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Development of a Posted Speed Limit Setting Procedure and Tool (2021)

Chapter: APPENDIX F. DECISION RULE DEVELOPMENT

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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
×
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"APPENDIX F. DECISION RULE DEVELOPMENT." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 107 APPENDIX F. DECISION RULE DEVELOPMENT SLSG = LIMITED ACCESS Overview This section provides an overview of the decision rules selected for the SLSG of Limited Access. The research team considered the following sources in creating the decision rules for limited-access roadways:  Rules used in USLIMITS2 (11).  Information included in the updated User Guide for USLIMITS2 (15).  Findings from the literature, particularly the final report from NCHRP Project 17-45 (219) and NCHRP Report 783 (158).  Guidance from the Green Book (17) and the HSM (16).  Research team expert opinions.  Feedback from experts, including the project panel. Decision Rule Development The research team reviewed several options and decided to start with the rules used within USLIMITS2 and then refine or add to those rules based on findings in the literature. For roadways within the Limited-Access SLSG, measured operating speeds are used to identify the suggested posted speed limit. In general, the relationship of a variable with crashes is used to suggest whether the suggested posted speed limit should reflect:  The 85th percentile speed rounded to the closest 5-mph increment (C85).  The 85th percentile speed rounded down to the nearest 5-mph increment (RD85).  The 50th percentile speed rounded to the closest 5-mph increment (C50). When the roadway conditions are optimal, the suggested speed limit should reflect the 85th percentile speed. When roadway conditions are not favorable to all users or when crashes are a significant concern, then the suggested speed limit should reflect the 50th percentile speed. An RD85 speed limit is suggested when conditions are between those extremes. In rare cases, the RD85 will be less than the C50 due to rounding. As an example, if the 50th percentile speed were 58 mph and the 85th percentile speed were 59 mph, then the C50 would equal 60 mph and the RD85 would equal 55 mph. This situation only occurs when the 85th and 50th percentile speeds are within 1 mph of each other. Variables Included in SLS-Tool Table 46 provides an overview of the variables along with the variable value that would trigger using C85, RD85, or C50. Following is a justification for selecting those variables and break points.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 108 Table 46. Overview of decision rules for SLSG = Limited Access. Variable Basis for Suggested Posted Speed Limit Closest 50th (C50) Rounded Down 85th (RD85) Closest 85th (C85) Interchange spacing (length/number of interchanges) in miles and AADT (two-way total) in vehicles per day Inter_spac ≤ 0.5 mi and AADT ≥ 180,000 veh/d 0.5 mi < Inter_spac ≤ 1 mi and AADT ≥ 180,000 veh/d All other cases Grade in percent and design speed in miles per hour {Not applicable, see criteria in other cells}  Design speed ≥ 60 mph and grade > 4%  Design speed ≤ 55 mph and grade > 5% All other cases Outside shoulder width (SW) in feet {Not applicable, see criteria in other cells} SW < 8 ft SW ≥ 8 ft Inside shoulder width (ISW) in feet, number of lanes (N), and directional design-hour truck volume in trucks per hour {Not applicable, see criteria in other cells}  Truck_vol > 250 trk/hr and ISW < 12 ft  Truck_vol ≤ 250 trk/hr, N ≥ 6, and ISW < 10  Truck_vol ≤ 250 trk/hr, N < 6, and ISW < 4 All other cases All (KABCO) crash rate  High: Crash_rate > critical crash rate  Medium: Crash_rate > 1.3 average crash rate  Low: neither of the above is true High Medium Low Fatal and Injury (F&I) (KABC) crash rate  High: F&I_rate > critical crash rate  Medium: F&I_rate > 1.3 average crash rate  Low: neither of the above is true High Medium Low Interchange Spacing USLIMITS2 includes the variable “interchange spacing” with the break points shown in Table 46. The analyst enters the section length, number of interchanges, and AADT in vehicles per day. The program computes interchange spacing as length per interchange and calls for lower posted speed limits for the specified levels of interchange spacing if the AADT equals or exceeds 180,000 veh/d. The research team suggests retaining this approach. Grade and Design Speed USLIMITS2 calls for the analyst to enter the terrain type and caps the posted speed limit at 70 mph if the terrain type is mountainous. The research team suggests a similar special consideration for mountainous terrain based on the Green Book guidance for maximum grade

