Design of Construction Work Zones on High-Speed Highways (2007) / Chapter Skim
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From page 51... ...
51 CHAPTER 4 METHODOLOGY AND FINDINGS 4.1 DEVELOPMENT OF ROADSIDE DESIGN AND TEMPORARY BARRIER PLACEMENT GUIDANCE FOR CONSTRUCTION WORK ZONES Task 3 of NCHRP Project 3-69 called for "a survey of the states to collect guidance related to construction work zones and traffic control." At the time that the survey was created, it appeared that previous research and the associated literature would not provide direction as to which design features are high risk factors in work zones and should be prioritized for selected research during phase II of the project. This indeed was the case (see Chapter 2 of this report)
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52 4.1.2 Roadside Principles and Practices for Permanent Roadways Adoption of the roadside safety principles and the implementing procedures outlined in the Roadside Design Guide has significantly enhanced highway safety. The forgiving roadside concept, clear zone, prioritized treatment of hazards, and crashworthiness are applicable to work zones as well as permanent roads.
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53 The clear zone concept has been widely accepted because of its perceived simplicity. In general, the idea has been to observe the clear zone of a roadway segment.
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54 A benefit-cost ratio greater than 1 does not alone justify the implementation of a particular alternative. However, observing the ratios provides designers or other decision makers with quantitative information to help in making the best investment for safety and mobility needs.
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The adaptation and application of the benefit-cost analysis procedures discussed above to construction work zones is discussed in Section 4.1.5. 4.1.2.2 Use of Roadside and Median Barriers for Permanent Roadways A roadside barrier "is a longitudinal barrier used to shield motorists from natural or man-made obstacles located on either side of a traveled way.
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From page 56... ...
A temporary concrete barrier is the option most preferred by state transportation agencies for this purpose. One objective of this research was to develop design aids for commonly occurring construction work zone scenarios that are easy to use, similar to Figures 5.1 and 6.1 of the Roadside Design Guide.
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. 4.1.3 Existing and Ongoing Research on Construction Work Zone Roadside Design and Safety The two most relevant studies to the development of roadside design and barrier placement guidance were conducted by Sicking and Ross (46)
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His methodology utilized the AASHTO ROADSIDE computer program (available with the 1996 Roadside Design Guide) to generate estimates for the number of collisions in a work zone with and without placement of positive barrier.
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The design aids based on benefit-cost analysis are combined with information from various other sources to provide roadside design and temporary barrier placement guidance for construction work zones on high-speed highways.
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60 The Roadside Safety Analysis Program (RSAP) was considered by the research team to be the best available tool for developing work zone barrier placement guidance based on benefit-cost analysis.
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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.
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From page 62... ...
Final Report for NCHRP Report 581: Design of Construction Work Zones on High-Speed Highways Copyright National Academy of Sciences. All rights reserved.
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Compared to permanent roadside situations, there are many factors that are inherently different in construction work zones; the most important of these factors are the number and types of safety hazards and the level of exposure to particular hazards. For example, barrier placement decisions for permanent roadside hazards may be evaluated over a 25-year analysis period, whereas construction work zone hazards may exist only for a few days to 24 months.
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The latter three objects were common to most work zone scenarios and will be discussed here. Figure 12.
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Severity index and repair costs for motor vehicle-heavy equipment crashes.
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Severity index and repair costs for motor vehicle-light equipment crashes. The RSAP base encroachment rate was also adjusted to reflect differences between permanent roads and work zones.
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Therefore, given a work zone roadside scenario, benefit cost ratios will change as these three variables (average daily traffic, project duration, posted speed) change.
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= Two-way ADT (veh/day) x Work Zone Duration (days)
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From page 69... ...
The scenarios were developed using MUTCD Part 6H Typical Applications, as well as input from NCHRP Project 3-69 panel members and other practitioners. Each scenario contains a short description, an illustration of the work zone layout and cross sections, and the design guidance resulting from the RSAP
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A number of combinations can often exist. For the tools used to develop barrier placement guidance, different combinations of lane and shoulder widths did not affect the resulting guidance in the referenced sections.
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Section 5.2 of Appendix A discusses the clear zone concept and its applicability to construction work zones. Several disadvantages and shortcomings of the clear zone concept are provided.
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Section 5.5 presents the results of benefit-cost analysis to develop barrier placement guidance for a series of construction work zone scenarios. Study methodology detail is provided in section 4.1.5, some of which is repeated in the Appendix A to provide sufficient background detail for a designer using the aids.
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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.
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From page 74... ...
The final set of model inputs are discussed in section 4.2.4. 4.2.3 Data Collection and Descriptive Statistics High-speed highways are defined as "roads and highways with free-flow operating speeds of 50 mph and higher." Since this research involves construction work zones on high-speed highways, data was collected only for high-speed facilities.
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Data were collected in a total of 17 construction work zones; 11 single lane closures and 6 median crossovers in Pennsylvania and Texas. For crossovers, data were only collected in the travel direction containing the crossover.
