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

Chapter: Section 4 - Decision-Making Steps Within the Suggested Speed Limit Procedure

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Suggested Citation:"Section 4 - Decision-Making Steps Within the Suggested Speed Limit 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 4 - Decision-Making Steps Within the Suggested Speed Limit 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 4 - Decision-Making Steps Within the Suggested Speed Limit 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 4 - Decision-Making Steps Within the Suggested Speed Limit 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 4 - Decision-Making Steps Within the Suggested Speed Limit 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 4 - Decision-Making Steps Within the Suggested Speed Limit 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 4 - Decision-Making Steps Within the Suggested Speed Limit 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 4 - Decision-Making Steps Within the Suggested Speed Limit 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 4 - Decision-Making Steps Within the Suggested Speed Limit 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 4 - Decision-Making Steps Within the Suggested Speed Limit 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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11   Roadway Segment Context and Type The initial step in decision-making is identifying the roadway segment content and type. The Expanded Functional Classification System (Expanded FCS) aides in that determination. The Expanded Functional Classification System was developed to replace the existing functional classification scheme in order to facilitate optimal geometric design solutions with consideration of context, road functions, and user needs. The scheme was introduced in NCHRP Research Report 855 (33) and is intended to build upon existing efforts from state departments of trans- portation that have initiated and implemented a new classification system to address contextual multimodal deficiencies of the existing classification system. As stated in NCHRP Research Report 855 (33), “the major objective of the Expanded FCS is to provide enhanced information to designers to better inform the design decision process. . . . This enhanced information is provided by increasing the resolution of roadway’s design con- text to enable understanding of the role the roadway plays within the community; identifying the role of the roadway within the local, city, and regional transportation network; and identi- fying the multiple roadway user groups and their priority within the design corridor.” The goal of the Expanded FCS is to provide practitioners with a practical tool for deter- mining appropriate design criteria and elements to help better understand the impacts of the tradeoffs necessary to balance user needs and safety and to address other community issues. The Expanded FCS and associated design matrix can be used to identify preliminary require- ments for proper consideration of roadway context and user needs. As presented in NCHRP Research Report 855, the Expanded FCS considers roadway con- text, roadway type, roadway users, and overlays. The SLS-Procedure uses the basic roadway context/roadway type matrix. NCHRP Research Report 855 provides additional information on the Expanded FCS. Roadway Context The Expanded FCS includes five distinct contexts. These were determined to represent unique land use that requires different geometric design practices in terms of desired operating speeds, mobility/access demands, and user groups. NCHRP Research Report 855 (33) describes the context categories as follows and provides the illustration shown in Figure 5: • Rural: areas with lowest density, few houses or structures (widely dispersed or no residential, commercial, and industrial uses), and usually large setbacks. • Rural Town: areas with low density but diverse land uses with commercial main street char- acter, potential for on-street parking and sidewalks, and small setbacks. S E C T I O N 4 Decision-Making Steps Within the Suggested Speed Limit Procedure

12 Posted Speed Limit Setting Procedure and Tool: User Guide • Suburban: areas with medium density, mixed land uses within and among structures (includ- ing mixed-use town centers, commercial corridors, and residential areas), and varied setbacks. • Urban: areas with high density, mixed land uses and prominent destinations, potential for some on-street parking and sidewalks, and mixed setbacks. • Urban Core: areas with highest density and mixed land uses within and among predomi- nately high-rise structures, and small setbacks. Table 1 summarizes the primary factors associated with each roadway context. Roadway Type The roadway types used in the Expanded FCS are based on their network function and the connectivity they provide among various centers of activity. The roadway types are as follows: • Interstates/Freeways/Expressways: corridors of national importance connecting large centers of activity over long distances. Figure 5. NCHRP Research Report 855 illustration of five roadway contexts. [Source: Transportation Research Board. 2018. NCHRP Research Report 855: An Expanded Functional Classification System for Highways and Streets. HTTPS://DOI.ORG/10.17226/ 24775. Reproduced with permission from the National Academy of Sciences. Figure 2, page 3. (33)]