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 109 and design speed of limited-access facilities (17, Table 8-1). The guidance is synthesized as follows:  If design speed is 60 mph or greater and maximum grade exceeds 4 percent, use rounded-down 85th percentile speed.  If design speed is 55 mph or less and maximum grade exceeds 5 percent, use rounded-down 85th percentile speed.  In all other cases, set the posted speed limit as the closest 85th percentile. The first two conditions are based on the break points between maximum grades for rolling and mountainous terrain specified by the Green Book. Outside Shoulder Width For limited-access facilities, the Green Book (17, Chapter 8) calls for outside shoulder widths of at least 12 ft if the truck volume exceeds 250 trucks per hour (trk/hr), and at least 10 ft otherwise. Examination of the outside shoulder width CMF for limited-access facilities in the HSM (16) shows that outside shoulder width can be reduced slightly without a significant increase in crash frequency. The CMF value computes as 1.21 for outside shoulder width of 7 ft and 1.14 for outside shoulder width of 8 ft. In other words, when the outside shoulder width (rounded down to the nearest foot) is less than 8 ft, crash frequency is expected to increase by about 21 percent. Thus, based on safety considerations, the research team suggests setting the posted speed limit based on the rounded-down 85th percentile if outside shoulder width is less than 8 ft, or the closest 85th percentile otherwise. Inside Shoulder Width, Number of Lanes, and Hourly Truck Volume For limited-access facilities, the Green Book (17, Chapter 8) calls for the following minimum ISW:  If directional design-hour truck volume ≤ 250 trk/hr and number of through lanes (two-way total) < 6, then ISW ≥ 4 ft.  If directional design-hour truck volume ≤ 250 trk/hr and number of through lanes ≥ 6, then ISW ≥ 10 ft.  If directional design-hour truck volume > 250 trk/hr, then ISW ≥ 12 ft. Examination of the inside shoulder width CMF for limited-access facilities in the HSM (16) shows that inside shoulder width has a minor effect on crash frequency. The CMF value computes as 1.07 for inside shoulder width of 2 ft. Thus, the research team suggests setting the posted speed limit based on the Green Book criteria. If the criteria are met, set the posted speed limit based on the closest 85th percentile. If the criteria are not met, set the posted speed limit based on the rounded-down 85th percentile. Crash Level USLIMITS2 can produce a suggested posted speed limit when crash data are and are not available. USLIMITS2 requests the following to be able to conduct an analysis of the crash data:  Length of the study period in years and months (at least 3 years of crash data is recommended; if less than 1 year of data are input, the program suggests that additional data should be collected and the process repeated).  Total number of crashes (KABCO) in the section.  Total number of injury and fatal crashes (KABC) in the section.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 110  Average AADT (two-way total) for the study period.  Average rate of total crashes and average rate of injury and fatal crashes (100 million vehicle miles [MVM]) for similar road sections in a jurisdiction. To determine the average crash/injury rate for similar sections, users should select a group of sections that have the same or similar geometry (i.e., number of lanes, median type, etc.) and similar traffic volumes and area type. The length of study, number of crashes, and average AADT are used to calculate the section crash rate for total crashes and for injury and fatal crashes per 100 MVM. If the user does not provide average rates, default values from the HSIS are used (see Table 47 and Table 48). The critical crash rate is calculated from: 𝑅 𝑅 𝐾 𝑅𝑀 1 2𝑀 Where: Rc = Critical crash rate for a given road type. Ra = Average crash rate for a given road type, provided by the user or obtained from Table 47 or Table 48. K = Constant associated with the confidence level (1.645 for 95 percent confidence). M = Exposure (100 MVM). When crash data are available, the program compares crash rate—both all and injury— for the section to critical crash rate and average crash rate and uses the worst-case scenario.  High: section crash_rate > critical crash rate.  Medium: section crash_rate > 1.3 average crash rate.  Low: neither of the above is true. USLIMITS permits the use of “low,” even if crash level is medium or high, when the user indicates that “traffic control and/or geometric treatments reduce crash/injury rate in this section.” The research team suggests retaining this approach. Table 47. Average KABCO crash rate per 100 MVM for Limited Access SLSG. AADT Category—Min AADT Category—Max Urban Limited- Access Facilities (Inter_spac > 1 mi) Rural Limited- Access Facilities (Inter_spac > 1 mi) 0 24,999 92.83 49.20 25,000 49,999 79.80 51.23 50,000 74,999 76.96 44.16 75,000 99,999 88.34 100,000 149,999 91.16 150,000 199,999 91.60 200,000 {no limit} 104.51 Note: Crash rates and injury rates were calculated using the latest 3 years of data that were available: California (2009–11), Minnesota (2010–12), North Carolina (2011–13), Ohio (2010–12), and Washington (2010–12). Source: Adapted from the 2017 version of the User Guide for USLIMITS2 (15), Table 1. Transportation Research Board. 2006. Expert System for Recommending Speed Limits in Speed Zones: NCHRP 03-67. Adapted and Reproduced with permission from the National Academy of Sciences, Washington, DC.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 111 Table 48. Average KABC crash rate per 100 MVM for Limited Access SLSG. AADT Category—Min AADT Category—Max Urban Limited- Access Facilities (Inter_spac > 1 mi) Rural Limited- Access Facilities (Inter_spac > 1 mi) 0 24,999 24.74 13.39 25,000 49,999 21.24 12.92 50,000 74,999 21.37 14.41 75,000 99,999 25.15 100,000 149,999 27.69 150,000 199,999 29.25 200,000 {no limit} 30.75 Note: Crash rates and injury rates were calculated using the latest 3 years of data that were available: California (2009–11), Minnesota (2010–12), North Carolina (2011–13), Ohio (2010–12), and Washington (2010–12). Source: Adapted from the 2017 version of the User Guide for USLIMITS2 (15), Table 1. Transportation Research Board. 2006. Expert System for Recommending Speed Limits in Speed Zones: NCHRP 03-67. Adapted and Reproduced with permission from the National Academy of Sciences, Washington, DC. Variables Considered but Not Included in SLS-Tool The research team considered the following variables but did not select them for the SLS-Tool:  Terrain. USLIMITS2 calls for the analyst to enter terrain type (mountainous, rolling, or level) and caps the posted speed limit at 70 mph for mountainous terrain. The research team suggests an alternate approach that accounts for terrain through consideration of grade and design speed. This approach provides the analyst with a more quantitative method to account for the terrain.  Illumination. Research has not consistently shown under what conditions illumination is needed on limited-access facilities for safety purposes or whether the presence of illumination notably affects vehicle speeds.  Lane width. Examination of the lane width CMF for limited-access facilities in the HSM (16) shows that lane width has a minor effect on crash frequency. The CMF value is only 1.06 for a lane width of 10.5 ft, and lanes more narrow than 10.5 ft are rare in practice.  Horizontal alignment. The tool should continue to provide a warning when the user indicates that an adverse alignment is present, but this variable should not form the basis for adjusting the posted speed limit.  Number of high-volume or congested hours per day. While a notable volume of research exists to suggest implementing variable speed limits by time of day based on hourly volumes or onset of congestion, this variable should not form the basis for adjusting the base posted speed limit. Variable speed limits are outside the scope of this research project.  Presence of HOV or managed lanes (and type of separation from mixed-flow lanes). This variable is outside the scope of this research project.  Longitudinal barrier presence (median or roadside). This variable significantly affects safety performance but does not have a direct bearing on vehicle speeds. Thus, a longitudinal barrier presence should be considered in the analysis of crash rates. Additionally, the amount of effort needed to describe longitudinal barrier presence

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 112 (both length and offset are required) is greater than desired for analysis of the posted speed limit.  Clear zone width. The safety effect of this variable is minor compared to that of outside shoulder width. Input Variables The user is asked to provide the following variables:  Maximum speed limit (mph).  85th percentile speed (mph).  50th percentile speed (mph).  Section length (mi).  Number of lanes (two-way total).  Average annual daily traffic (veh/d).  Directional design-hour truck volume (trk/hr).  Design speed (mph).  Number of interchanges in the section.  Roadway grade (percent).  Outside (right) shoulder width (ft).  Inside (left) shoulder width (ft).  Is there adverse alignment (yes/no)?  Are crash data available (yes/no)?  If crash data are available, provide: o Number of years of crash data. o Average AADT for crash data period (veh/d). o Number of all (KABCO) crashes for crash data period. o Number of fatal and injury (KABC) crashes for crash data period. o Average KABCO crash rate (per 100 million vehicle miles traveled [VMT]) for similar sections during the same time period. If not provided, average KABCO rate from HSIS is used. o Average KABC crash rate (per 100 million VMT) for similar sections during the same time period. If not provided, average KABC rate from HSIS is used. Warning Messages USLIMITS2 checks for several conditions and issues warnings as needed. The NCHRP Project 17-76 SLS-Tool also issues warnings. The guide discusses the warning messages included in the SLS-Tool. The suggested minimum section length used in the warning message is from USLIMITS2, which notes that the minimum length values are the same as used in the USLIMITS1.0 and Australian XLIMITS expert systems (see Table 49).

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 113 Table 49. Minimum section length for a particular speed limit. Speed Limit (mph) Minimum Length (mi) 30 0.30 35 0.35 40 0.40 45 0.45 50 0.50 55 0.55 60 1.20 65 3.00 70 6.20 75 6.20 Source: Adapted from the 2017 version of the User Guide for USLIMITS2 (15) Table 2. Transportation Research Board. 2006. Expert System for Recommending Speed Limits in Speed Zones: NCHRP 03-67. Adapted and Reproduced with permission from the National Academy of Sciences, Washington, DC. SLSG = UNDEVELOPED Overview This section provides an overview of the decision rules selected for the SLSG of Undeveloped. The research team considered the following sources when creating the decision rules:  Findings from the literature review, particularly Stapleton et al. (220), Gates et al. (221), Das et al. (222), and Gates et al. (223).  Rules used in USLIMITS2 (11).  Information included in the updated User Guide for USLIMITS2 (15).  Expert opinions from members of the research team.  Feedback from the project panel. Decision Rule Development The research team reviewed several options before deciding to start with the rules used within USLIMITS2 (11) and then refine or add to those rules based on findings from the literature. For roadways within the undeveloped group, the research team used measured operating speeds to identify the suggested posted speed limit. In general, the relationship of a variable with crashes is used to determine whether the suggested posted speed limit should reflect:  The 85th percentile speed rounded to the closest 5 mph increment (C85).  The 85th percentile speed rounded down to the nearest 5 mph increment (RD85).  The 50th percentile speed rounded to the closest 5 mph increment (C50). When the roadway conditions are optimal, the suggested speed limit should reflect the 85th percentile speed. When roadway conditions are not favorable to all users or when crashes are a significant concern, then the suggested speed limit should reflect the 50th percentile speed. An RD85 speed limit is suggested when conditions are between those extremes. In rare cases, the RD85 will be less than the C50 due to rounding. As an example, if the 50th percentile speed were 33 mph and the 85th percentile speed were 34 mph, then the C50 would equal 35 mph and the RD85 would equal 30 mph. This situation only occurs when the 85th and 50th percentile speeds are within 1 mph of each other.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 114 Variables Included in SLS-Tool Table 50 provides an overview of the variables along with the variable value that would trigger using either C85, RD85, or C50. Following the table is a justification for selecting those variables and break points. Access Points Previous studies have shown that roadway safety decreases as the number of access points increases (220, 221). The guidance for access points is synthesized as follows:  If the number of access points is greater than or equal to 40 per mile on undeveloped divided roadways or if the number of access points is greater than or equal to 30 per mile on undeveloped undivided roadways, then the posted speed limit should be set at the lower of the closest increment to the 50th percentile (C50) or rounded down to the closest increment to the 85th percentile (RD85).  If the number of access points is greater than 20 and less than 40 per mile on undeveloped divided roadways or if the number of access points is greater than 15 or smaller than 30 per mile on undeveloped undivided roadways, then the posted speed limit should be set at the closest 5-mph increment to the rounded-down 85th percentile (RD85).  If the number of access points is less than or equal to 20 per mile on undeveloped divided roadways or if the number of access points is less than or equal to 15 per mile on undeveloped undivided roadways, then the posted speed limit should be set at the closest increment to the 85th percentile (C85). Number of Lanes/Median Type (Divided or Undivided) Combinations Das et al. (222) found that undeveloped undivided four-lane roadways had a greater speed variability than undeveloped undivided two-lane roadways. A review of the HSM showed that the crash prediction for undivided four-lane roadways is greater than for divided four-lane roadways (see Figure 24). The figure uses the SPFs in the HSM. The SPFs for undivided roadway segments on rural multilane highways (4U) are applicable to the AADT range from 0 to 33,200 veh/d (HSM, pages 11–15). The SPFs for divided roadway segments on rural multilane highways (4D) are applicable to the AADT range from 0 to 89,300 veh/d (HSM, pages 11–18).

