Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
33Â Â Overview of SLS-Tool Requirements The SLS-Tool is designed to help practitioners assess and establish consistent speed limits for segments of streets and highways. The tool combines customary engineering studies with context-sensitive considerations to identify appropriate speed limits. The engineering studies typically include evaluating criteria such as 85th percentile speed, traffic volume, number of access points, bicyclist activity, pedestrian activity, crash history, and others. The SLS-Tool is designed to produce an unbiased and objective suggested speed limit value based on the 50th and 85th percentile speed, roadway characteristics, and safety. The SLS-Tool is an ExcelÂ®-based spreadsheet program that provides an objective suggested speed limit that traffic engineers can use to communicate with the public or government offi- cials to explain the methodology behind setting speed limits. The tool provides the rationale for setting the speed limit based on key site characteristics, including the statutory speed limit, the distribution of traffic speed, site characteristics, and crash data. Two versions of the SLS-Tool are available: â¢ N17-76 SLS-Tool (macro). â¢ N17-76 SLS-Tool (no macro). The N17-76 SLS-Tool (macro) uses macro code to display the required data input cells for the specified roadway context and type. This worksheet contains a single analysis worksheet that is used for all SLSGs. The macro code displays only the required data entry rows for the roadway context and type specified by the user. The macro code also includes control buttons that allow the user to clear the data from the Analysis worksheet or populate the data entry cells with a set of default values. When the user opens the tool, ExcelÂ® may display a security message indicat- ing that macro code has been disabled. The user must click the âEnable Contentâ button that appears in a yellow ribbon on the top of the screen. It may also be necessary to check the macro security settings as follows: 1. Select âFileâ in the upper ribbon. 2. Select âOptions.â 3. Select âTrust Center.â 4. Click the âTrust Center Settingsâ button. 5. Select âMacro Settings.â 6. If the option of âDisable all macros without notificationâ is selected, select a different option and click the âOKâ button. The other version of the SLS-ToolâN17-76 SLS-Tool (no macro)âdoes not use macro code. This version is available for users who are not able to use macro codes on their computers. The no-macro version contains one analysis worksheet for each SLSG (Limited-Access, Developed, S E C T I O N 6 Speed Limit Setting Tool
34 Posted Speed Limit Setting Procedure and Tool: User Guide Undeveloped, and Full-Access), and the user must select the appropriate worksheet for each analyzed segment. This version does not provide control buttons to clear data or populate the data entry cells with default values. Data Entry The main data entry area is located in columns AâF of the worksheet. The data entry area is organized with boxes for the following data categories: â¢ Site Description Data: Enter basic roadway characteristics such as the roadway context and type, indicate whether crash data are available for the analysis, and enter optional information such as the userâs name, analysis date, and roadway segment location. â¢ Analysis Results: The SLS-Tool provides the roadway group (Limited-Access, Developed, Undeveloped, or Full-Access) based on the specified roadway context and type, and displays the suggested speed limit. â¢ Speed Data: Enter the maximum (statutory) speed limit and the needed speed distribution values. The 50th percentile speed is needed for all roadway groups, and the 85th percentile speed is needed for all roadway groups except Full-Access. â¢ Site Characteristics: Enter data to specify the segment length, AADT (two-way total), number of lanes, and other attributes describing the segmentâs design and traffic control characteristics. â¢ Crash Data: If crash data are available, enter data to specify the time period, traffic volume, and crash counts. Most of the cells in the SLS-Tool are locked to prevent the user from altering equations and obtaining inaccurate results. Data entry cells are unlocked, and many of the cells have drop- down menus that contain the valid entry options for the cell. For example, the roadway context cell is limited to the categories listed in FigureÂ 7 (Rural, Rural Town, Suburban, Urban, and Urban Core). The data entry cells are color coded to help the user understand the type of data needed. The following colors are used: â¢ Aqua: basic input cell. â¢ Denim: basic input cell with drop-down menu. â¢ Orange: optional input cell (not needed for calculations). These cells include the userâs name, analysis date, roadway name and description, current speed limit, and notes. The user may enter this information for documentation purposes if desired. â¢ Green: optional input cell. These cells contain values that are used for calculations but should be left blank if values are not available. Specifically, the user may enter average crash rates for segments like the one being analyzed, but the SLS-Tool can also estimate average crash rates if the user lacks data to provide average crash rates. â¢ Rose: intermediate calculations. â¢ Purple: final analysis results (specifically, the suggested speed limit). â¢ Yellow: calibration coefficient or policy value. The user should change these cells only based on actual data (e.g., crash rates for specified roadway types) or documented policies (e.g., statutory minimum and maximum speed limits). Select values used in the analysis calculations are in the âSupport Tablesâ worksheet. That worksheet includes the assumed values for minimum segment lengths by speed limits, upper and lower speed limits by roadway group, SLSGs by roadway type and roadway context, and HSIS crash rates. Intermediate calculation cells are located to the right of the data entry area. Users will not need to use these cells.
