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Posted Speed Limit Setting Procedure and Tool: User Guide (2021)

Chapter: Section 5 - Variables for Decision-Making Procedure

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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Section 5 - Variables for Decision-Making Procedure." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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21   Roadway Context NCHRP Research Report 855 (33) provides the following two questions for determining a roadway segment’s context category: • For the most part, does it meet the category’s primary factors? • Does the landscape adjacent to the roadway look similar to the photographs/graphic examples in Figure 7? Roadway Type The Expanded FCS roadway types follow basic transportation system functions and are defined based on their network function and connectivity. NCHRP Research Report 855 (33) provides the following key characteristics for each roadway type: 1. Interstates/Freeways/Expressways: corridors of national importance providing long-distance travel. – Limited-Access. – Through traffic movements. – Primary freight routes. – Possible transit network support. – No pedestrian or bicycle traffic. – Guided by Federal Highway Administration (FHWA) design standards. 2. Principal Arterials: corridors of regional importance connecting large centers of activity. – Through-traffic movements. – Long-distance traffic movements. – Long-haul public transit buses. – Primary freight routes. 3. Minor Arterials: corridors of local importance connecting centers of activity. – Connections between local areas and network principal arterials. – Connections for through traffic between arterial roads. – Access to public transit and through movements. – Pedestrian and bicycle movements. 4. Collectors: roadways providing connections between arterials and local roads. – Traffic with trips ending in a specific area. – Access to commercial and residential centers. – Access to public transportation. – Pedestrian and bicycle movements. 5. Local: all other roads. – Direct property access—residential and commercial. – Pedestrian and bicycle movements. S E C T I O N 5 Variables for Decision-Making Procedure

22 Posted Speed Limit Setting Procedure and Tool: User Guide Roadway Segment Input Variables for Speed Limit Setting Groups Several variables are needed for use in the SLS-Procedure. The needed variables vary by the SLSG. The speed data variables are provided in Table 11. The table also indicates when the variable is needed based on the SLSG, for example, the 85th percentile speed is not needed for the Full-Access SLSG. Table 12 summarizes the variables and indicates when the variable is needed based on the SLSG. Table 13 shows the variables needed when crash data are available. Illustration Description Rural Ranges from no development (natural environment) to some light development (structures), with sparse residential and other structures mostly associated with farms. The land is primarily used for outdoor recreation, agriculture, farms, and/or resource extraction. In a rural setting, there are no or very few pedestrians, bicyclists are most likely of recreational nature, and transit is limited or nonexistent. Rural Town Characterized by low density (low-rise—one or two story— structures) but a concentrated development of diverse uses— residential and commercial. Rural towns are generally incorporated but have limited government services. Rural towns usually have a roadway section that has a main street character (or even a town square) with on-street parking and sidewalks and in some cases bicycle lanes. Suburban Diverse range of commercial and residential uses that have a medium density. The buildings tend to be multistory with off-street parking. Sidewalks are usually present, and bicycle lanes may exist. The range of uses encompasses health services, light industrial (and sometimes heavy industrial) uses, quick-stop shops, gas stations, restaurants, and schools and libraries. Typically, suburban areas rely heavily on passenger vehicles, but some transit may be present. Urban High density, consisting principally of multistory and low- to medium-rise structures for residential and commercial use. Areas usually exist for light and sometimes heavy industrial use. Many structures accommodate mixed uses: commercial, residential, and parking. Streets have minimal on-street parking. Wide sidewalks and plazas accommodate more intense pedestrian traffic, while bicycle lanes and transit corridors are frequently present. Urban Core Highest level of density with its mixed residential and commercial uses accommodated in high-rise structures. While there may be some on-street parking, it is usually very limited and time restricted. Most parking is in multilevel structures attached or integrated with other structures. The area is accessible to automobiles, commercial delivery vehicles, and public transit. Sidewalks and pedestrian plazas are present along with multilevel pedestrian bridges connecting commercial and parking structures. Bicycle facilities and transit corridors are typically common. Source: Transportation Research Board. 2018. NCHRP Research Report: An Expanded Functional Classification System for Highways and Streets. HTTPS://DOI.ORG/10.17226/24775. Reproduced with permission from the National Academy of Sciences, pages 10–16 (33). Figure 7. Roadway context illustrations and descriptions. Speed Data Variable Li m ite d- A cc es s U nd ev el op ed D ev el op ed Fu ll- A cc es s 50th percentile speed (mph) ü ü ü ü 85th percentile speed (mph) ü ü ü - Maximum speed limit (mph) ü ü ü ü Note: ü = variables used in SLSG, - = variables not used in SLSG. Table 11. Input variables for speed data.