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 115 Table 50. Overview of decision rules for SLSG = Undeveloped. Variable Basis for Suggested Posted Speed Limit Closest 50th (C50) Rounded-Down 85th (RD85) Closest 85th (C85) Access points (non- residential driveways and intersections per mile)  > 40 access points per mile (divided)  > 30 access points per mile (undivided)  > 20 and ≤ 40 access points per mile (divided)  > 15 and ≤ 30 access points per mile (undivided)  ≤ 20 access points per mile (divided)  ≤ 15 access points per mile (undivided) Number of lanes, median type, AADT (two-way total) combination {Not applicable, see criteria in other cells}  Four or more lanes with no median (undivided) and AADT > 2000 veh/d  Four or more lanes with divided median  Two lanes with any median type  Four or more lanes with no median (undivided) and AADT ≤ 2000 veh/d  Any number of lanes/median type combination when AADT ≤ 2000 veh/d Lane width (LW) and AADT (two-way total)  LW ≤ 9 ft and AADT > 2000 veh/d  9 ft < LW < 11 ft and AADT > 2000 veh/d  LW ≥ 11 ft and AADT > 2000 veh/d  Any lane width when AADT ≤ 2000 SW and AADT (two-way total)  SW < 2 ft and AADT > 2000 veh/d  2 ft ≤ SW < 6 ft and AADT > 2000 veh/d  SW ≥ 6 ft and AADT > 2000 veh/d  Any shoulder width when AADT ≤ 2000 veh/d All (KABCO) crash rate  High: crash_rate > critical crash rate  Medium: crash_rate > 1.3 average crash rate  Low: neither of the above is true High Medium Low F&I (KABC) crash rate  High: F&I_rate > critical crash rate  Medium: F&I_rate > 1.3 average crash rate  Low: neither of the above is true High Medium Low

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 116 Figure 24. Crash prediction using HSM equations for four-lane undivided and divided roadways. Four-lane undivided roads with AADT of 2,000 have about 35 percent more crashes compared to four-lane divided roads with the same AADT. The percent is smaller for roads with AADT less than 2,000 and more for AADT greater than 2,000. As will be discussed in the lane width and shoulder width sections below, the research team considered a CMF of more than 1.30 sufficient to justify using the rounded-down 85th percentile speed rather than the closest 85th percentile speed for that condition. Therefore, the research team suggests that the rounded- down 85th percentile speed be used when the road has four lanes, is undivided, and has 2,000 or more AADT. Other cases, such as two lanes, or AADT less than 2,000 would use the closest 85th percentile speed. Additional justification for using 2,000 veh/d is provided with the recent publication of the second edition of AASHTO’s low-volume road guidance (224). That document includes 2,000 veh/d or less as the threshold of traffic volume for low-volume roadways. The cut off between high-volume and low-volume roadways previously depended on context including roadway functional classification. AASHTO’s first edition of Guidelines for Geometric Design of Very Low-Volume Local Roads, published in 2001, considered 400 veh/d or less as the threshold of traffic volume for low-volume roadways (16). NCHRP Report 362 (Roadway Widths for Low-Traffic-Volume Roads) considered roadways with 2,000 veh/d or less as low- volume roadways (225). According to the sixth edition (in 2011) of AASHTO’s Green Book, “It may not be cost-effective to design local roads and streets that carry less than 400 vehicles per day using the same criteria applicable to higher volume roads or to make extensive traffic operational or safety improvements to such very low-volume roads” (226). The 2018 version of the AASHTO Green Book revised that sentence to use the 2,000 veh/d number: It may not be cost-effective to design local roads and streets that carry less than 2,000 vehicles per day using the same criteria applicable to higher volume roads or to make extensive improvements to such very low-volume roads. Alternate

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 117 design criteria may be considered for local and minor collector roads and streets that carry 2,000 vehicles per day or less in accordance with the AASHTO Guidelines for Geometric Design of Very Low-Volume Local Roads. (17, page 5- 2) The guidance for number of lanes/median type combination is synthesized as follows: if the undeveloped roadway has > 2,000 AADT, is four or more lanes, and is undivided, the posted speed limit should be set using the rounded-down 85th percentile speed (RD85). For other cases, such as when the roadway is divided, the closest 85th percentile speed is used. Roads with raised, depressed, or grass medians would be considered divided. Lane Width Examination of the lane width CMF for undeveloped facilities in the HSM (16) shows that a 12-ft lane width is assigned a CMF of 1.00 (see Table 10.8, Table 11.11, and Table 11.16 in the HSM). The CMF value computes as 1.05 for 11-ft and 1.30 for 10-ft lane width for two- lane roadways. For multilane undivided roadways, these values are 1.04 and 1.23 for 11-ft and 10-ft roadways, respectively. Stapleton et al. (220) found that rural two-lane roadway lane widths greater than 12 ft were found to have fewer F&I crashes. The guidance for lane width is synthesized as follows:  If lane width is less than 10 ft, the suggested posted speed limit should use the closest increment to the 50th percentile speed (C50).  If lane width is greater than 9 ft and less than 11 ft, the posted speed limit should be set using the rounded-down 85th percentile (RD85).  If lane width is equal to or greater than 11 ft, the posted speed limit should be set at the closest increment to the 85th percentile (C85). Shoulder Width Studies have consistently found that wider paved shoulders on undeveloped roadways result in fewer crashes (227, 228). Examination of the shoulder width CMF for undeveloped facilities in the HSM shows that a 6-ft shoulder width is assigned a CMF of 1.00 (see Table 10.9, Table 11.12, and Table 11.16 in the HSM). The CMF value computes as 1.15 for 4-ft and 1.30 for 2-ft shoulder width for two-lane roadways. For multilane undivided roadways, these values are 1.15 and 1.30 for 4-ft and 2-ft shoulder width, respectively. For multilane divided roadways, an 8-ft right shoulder width is assigned a CMF of 1.00 (see Table 11-17 in the HSM). The guidance for shoulder width is synthesized as follows:  If the shoulder width is less than 2 ft in undeveloped roadways with AADT > 2,000 veh/d, the suggested posted speed limit should use the 50th percentile speed (C50).  If the shoulder width is greater than or equal to 2 ft and less than 6 ft in undeveloped roadways (AADT > 2,000 veh/d), the posted speed limit should be set using the rounded-down 85th percentile (RD85).  If the shoulder width is greater than or equal to 6 ft in undeveloped roadways (AADT > 2,000 veh/d), the posted speed limit should be set at the closest increment to the 85th percentile (C85).