Speed Limit Setting Tool 35Â Â A legend is provided on the top portion of the main data entry area to summarize the color- coding patterns. A button labeled âClear all dataâ is also provided to allow the user to clear input data and restart the analysis with a blank worksheet. When this button is clicked, a message box appears and asks, âClear all input data?â Click yes to clear the data or no to cancel the operation. After clicking yes, a second message box appears and asks, âEnter default values into data entry cells?â Click yes to populate the data entry cells with default values or no to leave the cells blank. Advisory, Calculated, or Warning Messages The SLS-Tool checks for several conditions and issues messages as needed. TableÂ 20 lists the conditions along with the advisory, calculated, or warning messages. These messages are color coded to indicate the message types as follows: â¢ Advisory message: blue font, used to call attention to issues that are not errors but could be improved. â¢ Calculated message: purple font, used to describe calculation results. â¢ Warning message: red font, used to call attention to erroneous input data. Condition Message Missing required data Enter values for all variables marked with O. (An O will appear to the right of empty input cells.) Missing roadway context or roadway type Specify roadway context and roadway type in cells B5 and B6. Completed calculations This value is determined by <x>. (The quantity x is specified as the maximum speed limit, speed data, site characteristics, and/or crash data, depending on which variables governed the setting of the speed limit.) Completed calculations but with maximum speed limit out of range (too high) The calculated value exceeds the upper value for this speed limit setting group; therefore, the suggested speed limit reflects the assumed upper value. Completed calculations but with maximum speed limit out of range (too low) The calculated value is below the lower value for this speed limit setting group; therefore, the suggested speed limit reflects the assumed lower value. Maximum speed limit out of range (too high) The assumed upper value for this speed limit setting group is <max> mph. Maximum speed limit out of range (too low) The assumed lower value for this speed limit setting group is <min> mph. 50th percentile speed is greater than 85th percentile speed The 85th percentile must be greater than the 50th percentile. 85th percentile speed is only 1 mph greater than 50th percentile speed (suggesting a very tight speed distribution) The 85th percentile is only 1 mph greater than 50th percentile. Interpret results with caution. Segment length < Minimum_Segment_Length For a suggested speed limit of x mph, minimum segment length = y mi. Adverse alignment present Consider location-specific advisory speed warnings. Less than 1 year of crash data Calculations based on 1 year of crash data or less and should be interpreted with caution. Less than 3 years of crash data Consider collecting at least 3 years of crash data. Average crash rates are greater than computed critical crash rates Critical rates should be higher than average rates. The entered number of KABC crashes is greater than the entered number of KABCO crashes The number of KABC crashes must be less than or equal to the number of KABCO crashes. Crash rates are calculated from input data Observed/average KABCO/KABC crash rate = x crashes/100 MVM. (For average crash rates, the message will also specify âfrom Userâ if the user provided the rate or âfrom HSISâ if the user did not provide the rate.) Input data value justifies lowering the speed limit below the closest 85th percentile value Rounded-down 85th, closest 50th, or rounded-down 50th percentile value. TableÂ 20. SLS-Tool advisory, calculated, or warning messages.