Decision-Making Steps Within the Suggested Speed Limit Procedure 13   • Principal Arterials: corridors of regional importance connecting large centers of activity. • Minor Arterials: corridors of regional or local importance connecting centers of activity. • Collectors: roadways of lower local importance providing connections between arterials and local roads. • Locals: roads with no regional or local importance for local circulation and access only. Matrix Table 2 shows the roadway context/roadway type matrix along with the target speed for each context/type combination. Target operating speed is the desirable speed for motorists to travel along a roadway within the particular context/roadway type combination. NCHRP Research Report 855 grouped the target operating speed into three categories (33): • Low (25 mph and below). • Medium (30 to 45 mph). • High (50 mph and above). NCHRP Research Report 855 provides the following justification for the target speed values: The speed used in the Expanded FCS is the target operating speed of the roadway. The rationale for selecting operating speed in the Expanded FCS is the need to recognize the influence of driver desire and expectations. Moreover, the goal is to develop a facility where the operating speed is close to the design speed, resulting in an environment with smaller speed differences among drivers. Smaller speed differ- entials could improve safety, since they will eliminate discrepancies between design speed and operating speeds, creating a more uniform speed profile among drivers. These speeds need to be considered with both existing and future volumes and contexts. The limits for each category are based on established practices and extensive research. The speed of 25  mph was considered the limit for the low-speed environments based on current trends of several urban areas to facilitate a speed limit of 25 mph. Indeed, 20 mph is considered the survivability speed for pedestrians and bicyclists in the event of a collision with a vehicle. Such collisions typically result in injuries, and non-drivers have a high chance of surviving when speeds remain at or below 20 mph. As such, speeds of 20 mph or less should be considered in areas of higher pedestrian activity in the urban and urban core environments. Target speeds for urban and rural towns have been designated as Context Density Land Use Setback Rural Lowest (few houses or other structures) Agricultural, natural resource preservation, and outdoor recreation uses with some isolated residential and commercial uses Usually large setbacks Rural Town Low to medium (single- family houses and other single-purpose structures) Primarily commercial uses along a main street (some adjacent single-family residential uses) On-street parking and sidewalks with predominately small setbacks Suburban Low to medium (single- and multifamily structures and multistory commercial) Mixed residential neighborhood and commercial clusters (including town centers, commercial corridors, big-box commercial, and light industrial uses) Varied setbacks with some sidewalks and mostly off-street parking Urban High (multistory, low-rise structures with designated off-street parking) Mixed residential and commercial uses, with some intuitional and industrial uses, and prominent destinations On-street parking and sidewalks with mixed setbacks Urban Core Highest (multistory and high-rise structures) Mixed commercial, residential, and institutional uses within and among predominately high-rise structures Small setbacks with sidewalks and pedestrian plazas Source: Transportation Research Board. 2018. NCHRP Research Report 855: An Expanded Functional Classification System for Highways and Streets. HTTPS://DOI.ORG/10.17226/24775. Reproduced with permission from the National Academy of Sciences, Table 1, page 10 (33). Table 1. Characteristics of roadway contexts.

14 Posted Speed Limit Setting Procedure and Tool: User Guide low/medium because of the competing issues within these contexts and the varied pedestrian and road- side environment. The designer should examine the available speed range to select the operating speed most appropriate for all users given the facilities and context. The upper limit for high speeds is based on the American Association of State Highway and Transportation Officials’ (AASHTO’s) A Policy on Geometric Design of Highways and Streets (commonly known as the Green Book) (42) definition of high-speed roads, which are those with speeds of 50 mph and above. (33, page 26) Speed Limit Setting Groups The roadway context and type should be considered when identifying a posted speed limit for a facility. While the expanded functional classification matrix has 25 unique combinations of roadway types and roadway contexts (Table 2), there are combinations where a similar decision process would be employed. For example, the setting of posted speed limits for Limited-Access freeways may be the same for suburban or urban freeways. Table 3 shows the SLSGs by roadway context/roadway type and includes the following: • Limited-Access. • Undeveloped. • Developed. • Full-Access. Context and Type Rural Rural Town Suburban Urban Urban Core Limited- Access Freeway High 50 mph and above High 50 mph and above High 50 mph and above High 50 mph and above High 50 mph and above Principal Arterial High 50 mph and above Low to Medium 45 mph and below Medium to High 30 mph and above Low to Medium 45 mph and below Low 25 mph and below Minor Arterial High 50 mph and above Low to Medium 45 mph and below Medium 30 to 45 mph Low to Medium 45 mph and below Low 25 mph and below Collector Medium 30 to 45 mph Low 25 mph and below Medium 30 to 45 mph Low 25 mph and below Low 25 mph and below Local Medium 30 to 45 mph Low 25 mph and below Low 25 mph and below Low 25 mph and below Low 25 mph and below Source: Adapted from Transportation Research Board. 2018. NCHRP Research Report 855: An Expanded Functional Classification System for Highways and Streets. HTTPS://DOI.ORG/10.17226/24775. Reproduced with permission from the National Academy of Sciences, Figure 19 (33). Table 2. Suggested target speed by roadway context and type. Context and Type Rural Rural Town Suburban Urban Urban Core Freeways Limited- Access Limited- Access Limited- Access Limited- Access Limited- Access Principal Arterial Undeveloped Developed Developed Developed Full-Access Minor Arterial Undeveloped Developed Developed Developed Full-Access Collector Undeveloped Full-Access Developed Full-Access Full-Access Local Undeveloped Full-Access Full-Access Full-Access Full-Access Table 3. Suggested SLSGs.