Variables for Decision-Making Procedure 23   Table 13. Input variables when crash data are available. Crash Data Variable Li m ite d- A cc es s U nd ev el op ed D ev el op ed Fu ll- A cc es s Number of years of crash data ü ü ü ü Average AADT (two-way total) for crash data period (veh/d) ü ü ü ü All (KABCO) crashes for crash data period ü ü ü ü Fatal and injury (KABC) crashes for crash data period ü ü ü ü Average KABCO crash rate (crashes/100 MVM) and average KABC crash rate (crashes/100 MVM)? If not provided, the KABCO and KABC crash rates from HSIS is used ü ü ü ü Is the segment a one-way street? - - ü ü Number of lanes (pulled from the Site Characteristics section) ü ü ü ü Median type (pulled from the Site Characteristics section) - ü ü ü Note: ü = variables used in SLSG, - = variables not used in SLSG. Roadway Segment Variable Li m ite d- A cc es s U nd ev el op ed D ev el op ed Fu ll- A cc es s AADT (two-way total), annual average daily traffic (veh/d) ü ü - - Adverse alignment present (yes or no) ü ü ü ü Angle parking present (no, yes for at least 40 percent of the segment, or yes for less than 40 percent of the segment) - - ü ü Bicyclist activity (high or not high) - - ü ü Design speed (mph), used with grade to identify mountainous terrain ü - - - Directional design-hour truck volume (trk/hr) ü - - - Grade (%), used with design speed to identify mountainous terrain ü - - - Inside (left) SW (ft) ü - - - Lane width (ft) - ü - - Median type, developed or Full-Access (undivided, TWLTL, or divided) - - ü ü Median type, undeveloped (undivided or divided) - ü - - Number of access points (total of both directions) - ü ü ü Number of interchanges ü - - - Number of lanes (two-way total) ü ü ü ü Number of traffic signals - - ü ü On-street parking activity (high or not high) - - ü ü Outside (right) SW (ft) ü - - - Parallel parking permitted (yes or no) - - ü - Pedestrian activity (high, some, or negligible) - - ü ü Segment length (mi) ü ü ü ü SW (ft) - ü - - Sidewalk buffer (present or not present) - - ü ü Sidewalk presence/width (none, narrow, adequate, or wide) - - ü ü Note: ü = variables used in SLSG, - = variables not used in SLSG. Table 12. Roadway segment input variables.

24 Posted Speed Limit Setting Procedure and Tool: User Guide Speed Data Input Variables for Speed Limit Setting Groups Speed Data Variable: 50th Percentile Speed (All SLSGs) The user provides the 50th percentile speed. Speed Data Variable: 85th Percentile Speed (All SLSGs) The user provides the 85th percentile speed. Speed Data Variable: Maximum Speed Limit (All SLSGs) The user enters the maximum speed limit for the roadway segment in mph. Roadway Segment Data Input Variables for Speed Limit Setting Groups Roadway Segment Variable: AADT (Limited-Access, Undeveloped SLSGs) The user provides the AADT (two-way total) on the Limited-Access or Undeveloped segment. Roadway Segment Variable: Adverse Alignment Presence (All SLSGs) The user answers the question “Is an adverse alignment present?” as either yes or no. If yes, the SLS-Tool provides a warning to consider location-specific advisory speed warnings. This variable does not contribute to the calculation of the suggested speed limit. Roadway Segment Variable: Angle Parking Present (Developed and Full-Access SLSGs) Because the on-street parking characteristics may vary within a segment, the user provides the on-street parking characteristics that are predominant within the segment. The user indicates if angle parking is present (no, yes for at least 40 percent of the segment, or yes for less than 40 percent of the segment). Roadway Segment Variable: Bicyclist Activity (Developed and Full-Access SLSGs) The user indicates if the bicyclist activity is high or not high and whether there is a separated bike line present. Suggested examples of high bicyclist activity are: • Residential development with four or more housing units per acre interspersed with multi- family dwellings. • Bicycle treatments including marked bike lanes, bike boxes, etc. • Multiple transit stops within the segment. Roadway Segment Variable: Design Speed (Limited-Access SLSG) The user selects either ≥ 60 mph or ≤ 55 mph for the design speed of the freeway segment. This value along with the grade is used to identify mountainous terrain.