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 118 Crash Level USLIMITS2 can produce a suggested posted speed limit whether crash data are available or not (11). USLIMITS2 requests the following information in order to conduct an analysis of the crash data:  Length of the study period in years and months (at least 3 years of crash data is recommended; if less than 1 year of data are input, the program suggests that additional data should be collected and the process repeated).  Total number of crashes (KABCO) in the section.  Total number of F&I crashes (KABC) in the section.  Average AADT for the study period.  Average rate of total crashes and average rate of F&I crashes (100 MVM) for similar road sections in a jurisdiction. To determine the average crash/F&I rate for similar sections, users should select a group of sections that have the same or similar geometry (i.e., number of lanes, median type, etc.) and similar traffic volumes and area type. The length of study, number of crashes, and average AADT are used to calculate the section crash rate for total crashes and for F&I crashes per 100 MVM. If the user does not provide average rates, default values from the HSIS are used (see Table 51 and Table 52). The critical crash rate is calculated from: 𝑅 𝑅 𝐾 𝑅𝑀 1 2𝑀 Where: Rc = Critical crash rate for a given road type. Ra = Average crash rate for a given road type, provided by the user or obtained from Table 51 or Table 52. K = Constant associated with the confidence level (1.645 for 95 percent confidence). M = Exposure (100 MVM).

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 119 Table 51. Average crash rate per 100 MVM for Undeveloped SLSG (i.e., context = rural with roadway types of principal arterial, minor arterial, collector, or local). AADT Category—Min AADT Category—Max Two-Lane Roads Multilane Divided Multilane Undivided 0 1,249 206.56 102.55 153.35 1,250 2,499 166.00 102.55 153.35 2,500 3,749 147.23 102.55 153.35 3,750 4,999 133.96 102.55 153.35 5,000 6,249 128.57 76.77 145.63 6,250 7,499 121.91 76.77 145.63 7,500 8,749 125.70 76.77 145.63 8,750 9,999 123.35 76.77 145.63 10,000 14,999 98.16 73.90 124.54 15,000 19,999 98.16 70.83 124.54 20,000 24,999 98.16 70.59 124.54 25,000 {no limit} 98.16 65.56 124.54 Note: Crash rates and injury rates were calculated using the latest 3 years of data that were available: California (2009–11), Minnesota (2010–12), North Carolina (2011–13), Ohio (2010–12), and Washington (2010–12). Source: Adapted from the 2017 version of the User Guide for USLIMITS2 (15), Table 1. Transportation Research Board. 2006. Expert System for Recommending Speed Limits in Speed Zones: NCHRP 03-67. Adapted and Reproduced with permission from the National Academy of Sciences, Washington, DC. When crash data are available, the program compares crash rate—both total and F&I— for the section to critical crash rate and average crash rate and uses the worst-case scenario.  High: section observed crash_rate > critical crash rate.  Medium: section observed crash_rate > 1.3 average crash rate.  Low: neither of the above is true. USLIMITS2 (11) permits the use of “low,” even if the crash level is medium or high, when the user indicates that “traffic control and/or geometric treatments reduce crash/F&I rate in this section.” The research team suggests retaining this approach.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 120 Table 52. Average F&I crash rate per 100 MVM for Undeveloped SLSG (i.e., context = rural with roadway types of principal arterial, minor arterial, collector, or local). AADT Category—Min AADT Category—Max Two-Lane Roads Multilane Divided Multilane Undivided 0 1,249 65.21 28.93 50.00 1,250 2,499 54.01 28.93 50.00 2,500 3,749 47.73 28.93 50.00 3,750 4,999 43.89 28.93 50.00 5,000 6,249 43.29 22.14 42.08 6,250 7,499 41.46 22.14 42.08 7,500 8,749 44.14 22.14 42.08 8,750 9,999 43.46 22.14 42.08 10,000 14,999 35.60 20.77 41.14 15,000 19,999 35.60 20.79 41.14 20,000 24,999 35.60 23.11 41.14 25,000 {no limit} 35.60 21.28 41.14 Note: Crash rates and injury rates were calculated using the latest 3 years of data that were available: California (2009–11), Minnesota (2010–12), North Carolina (2011–13), Ohio (2010–12), and Washington (2010–12). Source: Adapted from the 2017 version of the User Guide for USLIMITS2 (15), Table 1. Transportation Research Board. 2006. Expert System for Recommending Speed Limits in Speed Zones: NCHRP 03-67. Adapted and Reproduced with permission from the National Academy of Sciences, Washington, DC. Variables Considered but Not Included in SLS-Tool The research team considered the following variables but did not select them for the SLS-Tool:  Roadside hazard rating. USLIMITS2 (11) calls for the analyst to enter roadside hazard rating (RHR) as shown in Table 53. RHR is ranked on a 7-point categorical scale from 1 (best) to 7 (worst). Since these ratings are subjective in nature and vary based on the analyst’s perception, the research team excluded RHR in developing the decision rules.  Illumination. The HSM (16) has developed a CMF for roadway lighting of undeveloped roadways (see Table 11-19 in the HSM). However, the effect of lighting on undeveloped roadway safety is not consistent.  Horizontal alignment. The tool should continue to provide a warning when the user indicates that an adverse alignment is present, but this variable should not form the basis for adjusting the posted speed limit.  Median width. The HSM has included several CMFs for median width of undeveloped roadways. For example, a 30-ft median for an undeveloped multilane divided roadway has a CMF of 1. The CMF is reduced to 0.97 if the median width increases by 20 ft, but this value has a minimal effect on safety improvement. Instead, the research team used divided/undivided criteria for the decision rule for highways with four lanes.  Clear zone width. The safety effect of this variable is minor compared to that of shoulder width.  Horizontal alignment on two-lane highways (horizontal curves). Gates et al. (223) conducted a study on Michigan federal-aid two-lane highways that found horizontal

