National Academies Press: OpenBook

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

Chapter: Section 7 - SLS-Tool Case Study Examples

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Suggested Citation:"Section 7 - SLS-Tool Case Study Examples." 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 7 - SLS-Tool Case Study Examples." 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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Page 40
Suggested Citation:"Section 7 - SLS-Tool Case Study Examples." 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.
×
Page 40
Page 41
Suggested Citation:"Section 7 - SLS-Tool Case Study Examples." 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.
×
Page 41
Page 42
Suggested Citation:"Section 7 - SLS-Tool Case Study Examples." 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.
×
Page 42
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Suggested Citation:"Section 7 - SLS-Tool Case Study Examples." 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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Page 43

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38 Example 1: Limited-Access Example 1 is a freeway in a large city. Crash data are not available. The following information is available for the site: • Segment length = 6.5 mi. • AADT (two-way total) = 130,000 veh/d. • Directional design-hour volume = 200 trucks/hr. • Number of lanes (total in both directions) = 6. • Number of interchanges = 5. • Design speed ≥ 60 mph. • Grade = 2 percent. • Outside SW = 10 ft. • ISW = 2 ft. • Maximum speed limit = 70 mph. • Current posted speed limit = 65 mph. • 85th percentile = 71 mph. • 50th percentile = 67 mph. • No adverse alignment present. With these input variables, the suggested speed limit is computed as 70 mph. The speed limit criterion is identified as the rounded-down 85th percentile because of the narrow 2-ft ISW. Figure 8 shows the calculations. Example 2: Undeveloped Example 2 is for a rural, two-lane highway with the following characteristics: • Segment length = 7.2 mi. • AADT (two-way total) = 2250 veh/d. • Number of lanes = 2. • Median type = none. • Number of access points (non-residential driveways and unsignalized intersections) = 14. • Lane width = 12 ft. • SW = 4 ft. • Current posted speed limit = 65 mph. • 85th percentile = 72 mph. • 50th percentile = 68 mph. • Adverse alignment is present. S E C T I O N 7 SLS-Tool Case Study Examples

SLS-Tool Case Study Examples 39   Crash data are available and include the following: • Number of years of crash data = 5 years. • Average AADT (two-way total) for crash data period = 2200 veh/d. • Number of all (KABCO) crashes for crash data period = 30 crashes. • Number of fatal and injury (KABC) crashes for crash data period = 20 crashes. With these input variables, the suggested speed limit is computed as 70 mph. The speed limit criterion is identified as the rounded-down 85th percentile because of the narrow 4-ft SW. Figure 9 shows the calculations. Example 3: Developed Example 3 is for a principal arterial in a suburban area with the following characteristics: • Current posted speed limit = 40 mph. • Maximum speed limit = 50 mph. • 85th percentile = 43 mph. • 50th percentile = 38 mph. • Segment length = 2 mi. Figure 8. Spreadsheet analysis of Example 1: Limited-Access Segment.

40 Posted Speed Limit Setting Procedure and Tool: User Guide • Number of lanes = 4. • Median type = TWLTL. • Number of traffic signals = 3. • Number of access points (non-residential driveways and unsignalized intersections) = 15. • Bicyclist activity = not high. • Sidewalk presence/width = none. • Sidewalk buffer = not applicable since sidewalk is not present. • Pedestrian activity = some. • On-street parking activity = not high. • Parallel parking permitted = yes. • Angle parking present = no. • Adverse alignment present = no. Crash data are available and include the following: • Number of years of crash data = 2 years. • Average AADT (two-way total) for crash data period = 20,000 veh/d. • The segment has two-way traffic. • Number of all (KABCO) crashes for crash data period = 25 crashes. • Number of fatal and injury (KABC) crashes for crash data period = 10 crashes. Figure 9. Spreadsheet analysis of Example 2: Undeveloped Segment.

SLS-Tool Case Study Examples 41   With these input variables, the suggested speed limit is computed as 40 mph. Figure 10 shows the calculations. The speed limit criterion is identified as the closest 50th percentile because no sidewalks are present. If sidewalks of adequate width were added, sidewalks with narrow width and a buffer were added, or pedestrian activity was negligible, the speed limit criterion would be the rounded-down 85th percentile. Because the years of crash data is less than desired (only 2 years rather than 3 years), the SLS-Tool provides an advisory message of “Consider collecting at least 3 years of crash data.” Example 4: Full-Access Example 4 is for a collector street in the urban core of a city. The following characteristics are available: • Current posted speed limit = 30 mph. • Maximum speed limit = 30 mph. • 50th percentile = 32 mph. Figure 10. Spreadsheet analysis of Example 3: Developed Segment.

42 Posted Speed Limit Setting Procedure and Tool: User Guide • Segment length = 1 mi. • Number of lanes = 2. • Median type = undivided. • Number of traffic signals = 3. • Number of access points, total of both directions (non-residential driveways and unsignalized intersections) = 10. • Bicyclist activity = not high. • Sidewalk presence/width = wide. • Sidewalk buffer = present. • Pedestrian activity = high. • On-street parking activity = high. • Angle parking present = no. • Adverse alignment present = no. Crash data are available and include the following: • Number of years of crash data = 5 years. • Average AADT (two-way total) for crash data period = 10,000 veh/d. • The segment has two-way traffic. • Number of all (KABCO) crashes for crash data period = 50 crashes. • Number of fatal and injury (KABC) crashes for crash data period = 25 crashes. With these input variables, the suggested speed limit is computed as 30 mph. The speed limit criterion is identified as the rounded-down 50th percentile because of the high number of KABC crashes on the segment. The observed KABC crash rate of 114.16 crashes/100 MVM exceeds the critical KABC crash rate of 105.5 crashes/100 MVM. The high level on-street parking activity also results in the suggested speed limit being the rounded-down 50th percentile value. Figure 11 shows the calculations.

SLS-Tool Case Study Examples 43   Figure 11. Spreadsheet analysis of Example 4: Full-Access Segment.

Next: Section 8 - Other Considerations When Setting Posted Speed Limits »
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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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