Variables for Decision-Making Procedure 25   Roadway Segment Variable: Directional Design-Hour Truck Volume (Limited-Access SLSG) The user enters the directional design-hour truck volume for the freeway segment in the units of trucks per hour. Roadway Segment Variable: Grade (Limited-Access SLSG) The user enters the grade for the freeway segment. Roadway Segment Variable: ISW (Limited-Access SLSG) The user enters the inside (left) SW for the freeway segment. Roadway Segment Variable: Lane Width (Undeveloped SLSG) The user enters the typical LW (ft) for the segment. Examination of the LW crash modifica- tion factor (CMF) for undeveloped facilities in the Highway Safety Manual (HSM) (43) 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 lane width 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. (34) found that rural two-lane roadway lane widths greater than 12 ft had fewer fatal and injury crashes (KABC) crashes. The guidance for lane width is synthesized as follows: • If the LW is less than 10 ft, 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 LW is less than 11 ft, the posted speed limit should be set at the higher of the closest increment to the 50th percentile (C50) or rounded down to the closest increment to the 85th percentile (RD85). • If the LW is equal to or greater than 11 ft, the posted speed limit should be set at the closest increment to the 85th percentile. Roadway Segment Variable: Median Type (Undeveloped, Developed, and Full-Access SLSGs) With respect to Developed and Full-Access SLSGs, the safety analyses conducted as part of NCHRP Project 17-76, published as Web-Only Document 291 (2) (Appendix D on Austin, Texas, and Appendix E on Washtenaw County/Greater Ann Arbor, Michigan) found fewer crashes for a raised (divided) median compared to 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 (35, 36). The research team suggested that the presence of a divided (raised or depressed) median or a TWLTL on a road with four or more lanes be considered the baseline condition, and for undivided four-lane roads to 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.

26 Posted Speed Limit Setting Procedure and Tool: User Guide How median type is used for the Undeveloped SLSG is discussed in the “Number of Lanes/ Median Type Combination” section that follows. Roadway Segment Variable: Number of Access Points (Undeveloped, Developed, and Full-Access SLSGs) The user provides the number of non-single-family residential driveways and unsignalized intersections within the segment, and the SLS-Tool calculates the access density (access point per mile). The variable is called access density to avoid the question of whether driveways per mile should include unsignalized intersections, which it should. For the Developed and Full-Access SLSGs, the findings from the NCHRP Project 17-76 research supports the breakpoints used in USLIMITS2 (37). All types of non-single-family home driveways, such as multifamily residential, commercial, etc., along with unsignalized inter sections, should be counted. The guidance for access points is provided in Table 7 for the Developed SLSG and Table 9 for the Full-Access SLSG, and can be synthesized as follows: • If the number of access points is less than 40 per mile on Developed or Full-Access streets, the suggested posted speed limit should be the 5-mph increment closest to the 85th percentile speed. • If the number of access points is greater than 40 per mile or less than or equal to 60 per mile, then the suggested posted speed limit should use RD85. • If the number of access points is more than 60 per mile, then the suggested posted speed limit should be the 5-mph increment closest to the 50th percentile speed. Previous studies for undeveloped facilities have shown that roadway safety decreases as the number of access points increases (34, 38). Access density for undeveloped conditions should also include any signalized intersection within the corridor. Table 6 provides guidance for access points. Roadway Segment Variable: Number of Interchanges (Limited-Access SLSG) The user enters the number of interchanges within the segment. This information is used with the segment length and AADT (two-way total) in veh/d. The program computes interchange spacing as length per interchange and calls for lower suggested speed limits for the specified levels of interchange spacing if the AADT equals or exceeds 180,000 veh/d. Roadway Segment Variable: Number of Lanes (All SLSGs) The user enters the number of lanes for both directions of travel. Roadway Segment Variable: Number of Traffic Signals (Developed and Full-Access SLSGs) The user provides the number of signals within the segment and the program calculates the number of signals/segment length. Previous research used breakpoints at three and four signals per mile and these values were supported by the findings from the analyses conducted in this research [see NCHRP Web-Only Document 291 (2)]. A revised breakpoint was needed for use in the Full-Access SLSG, and the value of eight signals per mile was selected based on feedback from the research project panel.

