research products and work zone safety in the future.