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76 • Travel lane width; • Right and left shoulder width; • Right and left shoulder type; • Presence of and offset to roadside objects (e.g. temporary or permanent barrier, work zone channelizing devices, other roadside conditions)
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Table 9 Breakdown of speed data by location and work zone configuration for passenger cars (PC) and heavy vehicles (HV)
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Variable Categories Frequency Percent Frequency Percent Left 7 30.4% 9 9.4% Lane closed Right 16 69.6% 87 90.6% 50 4 17.4% 25 26.0% 55 2 8.7% 8 8.3% 60 4 17.4% 31 32.3% 65 7 30.4% 16 16.7% Posted speed 70 6 26.1% 16 16.7% no 6 26.1% 40 41.7% yes 2 8.7% 4 4.2% Police presence missing 15 65.2% 52 54.2% Permanent 23 100.0% 66 68.8% Roadway type Temporary 0 0.0% 30 31.3% Tangent 19 82.6% 52 54.2% Curve to the left 2 8.7% 27 28.1% Horizontal alignment Curve to the right 2 8.7% 17 17.7% Flat (-1 to 1) 12 52.2% 41 42.7% Upgrade 2 8.7% 21 21.9% Downgrade 1 4.3% 25 26.0% Crest curve 7 30.4% 4 4.2% Vertical alignment Sag curve 1 4.3% 5 5.2% Incoming grade 2 8.7% 0 0.0% Middle 1 4.3% 5 5.2% Outgoing grade 1 4.3% 2 2.1% N/A 15 65.2% 87 90.6% Location in vertical curve Missing 4 17.4% 2 2.1% None 15 65.2% 31 32.3% Drum 7 30.4% 7 7.3% Panel 0 0.0% 2 2.1% Guardrail 0 0.0% 4 4.2% Concrete barrier 1 4.3% 50 52.1% Traffic control device (TCD)
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F inal R eport for N C H R P R eport 581: D esign of C onstruction W ork Z ones on H igh-S peed H ighw ays C opyright N ational A cadem y of S ciences. A ll rights reserved.
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F inal R eport for N C H R P R eport 581: D esign of C onstruction W ork Z ones on H igh-S peed H ighw ays C opyright N ational A cadem y of S ciences. A ll rights reserved.
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F inal R eport for N C H R P R eport 581: D esign of C onstruction W ork Z ones on H igh-S peed H ighw ays C opyright N ational A cadem y of S ciences. A ll rights reserved.
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From page 82... ...
F inal R eport for N C H R P R eport 581: D esign of C onstruction W ork Z ones on H igh-S peed H ighw ays C opyright N ational A cadem y of S ciences. A ll rights reserved.
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From page 83... ...
F inal R eport for N C H R P R eport 581: D esign of C onstruction W ork Z ones on H igh-S peed H ighw ays C opyright N ational A cadem y of S ciences. A ll rights reserved.
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Some of these variables, like work zone type, are constant for a
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For the first speed predicted in a work zone, the previous predicted speed is the upstream speed, ν(0)
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choices and ranges Work Zone Configuration; discrete choices: Lane Closure or Median Crossover Upstream Speed (estimated or measured 85th percentile speeds upstream of work zone) any value between 48 and 72 mph Location; discrete choices: Taper or Within Work Zone Distance (location of analysis/prediction point measured from the taper)
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Measured Speed Predicted Speed Figure 23. ANN results for mean speed data for cars.
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Measured Speed Predicted Speed Figure 24. ANN results for mean speed data for trucks.
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Measured Speed Predicted Speed Figure 25. ANN results for mean speed data for all vehicles.
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90 4.2.6 Excel Implementation The spreadsheet then calculates the predicted speed profiles for 15th percentile speed, mean speed and 85th percentile speed and plots them on one graph. The 15th and 85th percentile speeds are calculated as follows: • 15th Percentile speed = Mean speed – 1.036 S.D; • 85th Percentile speed = Mean speed + 1.036 S.D.
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From page 91... ...
4.2.7 Conclusions The goal of this part of the research was to develop a speed profile model that will enable designers to detect design inconsistencies in construction work zone designs for high-speed highways before their implementation. An ANN was selected for model generation because of the advantages it offers.
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Definitions and basic information for specific work zone configurations and mitigation strategies is provided. Definitions were developed by the research team for work zone types that appeared in state DOT design guides and literature.
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Speed, and especially speed in work zones, has been a controversial subject for some time. The research team attempted to develop guidance that is consistent with general speed management doctrine, reinforces the MUTCD, is generally consistent with the practices of some DOTs, and is useful to practitioners at the project level.
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From page 94... ...
Guidance on roadside safety, roadside design and barrier placement in construction work zones builds on previous research and design guidance, notably that reflected in the Roadside Design Guide and state DOT design practices, as outlined in section 4.2 of Appendix A An extensive effort was made to the development of objective guidance for commonly occurring work zone design scenarios.
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From page 95... ...
. 4.3.2.4 Vertical Alignment Guidance on maximum grades was developed primarily from a review of state DOT work zone design guidance, which is often based on design of permanent roads.
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96 these provisions are referenced, rather than repeated or summarized, when applicable. The following discussion outlines the basis for work zone types and strategies.
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4.3.3.3 Median Crossover Median crossovers are a common freeway work zone configuration comprised of temporary and permanent roadway elements, with design decisions required for each. General guidance provisions for work zone design speed selection, sight distance, vertical alignment, horizontal alignment and superelevation are considered applicable and referenced.
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From page 98... ...
The design recommendations for multi-lane temporary roads, including shoulders, are based on limited published guidance. No research has been published specifically on safety consequences of multi-lane traveled way or shoulder widths for temporary roads in work zones.
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From page 99... ...
at entrance ramps within work zones. Maryland State Highway Administration has developed guidance on TTC (i.e., stop and yield control with related signage)
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From page 100... ...
100 A characterization of use and spacing of emergency turnouts was based on responses from 12 state DOTs to the survey. The schematic geometry layout is based on the guidance of New York and Wisconsin DOTs and a Pennsylvania DOT example project.
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