Decision-Making Steps Within the Suggested Speed Limit Procedure 15   Speed Distribution The distribution of individual vehicle speeds within the traffic stream is dependent on several factors. Speeds tend to be relatively uniform (i.e., narrowly distributed) during periods of heavy congestion and more broadly distributed during free-flow conditions. Typically, for speed limit setting purposes, the speed distribution should only include free-flowing vehicles. The distri- bution of individual vehicle speeds may be characterized by variables that include the average, 50th percentile, 85th percentile, standard deviation, and pace of the measured speeds, each of which is defined in Table 4. Figure 6 illustrates key speed terms within a speed distribution plot. For speed setting purposes within the SLS-Tool, the primary variables of interest related to speed are the 50th percentile and the 85th percentile speed. While not used within the SLS-Tool, minimizing the standard deviation or maximizing the pace (largest percent of vehicles within a 10-mph range) is associated with fewer crashes; therefore, other tools such as enforcement or changes in roadway design could be considered. Consideration of Geometric Variables, Human Factors, and Safety Geometry, human factors, and safety are all considerations that are utilized within a set of deci- sion rules for each SLSG to determine the suggested speed limit. The possible suggested speed limit options are as follows, listed in order from highest to lowest speed within the distribution: • 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). • The 50th percentile speed rounded down to the nearest 5-mph increment (RD50). Table 4. Speed definitions. Term Definition 50th percentile (median) The speed at or below which 50 percent of the total observed values fall in a sample of measured spot speeds. 85th percentile The speed at or below which 85 percent of the total observed values fall in a sample of measured spot speeds. Average travel speed The average speed of the traffic stream over a specified section of highway. Free-flow speed The average speed of vehicles on a given segment, measured under low-volume conditions, when drivers are free to drive at their desired speed and are not constrained by the presence of other vehicles or downstream TCDs (e.g., traffic signals, roundabouts, or stop signs). Operating speed The operating speed of a road is the speed at which motor vehicles generally operate on that road. In a general sense, the term operating speed refers to the speed at which drivers are observed operating their vehicles. The 85th percentile of a sample of observed speeds has been typically used as a descriptive statistic for establishing the operating speed associated with a particular road segment; however, other percentiles have also been used. Pace The 10-mph range that contains the greatest percentage of observations, expressed as a percentage of the number of speed measurements within the 10-mph range divided by the total number of speed measurements. Posted speed Numeric speed limit value displayed on regulatory speed limit signs. Space-mean speed Harmonic mean of several spot speed measurements (or calculated using the average travel times of vehicles measured over a given length of roadway). Speed Rate of movement of a vehicle in mph. Spot speed Instantaneous measure of speed at a specific location on a roadway. Standard deviation Spread of individual speeds around the mean, calculated as the square root of the sum of squares of the deviations of the individual spot speeds from the mean divided by the number of measurements less one. Statutory speed limit Statutory speed limits are established by state legislatures and are enforceable by law. Such limits typically vary by highway type (e.g., interstate) or by location (e.g., urban district). Target speed The highest speed at which vehicles should ideally operate on a roadway. Time-mean speed Arithmetic mean or average of several spot speed measurements (or the average of speeds of vehicles passing a given point along a roadway over a certain time period).

