Step 1. Determine Roadway Context and Type The SLS-Procedure starts with determining the roadway context and type. Roadway context includes Rural, Rural Town, Suburban, Urban and Urban Core and roadway type includes Interstates/Freeways/Expressways, Principal Arterials, Minor Arterials, Collectors, and Locals . Based on the roadway context and type, the speed limiting setting group (SLSG) a facility belongs to can be identified using the table shown in Table 1. For facilities within the same SLSG, a similar decision process would be employed to determine the speed limit in the following steps.
Table 1: Suggested SLSGs based on roadway context and type
Step 2: Determine Speed Distribution and Crash Rate 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, including:
• 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).
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).
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