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 121 curvature to significantly affect crash occurrence. These data were reanalyzed for purposes of guideline development here. The findings are: o For county federal-aid rural two-lane 55-mph roadways, total and F&I segment crash frequency (non-deer) increased when the frequency of horizontal curves with design speeds below 60 mph exceeded 0.5 per mile (one curve every 2 mi). o For state-owned rural two-lane 55-mph roadways, total and F&I segment crash frequency (non-deer) increased when the frequency of horizontal curves with design speeds below 55 mph exceeded 0.5 per mile (one curve every 2 mi). Since undeveloped roadways have a variety of design speeds, it would be necessary to generalize the research findings when developing guidance for such roadways. Therefore, the decision was to not include this criterion as a factor in identifying a suggested speed limit. Rather, the tool includes a warning message when the user indicates that adverse alignment is present. Input Variables The user is asked to input the following variables:  Maximum speed limit (mph).  85th percentile speed (mph).  50th percentile speed (mph).  Section length (mi).  Two-way vehicle volume, AADT (veh/d).  Number of lanes (two-way total).  Median type (undivided or divided).  Number of access points (non-residential driveways and unsignalized intersections).  Lane width (ft).  Shoulder width (ft).  Is there adverse alignment (yes/no)?  Are crash data available (yes/no)?  If crash data are available, provide: o Number of years of crash data. o Average AADT for crash data period (veh/d). o Number of all (KABCO) crashes for crash data period. o Number of fatal and injury (KABC) crashes for crash data period. o Average KABCO crash rate (per 100 million VMT) for similar sections during the same time period. If not provided, average KABCO rate from HSIS is used. o Average KABC crash rate (per 100 million VMT) for similar sections during the same time period. If not provided, average KABC rate from HSIS is used.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 122 Table 53. RHR for undeveloped roadways. RHR Description 1  Wide clear zones free from obstacles greater than or equal to 9 m (30 ft) from the pavement edgeline.  Sideslope flatter than 1:4.  Recoverable in a run-off-road situation. 2  Clear zone free from obstacles between 6 and 7.5 m (20 and 25 ft) from pavement edgeline.  Sideslope about 1:4.  Recoverable in a run-off-road situation. 3  Clear zone free from obstacles about 3 m (10 ft) from pavement edgeline.  Sideslope about 1:3 or 1:4.  Rough roadside surface.  Marginally recoverable in a run-off-road situation. 4  Clear zone free from obstacles between 1.5 and 3 m (5 to 10 ft) from pavement edgeline.  Sideslope about 1:3 or 1:4.  May have guardrail (1.5 to 2 m [5 to 6.5 ft] from pavement edgeline).  May have exposed trees, poles, or other objects (about 3 m or 10 ft from pavement edgeline).  Marginally forgiving in a run-off-road situation, but increased chance of a reportable roadside collision. 5  Clear zone free from obstacles between 1.5 and 3 m (5 to 10 ft) from pavement edgeline.  Sideslope about 1:3.  May have guardrail (0 to 1.5 m [0 to 5 ft] from pavement edgeline).  May have rigid obstacles or embankment within 2 to 3 m (6.5 to 10 ft) of pavement edgeline.  Virtually non-recoverable in a run-off-road situation. 6  Clear zone free from obstacles less than or equal to 1.5 m (5 ft).  Sideslope about 1:2.  No guardrail.  Exposed rigid obstacles within 0 to 2 m (0 to 6.5 ft) of the pavement edgeline.  Non-recoverable in a run-off-road situation. 7  Clear zone free from obstacles less than or equal to 1.5 m (5 ft).  Sideslope 1:2 or steeper.  Cliff or vertical rock cut.  No guardrail.  Non-recoverable in a run-off-road situation with a high likelihood of severe injuries from roadside collision. Source: Adapted from the 2017 version of the User Guide for USLIMITS2 (15), pages 23–24. Transportation Research Board. 2006. Expert System for Recommending Speed Limits in Speed Zones: NCHRP 03-67. Adapted and Reproduced with permission from the National Academy of Sciences, Washington, DC. Warning Messages USLIMITS2 (11) checks for several conditions and issues warnings as needed. The NCHRP Project 17-76 SLS-Tool also issues warnings. The guide discusses the warning messages included in the SLS-Tool. The suggested minimum section length used in the warning message is from USLIMITS2, which notes that the minimum length values are the same as used in the USLIMITS1.0 and Australian XLIMITS expert systems (see Table 49).

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 123 SLSG = DEVELOPED Overview This section provides an overview of the decision rules selected for the SLSG of Developed. The research team considered the following sources in creating the decision rules for developed roadways:  Rules used in USLIMITS2 (11).  Information included in the updated User Guide for USLIMITS2 (15).  Findings from the literature.  Findings from the analyses conducted using data from Austin, Texas (see Appendix D) and Washtenaw County, Michigan (see Appendix E).  Research team expert opinions.  Feedback from experts, including the project panel. Decision Rule Development After review of several potential options, the research team decided to use the rules of USLIMITS2 as a basis, with further refinement to those rules made based on findings in the literature or findings from the safety analyses conducted as part of NCHRP Project 17-76 (see Appendix D on Austin, Texas, and Appendix E on Washtenaw, Michigan). For roadways within the Developed SLSG, measured operating speeds are used to identify the suggested posted speed limit. In general, the relationship of a variable with roadway safety is used to determine whether the suggested posted speed limit should reflect:  The 85th percentile speed rounded to the closest 5 mph increment (C85).  The 85th percentile speed rounded down to the nearest 5 mph increment (RD85).  The 50th percentile speed rounded to the closest 5 mph increment (C50). When the roadway conditions are optimal, the suggested speed limit should reflect the 85th percentile speed. When roadway conditions are not favorable to all users or when crashes are a significant concern, then the suggested speed limit should reflect the 50th percentile speed. An RD85 speed limit is suggested when conditions are between those extremes. In rare cases, the RD85 will be less than the C50 due to rounding. As an example, if the 50th percentile speed were 33 mph and the 85th percentile speed were 34 mph, then the C50 would equal 35 mph and the RD85 would equal 30 mph. This situation only occurs when the 85th and 50th percentile speeds are within 1 mph of each other. Variables Included in SLS-Tool Table 54 provides an overview of the variables along with the variable value that would trigger using C85, RD85, or C50. Table 55 shows the combinations of pedestrian treatments and pedestrian activity that would trigger the different speed percentage levels. The following sections provide justification for selecting those variables and the values provided for each.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 124 Table 54. Overview of decision rules for SLSG = Developed. Variable Closest 50th (C50) Rounded-Down 85th (RD85) Closest 85th (C85) Signal density > 4 signals/mile > 3 signals/mile ≤ 3 signals/mile Access density > 60 driveways/ unsignalized intersections per mile > 40 and <= 60 driveways/ unsignalized intersections per mile <= 40 driveways/ unsignalized intersections per mile Number of lanes/median type (undivided, TWLTL, divided) {Not applicable, see criteria in other cells} Four or more lanes with undivided median  Four or more lanes with divided or TWLTL median  Fewer than four lanes with any median type Bicyclist activity—in motor vehicle lane, shoulder, or non-separated bike lane High {Not applicable, see criteria in other cells} Not high Bicyclist activity—in separated bike lane {Not applicable, see criteria in other cells} High Not high Sidewalk presence/width (none, narrow, adequate, wide), sidewalk buffer (present, not present), and pedestrian activity (high, some, negligible) See Table 55 See Table 55 See Table 55 On-street parking activity High {Not applicable, see criteria in other cells} Not high On-street parking type Angle parking present for 40 percent or more of section  Parallel parking permitted  Angle parking present for less than 40 percent of section None All (KABCO) crash rate  High: crash_rate > critical crash rate  Medium: crash_rate > 1.3 average crash rate  Low: neither of the above is true High Medium Low F&I (KABC) crash rate  High: F&I_rate > critical crash rate  Medium: F&I_rate > 1.3 average crash rate  Low: neither of the above is true High Medium Low