Variables for Decision-Making Procedure 27   Roadway Segment Variable: On-Street Parking Activity (Developed and Full-Access SLSGs) Because the on-street parking characteristics may vary within a segment, the user provides the on-street parking characteristics that are predominant within the segment. The user indicates if on-street parking activity is high or not high. A high level of on-street parking can be character- ized as having parking on both sides of the road with parking time limits. Roadway Segment Variable: Outside (Right) SW (Limited-Access SLSG) For Limited-Access facilities, the Green Book (42) (Chapter 8) calls for outside SWs of at least 12 ft if the truck volume exceeds 250 trk/hr, and at least 10 ft otherwise. Examination of the outside SW CMF for Limited-Access facilities in the HSM (43) shows that the outside SW can be reduced slightly without a significant increase in crash frequency. The CMF value computes as 1.21 for an outside SW of 7 ft and 1.14 for an outside SW of 8 ft. In other words, when the outside SW (rounded down to the nearest foot) is less than 8 ft, crash frequency is expected to increase by about 21 percent. Therefore, based on safety considerations, the research team suggested setting the posted speed limit based on the rounded-down 85th percentile if the outside SW is less than 8 ft, or the closest 85th percentile otherwise. Roadway Segment Variable: Parallel Parking Permitted (Developed SLSGs) Because the on-street parking characteristics may vary within a segment, the user provides the on-street parking characteristics that are predominant within the segment. The user indicates if parallel parking is permitted (yes or no). Permitted parallel parking on a street within the Developed SLSG results in using RD85. Roadway Segment Variable: Pedestrian Activity (Developed and Full-Access SLSGs) The user indicates if the pedestrian activity is high, some, or negligible. Suggested examples of high pedestrian activity are: • Residential development with four or more housing units per acre interspersed with multi- family dwellings. • Hotels located within one half mile of other attractions such as retail stores, recreation areas, or senior centers. • Paved sidewalks, marked crosswalks, and pedestrian signals. • Multiple transit stops within the segment. Roadway Segment Variable: Segment Length (All SLSGs) The user enters the length of the segment in miles. Roadway Segment Variable: SW (Undeveloped SLSG) The user enters the typical SW for the segment in feet. Studies have consistently found that wider paved shoulders on undeveloped roadways result in fewer crashes (39, 40). Examination of the SW CMF for undeveloped facilities in the HSM (43) shows that a 6-ft SW is assigned a CMF of 1.00 (see Table 10.9, Table 11.12, and Table 11.16 in the HSM). The CMF value

28 Posted Speed Limit Setting Procedure and Tool: User Guide computes as 1.15 for 4-ft and 1.30 for 2-ft lane widths for two-lane roadways. For multilane undivided roadways, these values are 1.15 and 1.30 for 4-ft and 2-ft SWs, respectively. For multilane divided roadways, an 8-ft right SW is assigned a CMF of 1.00 (see Table 11-17 in the HSM). Table 6 provides the guidance for SW within the SLS-Tool. Roadway Segment Variable: Sidewalk Buffer (Developed and Full-Access SLSGs) The user indicates if a sidewalk separation (or buffer) is present or not present. A sidewalk separation (or buffer) reflects the space between the road (the face of the curb when a curb and gutter are present, or the edge of the travel lane when a shoulder is present) and the sidewalk. A buffer could include a nature strip, a bike lane, or on-street parking. Because the type of sidewalk buffer may vary within a section, the user provides the type of sidewalk buffer treatment that is predominant within the section. Roadway Segment Variable: Sidewalk Presence/Width (Developed and Full-Access SLSGs) The FHWA University Level Course on Bicycle and Pedestrian Transportation (41) (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 segment, the user provides the side- walk characteristics that are predominant within the segment. The user indicates the pre- dominant width of the sidewalk within the following four categories for the segment: • None: no sidewalk is present on either side of the street. • Narrow: a narrow sidewalk is present (the sidewalk is less than 5 ft if set back from the curb or 6 ft if at the curb face). • Adequate: An adequate sidewalk is present (the sidewalk is between 8 ft and 5 ft if set back from the curb, or between 8 ft and 6 ft if at the curb face). • Wide: A wide sidewalk is present (the sidewalk is 8 ft or greater). Combination of Roadway Segment Variables Roadway Segment Combination of Variables: Grade and Design Speed (Limited-Access SLSG) Consideration for mountainous terrain based on Green Book guidance for maximum grade and design speed of Limited-Access facilities (42) (Table 8-1) generated the following guidance: • If the design speed is 60 mph or greater and the maximum grade exceeds 4 percent, set the posted speed limit as the higher of the closest 50th percentile or the rounded-down 85th percentile. • If the design speed is 55 mph or less and the maximum grade exceeds 5 percent, set the posted speed limit as the higher of the closest 50th percentile or the rounded-down 85th percentile. • In all other cases, set the posted speed limit as the closest 85th percentile. The first two conditions are based on the breakpoints between maximum grades for rolling and mountainous terrain specified by the Green Book.

