Final Report for NCHRP Report 581: Design of Construction Work Zones on High-Speed Highways (2007) / Chapter Skim
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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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55 Repair costs consider the cost of repair of a safety treatment or other roadside object that has functional value after a crash has occurred with that object. Repair costs can be estimated from historical data, full scale crash testing, or simulation.
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56 Roadside and median barriers should only be installed where crashes with the barrier are likely to be less severe than crashes without the barrier. In addition, the roadside hazard being shielded should be exposed to a significant level of traffic over the performance period to justify the cost of providing and maintaining the barrier.
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57 Figure 10. Example design chart for cost-effective embankment warrants based on traffic speeds and volumes, slope geometry, and length of slope (Figure 5.3b from Roadside Design Guide)
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58 Figure 11. Suggested guidelines for median barriers on high-speed roadways (Figure 6.1 from Roadside Design Guide)
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59 The research reported in Section 4.1.5 attempts to build on the work of Sicking and Ross, and Michie. Primary differences were (1)
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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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61 of vertical grades. The user can also input a User Defined Adjustment Factor to account for unusual situations that could affect encroachment frequencies beyond the parameters incorporated into the program.
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62 Table 8 Relationship of severity index to crash severity (48) Injury Level (%)
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63 often introduce inaccuracies. However, these inaccuracies in repair costs are insignificant when compared to crash and installation costs and do not affect the overall benefit-cost analysis.
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64 change in severity index with impact speed, and average repair cost per crash. This information was modified in the si5.dat file for fixed objects.
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65 Severity Index vs. Impact Speed (Workers)
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66 Severity Index vs. Impact Speed (Light Equipment)
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67 The other assumptions inherent in all scenarios were the crash costs and direct costs for the safety treatments. FHWA KABCO crash costs from technical advisory T 7570.2, Motor Vehicle Accident Costs (51)
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68 It is apparent that in some cases benefit-cost ratios for equal values of exposure and speed are different. The encroachment rate versus ADT function is not a linear for certain ranges of ADT.
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69 higher ratio) was used.
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70 runs. It should be noted here that in situations where there is a lane or shoulder closure with some encroachment on the remaining travel lanes, a design decision that must be made is how to distribute the remaining paved roadway for temporary lanes and shoulders.
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71 encroachment rate for this facility type occurs for an ADT of approximately 5000 vehicles per day. Therefore, this ADT was used for all runs, and different levels of exposure were computed by only varying project durations.
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72 Section 5.4 of Appendix A discusses roadside safety and economics and is largely based on Chapter 2 of the Roadside Design Guide and on NCHRP Report 492. The main discussion item is the use of benefit-cost analysis for roadside safety treatment decisions.
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73 Figure 18. Schematic drawing of biological neurons.
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74 Some of the advantages of using an ANN are: • No assumptions need to be made as to the form of the model; • It is capable of extracting non-linear variable interactions; • It is able to generalize from small training data sets. Figure 20.
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75 Furthermore, the scope of the speed model was limited to single lane closures (with traffic using the travel lane adjacent to the closed lane) and lane closures with median crossovers on four lane divided facilities.
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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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78 Table 10 Descriptive statistics of candidate categorical predictor variables Lane Taper (23 locations) Work Area (96 locations)
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Table 11 Descriptive statistics of candidate continuous predictor variables Lane Taper (23 locations) Work Area (96 locations)
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Table 12 Descriptive statistics of 15th percentile speed (aggregated by location) for each vehicle type Aggregated 15th Percentile Speed for All Vehicles (mph)
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Table 13 Descriptive statistics of mean speed (aggregated by location) for each vehicle type Aggregated Mean Speed for All Vehicles (mph)
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Table 14 Descriptive statistics of 85th percentile speed (aggregated by location) for each vehicle type Aggregated 85th Percentile Speed for All Vehicles (mph)
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Table 15 Descriptive statistics of standard deviation of speed (aggregated by location) for each vehicle type Aggregated Standard Deviation of Speed for All Vehicles (mph)
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84 After an initial analysis of the dataset, several geometric variables were eliminated from the model based on missing data and lack of variation. The following variables were eliminated due to missing data: superelevation (e)
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85 particular site, while most variables change depending on the particular point within the site. The second input, ν(0)
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86 Table 16 Final model inputs Predicted network outputs are compared to the actual measured speeds or target values. Network parameters or weight and bias values are adapted in the training process to minimize error.
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87 The results obtained using the mean speed datasets for cars, trucks and all vehicles are shown in Figures 23, 24, and 25. A summary of the mean square errors (MSE)
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88 30 40 50 60 70 30 40 50 60 70 Measured Speed P re di ct ed S pe ed Best Linear Fit: A = (0.734)
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89 30 40 50 60 70 30 40 50 60 70 Measured Speed P re di ct ed S pe ed Best Linear Fit: A = (0.717)
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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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91 Figure 27. Excel speed profile model detail.
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92 factors have been the subjects of previous research, while many others have not. Therefore, guidance that provides for documented relationships among all design factors and performance is not achievable.
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93 number of lanes. Another enhancement is the development of separate exhibits for twolane and multi-lane facilities.
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94 Linkages were identified between construction work zones, desirable speed behavior, work zone design, and implementing speed management and control techniques. This guidance (section 2.2.4 of Appendix A)
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95 mph, Green Book stopping sight distance values are recommended. Green Book values for driver eye height and object height are recommended.
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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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97 summarized in Chapter 2, section 2.2.4, reports on crash rates for projects with and without reduced lane widths during construction. The results suggest reducing lane widths increase crash rates.
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98 The design guidance of Illinois, Indiana and Wisconsin DOTs all provide for a 16-foot traveled way width. Oklahoma DOT's guidance calls for a 12-foot minimum traveled way on median crossovers.
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99 4.3.3.5 Interchange Ramps One study on safety at interchanges within work zones was identified and is summarized in section 2.2.3 of this report. The conclusions are not useful in developing interchange ramp design work zone design guidance.
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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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