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 125 Table 55. Decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Developed SLSG. Pedestrian Activity Sidewalk Presence/Width Sidewalk Buffer Speed% High Adequate Not present RD85 High Adequate Present C85 High Narrow Not present C50 High Narrow Present RD85 High None Not applicable C50 High Wide Not present C85 High Wide Present C85 Some Adequate Not present C85 Some Adequate Present C85 Some Narrow Not present C50 Some Narrow Present C85 Some None Not applicable C50 Some Wide Not present C85 Some Wide Present C85 Negligible Adequate Not present C85 Negligible Adequate Present C85 Negligible Narrow Not present C85 Negligible Narrow Present C85 Negligible None Not applicable RD85 Negligible Wide Not present C85 Negligible Wide Present C85 Note: See text for additional discussion on sidewalk presence/width and sidewalk buffer characteristics. Signal Density USLIMITS2 includes the variable “signals per mile” with the break points shown in Table 46. The findings from the analyses conducted using City of Austin, Texas, data (see Appendix D) and Washtenaw County, Michigan, data (see Appendix E) support the inclusion of this variable. Those analyses also appear to support the break points of 3 and 4 signals per mile; however, additional research may be needed to refine those values. The segments used in the 17-76 analyses were created when a signal was present; in other words, the segments began and ended with a signalized intersection or with a control that would have resulted in a change of speed (e.g., four-way stop or end of road). The user provides the number of signals within the section, and the program calculates the signals density (signals/section length). Access Density USLIMITS2 includes the variable “driveways per mile.” The research team renamed the variable “access density” to avoid the question of whether the driveways per mile variable should include unsignalized intersections (which it should). The findings from NCHRP Project 17-76 support the break points used in USLIMITS2. All types of non-single-family driveways (e.g., multifamily residential, commercial, etc.) along with unsignalized intersections should be counted. The user is asked to provide the number of driveways and unsignalized intersections within the section, and the tool calculates the access density.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 126 Number of Lanes/Median Type (Undivided, TWLTL, Divided) Combinations The safety analyses conducted as part of NCHRP Project 17-76 (see Appendix D on Austin, Texas, and Appendix E on Washtenaw, Michigan) found fewer crashes for a raised (divided) median compared to a TWLTL or no median. A review of the literature found studies that documented reduction in crashes when a TWLTL was added to a four-lane undivided roadway (70, 71). The research team recommends that the presence of a divided (raised or depressed) median or a TWLTL on a road with four lanes or more be considered the baseline condition, and for undivided four-lane roads, be associated with suggested posted speed limits that reflect the rounding down of the 85th percentile speed. Because the type of median may vary within a section, the user is asked for the type of median treatment that is predominant within the section. Pedestrian Activity In USLIMITS2, the user is asked to select between “high” and “not high” for pedestrian activity. USLIMITS2 provides the following examples for high pedestrian activity:  Residential developments with four or more housing units per acre interspersed with multifamily dwellings.  Hotels located with ½ mi of other attractions such as retail stores, recreation areas, or senior centers.  Downtown or CBD areas.  Areas that usually have paved sidewalks, marked crosswalks, and pedestrian signals. The research team recommends that an appreciation for the pedestrian activity level be retained within the SLS-Tool. The examples for high pedestrian activity used in USLIMITS2 should be refined to reflect the use of the Expanded Functional Classification System in that downtown or CBD areas now fit within the urban core context. The suggested examples for when pedestrian activity is high are:  Residential developments with four or more housing units per acre interspersed with multifamily dwellings.  Hotels located with ½ mi of other attractions such as retail stores, recreation areas, or senior centers.  Areas with paved sidewalks, marked crosswalks, and pedestrian signals.  Areas with multiple transit stops within the section. The level of pedestrian activity ties into the decision rule that considers pedestrian facilities such as sidewalk presence/width and sidewalk buffer. Bicyclist Activity In USLIMITS2, the user is asked to select between “high” and “not high” for pedestrian and bicycle activity. USLIMITS2 provides the following examples for high activity:  Residential developments with four or more housing units per acre interspersed with multifamily dwellings.  Hotels located with ½ mi of other attractions such as retail stores, recreation areas, or senior centers.  Downtown or CBD areas.  Areas that usually have paved sidewalks, marked crosswalks, and pedestrian signals.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 127 The research team recommends that an appreciation for the bicycle activity level be retained within the SLS-Tool but separated from pedestrian activity. The examples for high bicyclist activity used in USLIMITS2 should be refined to reflect the use of the Expanded Functional Classification System in that downtown or CBD areas now fit within the urban core context. The suggested examples for when bicyclist activity is high are:  Residential developments with four or more housing units per acre interspersed with multifamily dwellings.  Areas with bicycle treatments including marked bike lanes, bike boxes, etc.  Areas with multiple transit stops within the section. Based on feedback from the panel, the research team subdivided bicyclist activity for when a separated bike lane is present. When bicycle activity is high and bicyclists are with, or not separated from, motor vehicles (i.e., the bicyclists are in the travel lane or shoulder, or the bike lane does not have vertical separations), the suggested speed limit is lower than when the bicyclists are within a separated bike lane. Sidewalk Presence/Width, Sidewalk Buffer (Separation Distance between Pedestrian and Vehicles), and Pedestrian Activity Combination When there is a reasonable expectation of pedestrians on or very near a roadway, selection of a lower operating speed can be justified (i.e., the analyst selects high or some as the level of pedestrian activity) depending upon the quality of the pedestrian facility present. The following conditions were assigned to rounding the operating speed to the 5-mph increment that is nearest the 50th percentile: narrow sidewalk that has no buffer between the sidewalk and the road or when there is a reasonable expectation of a pedestrian and no sidewalk is present. These conditions place the pedestrian very close to the moving vehicles. When wide sidewalks are present, pedestrians can shift away from the moving traffic; thus, that condition was assigned to the closest 85th percentile operating speed. Those cases when a sidewalk is not present and there is minimal expectation of a pedestrian were assigned to the rounding down of the 85th percentile speed. When there could be an expectation of some pedestrians but not a high number of pedestrians and a sidewalk facility exists but may be considered poor, the recommendation is C85. When there is a high level of pedestrians present and the sidewalk facility is poor, then the suggest speed limit reflects the 5-mph increment associated with RD85. The FHWA Course on Bicycle and Pedestrian Transportation (229, page 13-1) states that “sidewalks require a minimum width of 5.0 feet if set back from the curb or 6.0 feet if at the curb face. Any width less than this does not meet the minimum requirements for people with disabilities.” Because the sidewalk characteristics may vary within a section, the user is asked for the sidewalk characteristics that are predominant within the section. The user is asked:  What is the predominate width of the sidewalk within the section? o No sidewalk (none) is present on either side of the street. o A narrow sidewalk is present (sidewalk that is < 5 ft if set back from curb or 6 ft if at the curb face). o An adequate sidewalk is present (sidewalk is between 5 ft and 8 ft if set back from curb or between 6 ft and 8 ft if at curb face). o A wide sidewalk is present (sidewalk is 8 ft or greater).