Variables for Decision-Making Procedure 29   Roadway Segment Combination of Variables: ISW, Number of Lanes, and Hourly Truck Volume (Limited-Access SLSG) For Limited-Access facilities, the Green Book (42) (Chapter 8) calls for the following min- imum ISW: • Directional design-hour truck volume ≤ 250 trk/hr and number of lanes (two-way total) < 6 then ISW ≥ 4 ft. • Directional design-hour truck volume ≤ 250 trk/hr and number of lanes ≥ 6 then ISW ≥ 10 ft. • Directional design-hour truck volume > 250 trk/hr then ISW ≥ 12 ft. Examination of the ISW CMF for Limited-Access facilities in the HSM (43) shows that the ISW has a minor effect on crash frequency. The CMF value computes as 1.07 for the ISW of 2 ft. Therefore, the research team suggested setting the posted speed limit based on the Green Book criteria. If the criteria are met, the posted speed limit is based on the closest 85th per- centile. If the criteria are not met, set the posted speed limit based on the rounded-down 85th percentile. Roadway Segment Combination of Variables: Number of Lanes, Median Type, AADT Combination (Undeveloped SLSG) With respect to the Undeveloped SLSG, a review of the HSM showed that the crash predic- tion for undivided four-lane roadways is greater than that for divided four-lane roadways. Four-lane undivided roads with AADT value (two-way total) of 2,000 has about 35  per- cent more crashes as four-lane divided roads with the same AADT value. The percentage is smaller for roads with AADT values less than 2,000 and larger for AADT values greater than 2,000. Therefore, the research team suggested the rounded-down 85th percentile speed be used when the road has four lanes, is undivided, and has an AADT value of 2,000 or more. Other cases, such as two-lane roads or AADT values less than 2,000, would use the closest 85th percentile speed. The guidance for the number of lanes/median type combination is synthesized as follows: • If the undeveloped roadway has an AADT value more than 2,000 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. Roadway Segment Variable: Sidewalk Presences/Width, Sidewalk Buffer, and Pedestrian Activity (Developed and Full-Access SLSGs) When there is a reasonable expectation of pedestrians on or very near a roadway, selection of a lower operating speed can be justified. Sidewalk conditions (width and buffer) and the level of pedestrian activity are used in combination to select the speed percentile; those values are provided in Table 8 for the Developed SLSG and Table 10 for the Full-Access SLSG. Crash Data Input Variables for Speed Limit Setting Groups Table 13 shows the variables needed when crash data are available.

30 Posted Speed Limit Setting Procedure and Tool: User Guide Crash Variables The following variables are needed to be able to conduct an analysis of the crash data: • Length of the study period in years and months (least 3 years of crash data is recommended; if less than 1 year of data is input, the program suggests that additional data be collected and the process repeated). • Total number of all crashes (KABCO) in the segment. • Total number of fatal and injury crashes (KABC) in the segment. • AADT (two-way total) for the study period. • Average rate of all (KABCO) crashes and average rate of fatal and injury (KABC) crashes [100 million vehicle miles (MVM)] for similar road segments in their jurisdiction. To deter- mine the average crash/injury rate for similar segments, users should select a group of seg- ments that have the same or similar geometry (i.e., the number of lanes, median type, etc.) and similar traffic volumes and area type. • For Developed and Full-Access SLSGs, the user also indicates if the road is a one-way street. Average Crash Rate The length of study, number of crashes, and AADT are used to calculate the segment crash rate for all (KABCO) crashes and for fatal and injury (KABC) crashes per 100 MVM. If the user does not provide average rates, default values from the HSIS are used (44). Table 14 and Table 15 provide the values for the Limited-Access SLSG, Table 16 and Table 17 provide the values for the Undeveloped SLSG, and Table 18 and Table 19 provide the values for the Developed and Full-Access SLSGs. AADT Category— Minimum AADT Category— Maximum 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 44.16 100,000 149,999 91.16 44.16 150,000 199,999 91.60 44.16 200,000 No Limit 104.51 44.16 Note: Crash rates and injury rates were calculated using the latest 3 years of data that were available: California (2009–2011), Minnesota (2010–2012), North Carolina (2011–2013), Ohio (2010–2012), and Washington State (2010–2012). Source: Adapted from User Guide for USLIMITS2 (44), Table 1. Table 14. Average KABCO rate per 100 MVM for Limited-Access SLSG. AADT Category— Minimum AADT Category— Maximum 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 14.41 100,000 149,999 27.69 14.41 150,000 199,999 29.25 14.41 200,000 No Limit 30.75 14.41 Note: Crash rates and injury rates were calculated using the latest 3 years of data that were available: California (2009–2011), Minnesota (2010–2012), North Carolina (2011–2013), Ohio (2010–2012), and Washington State (2010–2012). Source: Adapted from User Guide for USLIMITS2 (44), Table 1. Table 15. Average KABC crash rate per 100 MVM for Limited-Access SLSG.

