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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. 101 CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 5.1 CONCLUSIONS The conclusions of this research are: 1. Many aspects of work zone design are not covered by any nationally recognized guidance publication. 2. Work zone design embraces many subjects and decisions beyond temporary traffic control (TTC); however, TTC remains important to work zone operations and safety. 3. Despite extensive work zone research, there are a very limited number of findings that relate design decisions to probable safety and operational consequences. 4. Databases that can be analyzed to determine relationships between work zone design decisions and probable safety consequences do not exist. 5. Highway safety is a relative, rather than categorical or discrete, characteristic and should be so recognized in developing design decision guidance for work zones. 6. Development of comprehensive design decision guidance for work zones requires extending and adapting numerous design principles and information developed for permanent roads to roads in work zones, despite some significant differences between roads in work zones and permanent roads. 7. Exposure is a key factor in highway safety and differs substantially between permanent roads and roads in work zones; the difference should be accounted for in design guidance and design decisions. 8. Roadside design and barrier placement guidance currently in widespread use for work zones do not explicitly account for exposure. 9. Development of generalized barrier placement and roadside design guidance for practitioners that is based on cost-effectiveness principles is feasible. The Roadside Safety Analysis Program (RSAP) (48), despite some current flaws, increases the feasibility of developing guidance for generalized conditions. (Design aids for generalized conditions were developed and are included in Appendix A of this report). 10. RSAP is amenable to analysis of specific roadside hazard conditions in work zones, but it could be enhanced for this purpose by eliminating program errors and adding additional functions specifically related to work zones. 11. An encroachment model and a severity prediction tool are critical elements of roadside safety analysis and roadside design decisions. Current encroachment models and crash severity indices may not be well suited for construction work zones on high-speed highways. 12. Work zone speed is a subject of widespread interest and research. Despite this interest, there is very little guidance addressing the subject as a related set of
Final Report for NCHRP Report 581: Design of Construction Work Zones on High-Speed Highways Copyright National Academy of Sciences. All rights reserved. 102 regulatory, driver information, geometric and enforcement factors. Basic speed management principles should be applied to work zones. 13. An Artificial Neural Network speed model for work zones, capable of making useful predictions of vehicle speeds in two types of work zones on high-speed highways, was developed and is included in Appendix B of this report. The model is appropriate for use where actual conditions are within, but not beyond, the limits of the data used for its development. The model may be used toward integrating work zone design into work zone speed management. 14. Work zone design criteria and guidance of various state DOTs vary substantially on several topics. Examples of disparity are in the areas of sight distance; superelevation rates; minimum travel lane widths; traveled way and shoulder widths for temporary roads; speed change lanes at interchanges; barrier placement; and ancillary items. 15. Several state DOTs have comprehensive work zone design guidance. The most comprehensive guidance publications are similar and based on the same information. 5.2 RECOMMENDATIONS The recommendations of this research are: 1. Guidance for construction work zones on high-speed highways in Appendix A of this report should be considered for adoption and implementation by AASHTO and/or individual transportation agencies. 2. Current coordination efforts related to the contents of the Manual on Uniform Traffic Control Devices and Roadside Design Guide should be extended to recognize the contents of Appendix A of this report. 3. A robust database of work zone characteristics, exposure and corresponding safety performance should be planned, collected and analyzed to relate work zone design decisions to probable safety consequences. The database should include characteristics of the underlying facility (i.e., permanent condition) and work zone (i.e., work zone type, cross-sectional features and dimensions, alignment characteristics, access type and density, traffic control). A location reference system (e.g., milepost marker) should be used in both the work zone and safety records to enable matching of crash locations with specific work zone characteristics. Safety records should be matched to work zone conditions and appropriate statistical procedures used to account for vehicle and time-of-day exposure differences, regression-to-the-mean phenomena, and other external influences that can also affect crash frequency. Relationships between work zone decisions and safety consequences should be developed and integrated into future work zone design guidance. 4. The roadside design and barrier placement guidance for construction work zones on high-speed highways included in Appendix A of this report should be considered for adoption and implementation by AASHTO and/or individual transportation agencies and serve as a foundation for future progress.
Final Report for NCHRP Report 581: Design of Construction Work Zones on High-Speed Highways Copyright National Academy of Sciences. All rights reserved. 103 5. An encroachment and crash consequence prediction model(s) for work zones on high-speed highways should be developed to improve the cost- effectiveness of roadside safety and design decisions. 6. RSAP should be enhanced to eliminate current programming errors and include roadside conditions in common work zones (e.g., median crossover, construction in a closed lane adjacent to an active lane), hazards (e.g., equipment, opposing-direction vehicles, partial bridge and pavement structures), and countermeasures. A model with these capabilities would facilitate analysis of very specific conditions and diminish use of or replace generalized design aids, such as those provided in Appendix A of this report. 7. The Artificial Neural Network speed model for work zones developed under this project and included as Appendix B of this report is recommended for use within the limits of the data used for its development. The model was developed for two types of work zones using data within observed ranges. These work zone types and data ranges might not encompass the values common in some locales and jurisdictions. Expansion of the modelâs capability is recommended. 8. For work zones within the range of its applicability, the Artificial Neural Network speed model for work zones should be used for design and regulatory (speed limit) decisions.