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 128  Is a sidewalk separation (or buffer) present or not present between road (face of curb when curb and gutter are present or edge of travel lane when a shoulder is present) and sidewalk? A buffer could include a nature strip, a bike lane, or on-street parking.  What is the level of pedestrian activity (negligible, some, high)? See previous section discussion for advice regarding these levels. On-Street Parking Activity and On-Street Parking Type In USLIMITS2, the user is asked to select between “high” and “not high” for parking activity. USLIMITS2 notes that high parking activity typically occurs in high-turnover areas often found in downtown and/or CBD areas. In addition, these areas usually have parking on both sides of the road with parking time limits that do not exceed 60 minutes, with at least 30 percent of parking spaces occupied during weekdays. USLIMITS2 says the high level of on-street parking activity should result in using C50. The findings from NCHRP Project 17-76 (see Appendix D and Appendix E) along with other studies—see Greibe (20) and Islam and El-Basyouny (36)—support this variable and consideration of a variable that reflects the characteristics of the on-street parking. For example, the findings from NCHRP Project 17-76 indicate that the parking width available has a relationship to number of crashes, with more narrow widths associated with more crashes. A study by Bonneson et al. (230) synthesized several prior studies to formulate a curb parking CMF. The curb parking CMF was dependent on type of parking (angle versus parallel), type of adjacent land use, type of cross-section, and proportion of segment length with parking. Based on a review of those findings, the research team suggests the following with regards to angle parking:  If angle parking is present for 40 percent or more of the section, use the closest 50th percentile speed. Angle parking results in a much larger increase in crash frequency than parallel parking.  If angle parking is present for less than 40 percent of the section, use the rounded- down 85th percentile speed. Because the on-street parking characteristics may vary within a section, the user is asked for the on-street parking characteristics that are predominant within the section. The user is asked:  Is on-street parking activity “high” or “not high”? A high level of on-street parking can be characterized as having parking on both sides of the road with parking time limits.  Is angle parking present for at least 40 percent of the section?  Is parallel parking (marked or unmarked) permitted in the section? Crash Level USLIMITS2 can produce a suggested posted speed limit when crash data are and are not available. USLIMITS2 requests the following to be able to conduct an analysis of the crash data:  Length of the study period in years and months (at least 3 years of crash data is recommended; if less than 1 year of data are input, the program suggests that additional data should be collected and the process repeated).  Total number of crashes in the section.  Total number of injury and fatal crashes in the section.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 129  Average AADT for the study period.  Average rate of total crashes and average rate of injury and fatal crashes (per 100 MVM) for similar road sections in a jurisdiction. To determine the average crash/injury rate for similar sections, users should select a group of sections that have the same or similar geometry (i.e., number of lanes, median type, etc.) and similar traffic volumes and area type. The length of study, number of crashes, and average AADT are used to calculate the section crash rate for total crashes and for injury and fatal crashes per 100 MVM. If the user does not provide average rates, default values from the HSIS are used (see Table 56 and Table 57). The critical crash rate is calculated from: 𝑅 𝑅 𝐾 𝑅𝑀 1 2𝑀 Where: Rc = Critical crash rate for a given road type. Ra = Average crash rate for a given road type, provided by the user or obtained from Table 56 and Table 57. K = Constant associated with the confidence level (1.645 for 95 percent confidence). M = Exposure (100 MVM). When crash data are available, the program compares crash rate—both all and fatal/injury—for the section to critical crash rate and average crash rate and uses the worst-case scenario.  High: section crash_rate > critical crash rate.  Med: section crash_rate > 1.3 average crash rate.  Low: neither of the above is true. USLIMITS permits the use of “low,” even if crash level is medium or high, when the user indicates that “traffic control and/or geometric treatments reduce crash/injury rate in this section.” No changes are suggested to this approach.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 130 Table 56. Average KABCO crash rate per 100 MVM from USLIMITS2 for Developed SLSG (i.e., context = urban, suburban, or rural town with roadway types of principal arterial, minor arterial, or collector). AADT Category— Min AADT Category— Max Two-Lane Roads Multilane Divided Multilane Undivided One-Way Streets 0 2,499 263.17 226.43 452.14 245.12 2,500 4,999 209.14 226.43 452.14 245.12 5,000 7,499 205.37 226.43 452.14 139.27 7,500 9,999 229.55 226.43 452.14 139.27 10,000 14,999 246.62 202.46 452.26 72.18 15,000 19,999 253.25 202.46 452.26 58.31 20,000 24,999 225.17 228.69 431.09 57.36 25,000 29,999 225.17 228.69 431.09 63.87 30,000 34,999 225.17 228.37 431.25 54.63 35,000 39,999 225.17 228.37 431.25 54.63 40,000 49,999 225.17 205.73 431.25 54.63 50,000 No limit 225.17 158.17 431.25 54.63 Note: Crash rates and injury rates were calculated using the latest 3 years of data that were available: California (2009–11), Minnesota (2010–12), North Carolina (2011–13), Ohio (2010–12), and Washington (2010–12). Source: Adapted from the 2017 version of the User Guide for USLIMITS2 (15), Table 1. Transportation Research Board. 2006. Expert System for Recommending Speed Limits in Speed Zones: NCHRP 03-67. Adapted and Reproduced with permission from the National Academy of Sciences, Washington, DC. Table 57. Average KABC crash rate per 100 MVM from USLIMITS2 for Developed SLSG (i.e., context = urban, suburban, or rural town with roadway types of principal arterial, minor arterial, or collector). AADT Category— Min AADT Category— Max Two-Lane Roads Multilane Divided Multilane Undivided One-Way Streets 0 2,499 67.32 72.02 131.02 60.21 2,500 4,999 64.31 72.02 131.02 60.21 5,000 7,499 63.75 72.02 131.02 37.29 7,500 9,999 70.26 72.02 131.02 37.29 10,000 14,999 73.14 66.16 131.98 22.79 15,000 19,999 78.14 66.16 131.98 18.19 20,000 24,999 71.82 75.37 129.00 17.72 25,000 29,999 71.82 75.37 129.00 20.07 30,000 34,999 71.82 74.01 131.10 15.03 35,000 39,999 71.82 74.01 131.10 15.03 40,000 49,999 71.82 70.84 131.10 15.03 50,000 No limit 71.82 56.32 131.10 15.03 Note: Crash rates and injury rates were calculated using the latest 3 years of data that were available: California (2009–11), Minnesota (2010–12), North Carolina (2011–13), Ohio (2010–12), and Washington (2010–12). Source: Adapted from the 2017 version of the User Guide for USLIMITS2 (15), Table 1. Transportation Research Board. 2006. Expert System for Recommending Speed Limits in Speed Zones: NCHRP 03-67. Adapted and Reproduced with permission from the National Academy of Sciences, Washington, DC.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 131 Variables Considered but Not Included in SLS-Tool The research team considered the following variables but did not select them for the SLS-Tool:  Presence of a midblock crosswalk. Whether the presence of a single midblock crossing warrants a speed reduction would need to consider whether additional supplemental treatments are present, such as a rectangular rapid flashing beacon or pedestrian hybrid beacon. Research studies on the safety relationship of these devices have indicated reduced crashes for both pedestrians and vehicles (231, 232).  Presence of school or school crossings. The SLS-Tool developed in NCHRP Project 17-76 does not apply to school zones.  Horizontal alignment. The tool should continue to provide a warning when the user indicates that an adverse alignment is present.  Bus stops. Pedestrian activity level associated with bus stops should be considered with the pedestrian activity variable.  Number of lanes or roadway surface width.  Roadside. Consideration of sidewalks is included.  Area type (residential collector, residential subdivision, commercial, large complexes). USLIMITS2 asks for the area type (residential collector, residential subdivision, commercial, or large complexes). The information is one of the variables used to determine if the suggested speed limit should be RD85 or C85. Because of the new Extended Functional Class System, area type is now handled within the SLSG. Input Variables The user is asked to provide the following:  Maximum speed limit (mph).  85th percentile speed (mph).  50th percentile speed (mph).  Section length (mi).  Number of lanes.  Predominant median type for the section (undivided, TWLTL, divided).  Number of traffic signals in the section.  Number of access points (i.e., driveways and unsignalized intersections) in the section.  Bicyclist activity (high or not high).  What is the predominate width of the sidewalk within the section? o None, no sidewalk is present on either side of the street. o A narrow sidewalk is present (sidewalk that is < 5 ft if set back from curb or 6 ft if at the curb face). o An adequate sidewalk is present (sidewalk is between 5 ft and 8 ft if set back from curb or between 6 ft and 8 ft if at curb face). o A wide sidewalk is present (sidewalk is 8 ft or greater).  Is a sidewalk separation (or buffer) present between road (face of curb when curb and gutter are present or edge of travel lane when a shoulder is present) and sidewalk (present or not present)? A buffer could include a nature strip, a bike lane, or on-street parking.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 132  Pedestrian activity (high, some, or negligible).  Is on-street parking activity “high” or “not high”? A high level of on-street parking can be characterized as having parking on both sides of the road with parking time limits.  Is parallel parking (marked or unmarked) permitted in the section?  Is angle parking present for at least 40 percent of the section?  Is there adverse alignment (yes/no)?  Are crash data available (yes/no)?  If crash data are available, provide: o Number of years of crash data. o Average AADT for crash data period (veh/d). o Number of all (KABCO) crashes for crash data period. o Number of fatal and injury (KABC) crashes for crash data period. o Is the segment a one-way street? o Average KABCO crash rate (per 100 million VMT) for similar sections during the same time period. If not provided, average KABCO rate from HSIS is used. o Average KABC crash rate (per 100 million VMT) for similar sections during the same time period. If not provided, average KABC rate from HSIS is used. Warning Messages USLIMITS2 checks for several conditions and issues warnings as needed. The NCHRP Project 17-76 SLS-Tool also issues warnings. The guide discusses the warning messages included in the SLS-Tool. The suggested minimum section length used in the warning message is from USLIMITS2, which notes that the minimum length values are the same as used in the USLIMITS1.0 and Australian XLIMITS expert systems (see Table 49). SLSG = FULL ACCESS Overview This section provides an overview of the decision rules selected for the SLSG of Full Access. The research team considered the following sources in creating the decision rules:  Rules used in USLIMITS2 (11).  Information included in the updated User Guide for USLIMITS2 (15).  Findings from the literature.  Findings from the analyses conducted using data from Austin, Texas (see Appendix D) and Washtenaw County, Michigan (see Appendix E).  Research team expert opinions.  Feedback from experts, including the project panel. Decision Rule Development After review of several potential options, the research team decided to use the rules for the SLSG of Developed as a basis, which were based on findings in the literature or findings from the safety analyses conducted as part of NCHRP Project 17-76 (see Appendix D on Austin, Texas, and Appendix E on Washtenaw, Michigan). Further refinements to those rules were made based on the research team and project panel’s expert opinions.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 133 For roadways within the Full-Access SLSG, measured operating speeds are used to identify the suggested posted speed limit within the following two methods:  The 50th percentile speed rounded to the closest 5-mph increment (C50).  The 50th percentile speed rounded down to the nearest 5-mph increment (RD50). Variables Included in SLS-Tool Table 58 provides an overview of the variables along with the variable value that would trigger using C50 or RD50. The following is a justification for selecting those variables and the values provided for each. Table 58. Overview of decision rules for SLSG = Full Access. Variable Rounded-Down 50th (RD50) Closest 50th (C50) Signal density > 8 signals/mile ≤ 8 signals/mile Access density > 60 driveways/unsignalized intersections per mile ≤ 60 driveways/unsignalized intersections per mile Bicyclist activity—in motor vehicle lane, shoulder, or non- separated bike lane High Not high Bicyclist activity—in separated bike lane High Not high Sidewalk presence/width (none, narrow, adequate, wide), sidewalk buffer (present, not present), and expectation of pedestrian activity (high, some, negligible) See Table 59 See Table 59 On-street parking activity High Not high On-street parking type Angle parking present for 40 percent or more of section No parking present Angle parking present for less than 40 percent of section All (KABCO) crash rate  High: crash_rate > critical crash rate  Medium: crash_rate > 1.3 average crash rate  Low: neither of the above is true High or Med Low F&I (KABC) crash rate  High: F&I_rate > critical crash rate  Medium: F&I_rate > 1.3 average crash rate  Low: neither of the above is true High or Med Low