16 Posted Speed Limit Setting Procedure and Tool: User Guide When the roadway conditions are optimal, the suggested speed limit would reflect the 5-mph increment closest to the 85th percentile speed except for segments within the Full- Access SLSG, where it would reflect the 5-mph increment closest to the 50th percentile speed in recognition of the anticipated users within those facilities. When roadway con- ditions are not favorable to all users or when crashes are a significant concern, then the suggested speed limit would reflect the 5-mph increment closest to the 50th percentile speed for Limited-Access, Developed, or Undeveloped SLSGs or the 5-mph increment rounded down from the 50th percentile speed for the Full-Access SLSG. An RD85 speed limit is suggested when conditions are between those extremes for Limited-Access, Developed, or Undeveloped SLSGs. In rare cases, the RD85 will be less than the C50 due to rounding. As an example, if the 50th percentile speed was 58 mph and the 85th percentile speed was 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. The results may appear (a) Cumulative distribution example (b) Histogram example 50 Figure 6. Example illustrations of speed distribution curves.

Decision-Making Steps Within the Suggested Speed Limit Procedure 17   unusual but are accurate given the provided speed data, and hence should be interpreted with caution. Decision Rules for Each Speed Limit Setting Group The following sections presents the decision rules for selecting the 5-mph increment that reflects C50, RD85, or C85 by SLSGs. Crashes are considered by comparing the crash rate [crashes/100 million vehicle miles (MVM)] for the segment with the crash rate for similar road sections in the jurisdiction or, if not available, with crash rates from the Highway Safety Information System (HSIS). KABCO is a crash severity scale where: • K = fatal. • A = incapacitating injury. • B = non-incapacitating injury. • C = possible injury. • O = no injury, property damage only. KABCO includes crashes for all severity levels and KABC includes crashes with fatal or injury severity levels. Speed Limit Setting Group: Limited-Access Table 5 provides an overview of the variables along with the variable value that would trigger using C85, RD85, or C50. Speed Limit Setting Group: Undeveloped Table 6 provides an overview of the variables along with the variable value that would trigger using either C85, RD85, or C50. Variable Closest 50th (C50) Rounded Down 85th (RD85) Closest 85th (C85) Average interchange spacing (Inter_spac) expressed as length/number of interchanges in miles (mi) and annual average daily traffic (AADT) (two-way total) in vehicles per day (veh/d) 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 Mountainous terrain as determined by grade in percent and design speed in mph {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 (trk/hr) {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 KABCO or KABC crash rate High Medium Low Table 5. Overview of decision rules for Limited-Access SLSG.

18 Posted Speed Limit Setting Procedure and Tool: User Guide Speed Limit Setting Group: Developed Table 7 provides an overview of the variables along with the variable value that would trigger using C85, RD85, or C50. Table 8 provides the decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Developed SLSG. Speed Limit Setting Group: Full-Access Table 9 provides an overview of the variables along with the variable value that would trigger using C50 or RD50. Table 10 provides the decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Full-Access SLSG. Variable 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 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 Lane width (LW) 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 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 SW when AADT ≤ 2000 KABCO or KABC crash rate High Medium Low Table 6. Overview of decision rules for Undeveloped area SLSG.

Decision-Making Steps Within the Suggested Speed Limit Procedure 19   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, two-way left-turn lane (TWLTL), or 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, or wide), sidewalk buffer (present or not present), and pedestrian activity (high, some, or negligible) See Table 8 See Table 8 See Table 8 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 KABCO or KABC crash rate High Medium Low Table 7. Overview of decision rules for Developed area SLSG. Table 8. Decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Developed SLSG. Pedestrian Activity Sidewalk Presence/Width Sidewalk Buffer Speed Percentage 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 RD85 Some Adequate Present C85 Some Narrow Not present C50 Some Narrow Present RD85 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 See text for additional discussion on sidewalk presence/width and sidewalk buffer characteristics.

20 Posted Speed Limit Setting Procedure and Tool: User Guide Pedestrian Activity Sidewalk Presence/Width Sidewalk Buffer SpeedPercentage 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 RD50 Some Adequate Present C50 Some Narrow Not present RD50 Some Narrow Present RD50 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 See text for additional discussion on sidewalk presence/width and sidewalk buffer characteristics. Table 10. Decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Full-Access Speed Limit Setting Group. Table 9. Overview of decision rules for Full-Access SLSG. 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, or wide), sidewalk buffer (present or not present), and pedestrian activity (high, some, or negligible) See Table 10 See Table 10 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 KABCO or KABC crash rate High or Medium Low

Next: Section 5 - Variables for Decision-Making Procedure »
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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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