Variables for Decision-Making Procedure 31   AADT Category— Minimum AADT Category— Maximum 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 (2009–2011), Minnesota (2010–2012), North Carolina (2011–2013), Ohio (2010–2012), and Washington State (2010–2012). Source: Adapted from User Guide for USLIMITS2 (44), Table 1. Table 16. Average KABCO rate per 100 MVM for Undeveloped SLSG. AADT Category— Minimum AADT Category— Maximum 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 (2009–2011), Minnesota (2010–2012), North Carolina (2011–2013), Ohio (2010–2012), and Washington State (2010–2012). Source: Adapted from User Guide for USLIMITS2 (44), Table 1. Table 17. Average KABC crash rate per 100 MVM for Undeveloped SLSG. AADT Category— Minimum AADT Category— Maximum 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 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 (2009–2011), Minnesota (2010–2012), North Carolina (2011–2013), Ohio (2010–2012), and Washington State (2010–2012). Source: Adapted from User Guide for USLIMITS2 (44), Table 1. Table 18. Average KABCO crash rate per 100 MVM for Developed and Full-Access SLSGs.

32 Posted Speed Limit Setting Procedure and Tool: User Guide Critical Crash Rate The critical crash rate is calculated from: R R K R M M c a a= + + 1 2 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 Tables 14 through 19. K = Constant associated with the confidence level (1.645 for 95 percent confidence). M = Exposure (100 MVM). Crash Rate Scenarios When crash data are available, the program compares the crash rate—both all (KABCO) and fatal and injury (KABC)—for the segment to the critical crash rate and average crash rate, and uses the worst-case scenario. The crash rate is put into one of three categories: • High: Segment crash_rate > critical crash rate. • Medium: Segment crash_rate > 1.3 average crash rate. • Low: neither of the above is true. AADT Category— Minimum AADT Category— Maximum 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 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 (2009–2011), Minnesota (2010–2012), North Carolina (2011–2013), Ohio (2010–2012), and Washington State (2010–2012). Source: Adapted from User Guide for USLIMITS2 (44), Table 1. Table 19. Average KABC crash rate per 100 MVM for Developed and Full-Access SLSGs.

Next: Section 6 - Speed Limit Setting Tool »
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Several factors are considered within engineering studies when determining the posted speed limit, including the 85th percentile speed, which is based on the driving behavior of most drivers (85 percent). The 85th percentile speed is believed to represent a safe speed that would minimize crashes.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide provides and explains a speed limit setting procedure (SLS-Procedure) that considers factors beyond the 85th percentile speed, including both driver speed choice and safety associated with the roadway. This report also provides instructions for using an automated version of the SLS-Procedure via a spreadsheet-based Speed Limit Setting Tool (SLS-Tool). Two versions of the SLS-Tool are available:

N17-76 SLS-Tool (with macros) and

N17-76 SLS-Tool (without macros).

The “without macros” version is made available for users who are not able to use macro codes on their computers. Please see the User Guide for more detailed information on using both versions of the SLS-Tool.

The report is also accompanied by NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool, which documents the research efforts of NCHRP Project 17-76 - Guidance for the Setting of Speed Limits and a Presentation that offers an overview of the project.

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