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 134 Signal Density USLIMITS2 includes the variable “signals per mile,” and the variable was included in the Developed SLSG with break points at 3 and 4 signals per mile. These break points were supported by the findings from the analyses conducted using City of Austin, Texas, data (see Appendix D) and Washtenaw County, Michigan, data (see Appendix E). A revised break point was needed for use in the Full-Access SLSG, and the value of 8 signals/mile was selected based on feedback from the panel. The user provides the number of signals within the section, and the program calculates the signal density. Table 59. Decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Full-Access SLSG. Pedestrian Activity Sidewalk Presence/Width Sidewalk Buffer Speed% High Adequate Not present RD50 High Adequate Present C50 High Narrow Not present RD50 High Narrow Present RD50 High None Not applicable RD50 High Wide Not present C50 High Wide Present C50 Some Adequate Not present C50 Some Adequate Present C50 Some Narrow Not present RD50 Some Narrow Present C50 Some None Not applicable RD50 Some Wide Not present C50 Some Wide Present C50 Negligible Adequate Not present C50 Negligible Adequate Present C50 Negligible Narrow Not present C50 Negligible Narrow Present C50 Negligible None Not applicable C50 Negligible Wide Not present C50 Negligible Wide Present C50 Note: See text for additional discussion on sidewalk presence/width and sidewalk buffer characteristics. Access Density USLIMITS2 includes the variable “driveways per mile.” The research team renamed the variable “access density” to avoid the question of whether the driveways per mile variable should include unsignalized intersections (which it should). The findings from NCHRP Project 17-76 support the break points used in USLIMITS2. All types of non-single-family home driveways (e.g., multifamily residential, commercial, etc.) along with unsignalized intersections should be counted. The user is asked to provide the number of non-single-family residential driveways and unsignalized intersections within the section, and the tool calculates the access density.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 135 Bicyclist Activity Like the Developed SLSG, the suggested posted speed for a corridor within the Full- Access SLSG is a function of the level of bicyclist activity and the presence of a separated bike lane. Sidewalk Presence/Width, Sidewalk Buffer (Separation Distance between Pedestrian and Vehicles), and Pedestrian Activity Combination Like the Developed SLSG, the suggested posted speed is a function of the quality of pedestrian facility. When there is a reasonable expectation of a high number of pedestrians on or very near a roadway, selection of a lower operating speed can be justified. See additional discussion on these variables in the Developed SLSG section. On-Street Parking Activity and On-Street Parking Presence Parking activity and on-street parking presence follow a similar approach as that suggested for the Developed SLSG. Because the on-street parking characteristics may vary within a section, the user is asked for the on-street parking characteristics that are predominant within the section. The user is asked:  Is on-street parking activity “high” or “not high”? A high level of on-street parking can be characterized as having parking on both sides of the road with parking time limits.  Is angle parking present for at least 40 percent of the section? Crash Level The suggested approach for crash level used with the Developed SLSG is suggested for the Full-Access SLSG, including using the default values from the HSIS for the Developed condition (see Table 56 and Table 57). For the Full-Access SLSG, a crash level of high or medium would trigger the use of rounding down from the 50th percentile; otherwise, using the closest 5-mph increment to the 50th percentile speed is suggested. See additional discussion in the Developed SLSG section. Variables Considered but Not Included in SLS-Tool The research team considered several other variables as part of either the Developed or Full-Access SLSG but did not select them for the SLS-Tool. See the discussion in the Developed SLSG section for additional details. Number of lanes and median type combination was included in the Developed SLSG but not in the Full-Access SLSG due to limited findings. Input Variables The user is asked to provide the following:  Maximum speed limit (mph).  50th percentile speed.  Section length (mi).  Number of traffic signals in the section.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 136  Number of access points (i.e., driveways and unsignalized intersections) in the section.  Bicyclist activity (high or not high).  What is the predominate width of the sidewalk within the section? o None, no sidewalk is present on either side of the street. o A narrow sidewalk is present (sidewalk that is < 5 ft if set back from curb or 6 ft if at the curb face). o An adequate sidewalk is present (sidewalk is between 5 ft and 8 ft if set back from curb or between 6 ft and 8 ft if at curb face). o A wide sidewalk is present (sidewalk is 8 ft or greater).  Is a sidewalk separation (or buffer) present between road (face of curb when curb and gutter are present or edge of travel lane when a shoulder is present) and sidewalk (present or not present)? A buffer could include a nature strip, a bike lane, or on-street parking.  Pedestrian activity (high, some, or negligible).  Is on-street parking activity “high” or “not high”? A high level of on-street parking can be characterized as having parking on both sides of the road with parking time limits.  Is parallel parking (marked or unmarked) permitted in the section?  Is angle parking present for at least 40 percent of the section?  Is there adverse alignment (yes/no)?  Are crash data available (yes/no)?  If crash data are available, provide: o Number of years of crash data. o Average AADT for crash data period (veh/d). o Number of all (KABCO) crashes for crash data period. o Number of fatal and injury (KABC) crashes for crash data period. o Average KABCO crash rate (per 100 million VMT) for similar sections during the same time period. If not provided, average KABCO rate from HSIS is used. o Average KABC crash rate (per 100 million VMT) for similar sections during the same time period. If not provided, average KABC rate from HSIS is used. Warning Messages USLIMITS2 checks for several conditions and issues warnings as needed. The NCHRP Project 17-76 SLS-Tool also issues warnings. The guide discusses the warning messages included in the SLS-Tool. The suggested minimum section length used in the warning message is from USLIMITS2, which notes that the minimum length values are the same as used in the USLIMITS1.0 and Australian XLIMITS expert systems (see Table 49).

Next: APPENDIX G. WORKSHOP SLIDES »
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Several types of speed limits exist, including statutory speed limit, posted speed limit, school zone speed limit, work zone speed limit, variable speed limit, and advisory speed.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool documents the research efforts and findings from an NCHRP Project 17-76 to identify factors that influence a driver’s operating speed and the development of a Speed Limit Setting Procedure and Tool.

The document is supplemental to NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide.

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