36 Posted Speed Limit Setting Procedure and Tool: User Guide SLSG Upper Speed Limit Checka Lower Speed Limit Check Limited-Access Depends on the state. The SLS-Tool has 85 mph as the upper limit because it is the highest currently allowed in the United States. 50 Undeveloped Depends on the state. The SLS-Tool has 70mph. 25 Developed 55 25 Full-Access 30 15 aUse the maximum speed provided by the user if the user-provided speed is lower than the value in this table. TableÂ 21. Upper and lower speed limit checks by Speed Limit Setting Group. Several messages refer to the upper and lower speed limit values for the relevant roadway group. TableÂ 21 provides these upper and lower values. The values can be altered in the yellow table in the âCalibration Tablesâ worksheet if needed. For example, if the segment of interest is an undeveloped facility in a jurisdiction that has a maximum speed limit of 75Â mph for these types of facilities, then the user can enter 75Â mph into the appropriate cell in the yellow table. How to Handle Situations When Data Are Not Available for One of the Variables Some of the variables are fundamental quantities that must be provided for all analysis cases. These variables include roadway context and roadway type. For all other variables, default values can be used if actual data are not available. TableÂ 22 shows these values for speed- and geometric- related variables, and TableÂ 23 shows values for crash-related variables. Variable Roadway Group Default Value 50th percentile speed All Maximum speed limit â 5 mph 85th percentile speed Limited-Access, Developed, or Undeveloped Maximum speed limit â 2 mph AADT (two-way total) Developed 30,000 veh/d AADT (two-way total) Full-Access 10,000 veh/d AADT (two-way total) Limited-Access (roadway context = rural) 25,000 veh/d AADT (two-way total) Limited-Access (roadway context = urban) 60,000 veh/d AADT (two-way total) Undeveloped 15,000 veh/d Angle parking present Developed or Full-Access No Bicyclist activity Developed or Full-Access Not high Design speed Limited-Access â¥ 60 mph Directional design-hour truck volume Limited-Access 200 trucks/hr Grade Limited-Access 0% ISW Limited-Access 6 ft Lane width Undeveloped 12 ft Maximum speed limit All See Table 21 Median type Developed or Full-Access Divided Median type Undeveloped Divided Number of access points Developed 40 access points Number of access points Full-Access 60 access points Number of access points Undeveloped 15 access points Number of interchanges Limited-Access 7 (1 interchange/mi Ã 7 mi) Number of lanes Developed, Undeveloped, or Full- Access 4 lanes Number of lanes Limited-Access 6 lanes TableÂ 22. Input data default values for speed and geometric-related variables.
Speed Limit Setting Tool 37Â Â The default values are chosen to reflect ideal conditions. That is, a site with conditions equal to the default values will have its speed limit set based on the closest 85th percentile speed. The user must enter any data values that deviate from ideal conditions, which may result in setting the speed limit based on a lower speed. TableÂ 23. Input data default values for crash-related variables. Variable Roadway Group Default Value Crash data availability All Yes Number of years of crash data All 3 years Is the segment a one-way street? Developed or Full-Access No Average AADT (two-way total) for crash data period All Same as AADT for site characteristics data All (KABCO) crashes for crash data period All Number needed to yield a crash rate equal to 1/3 that of the HSIS-based average rate Fatal and injury (KABC) crashes for crash data period All Number needed to yield a crash rate equal to 1/3 that of the HSIS-based average rate TableÂ 22. (Continued) Variable Roadway Group Default Value Number of traffic signals Developed 3 signals Number of traffic signals Full-Access 8 signals On-street parking activity Developed or Full-Access Not high Outside SW Limited-Access 10 ft Parallel parking permitted? Developed No Pedestrian activity Developed or Full-Access Negligible Segment length Developed or Full-Access 1 mi Segment length Limited-Access or Undeveloped 7 mi SW Undeveloped 10 ft Sidewalk buffer Developed or Full-Access Present Sidewalk presence/width Developed or Full-Access Adequate