Posted Speed Limit Setting Procedure and Tool: User Guide (2021) / Chapter Skim
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Pages 21-32
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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?
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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.
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Variables for Decision-Making Procedure 23 Table 13. Input variables when crash data are available.
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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.
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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.
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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)
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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.
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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.
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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)
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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)
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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)
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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.
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