National Academies Press: OpenBook

Design Guidelines for Horizontal Sightline Offsets (2019)

Chapter: Appendix B - Users Guide for Reliability Analysis Tool

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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Appendix B - Users Guide for Reliability Analysis Tool." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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71 A P P E N D I X B Users Guide for Reliability Analysis Tool This appendix presents a users guide for the spreadsheet-based tool that implements the reliability analysis model described in Chapter 5. The tool enables users to assess the horizontal curves with horizontal sightline obstructions present on the inside of the curve. The tool computes the ASSD profile for a horizontal curve and determines the minimum ASSD at any location within the site. The tool also provides measures of the likelihood that drivers on a particular horizontal curve may encounter stopped vehicles in the sight-restricted area. This measure of the annual number of vehicles that may be potentially affected by a sight-distance limitation can be used to prioritize horizontal curves for improvement. The users guide describes the hardware and software needed to run the spreadsheet tool, the entry of input data to the tool, the initiation of calculations, and output data provided. B.1 Hardware and Software Requirements The spreadsheet tool has been developed in Microsoft Excel® with supplementary programming in Visual Basic for Applications (VBA). All input data entry and output displays are in Microsoft Excel®. The spreadsheet tool is available on the TRB website by searching for NCHRP Research Report 910. Once this file has been copied to a user-selected folder on any computer with Microsoft Excel® installed, program execution can be initiated by clicking on the file name. The program should operate without modification on any Windows-based computer system with any version of Microsoft Excel® available at the time of the writing of this report. Program operation on non-Windows-based computer systems has not been verified. All input data entry is performed on a worksheet entitled UserInput. The key output results are also displayed on the worksheet entitled UserInput. One key set of supplementary output results, the ASSD profile for each lane on the roadway analyzed, can be viewed in the worksheet named Profile. Other worksheets can be viewed to review the results of intermediate calculations. All cells in the worksheets are locked except those in which the user is expected to supply input values. B.2 Input Data Entry This section presents the input data that the user is expected to enter to apply the reliability tool to horizontal curves with horizontal sightline obstructions. All of the input data items discussed are entered on the UserInput worksheet.

72 Design Guidelines for Horizontal Sightline Offsets characteristics for the study site are entered in a table on the UserInput worksheet shown in Figure B-1. Figure B-1. Selecting facility type and general characteristics. • Facility type: Select the facility type on which the analysis will be performed using a drop-down menu. The facility types included in the dropdown menu are: - Rural two-lane highway - Rural four-lane undivided highway - Rural four-lane divided highway - Rural freeway (4 lanes) - Rural freeway (6 lanes) - Urban freeway (4 lanes) - Urban freeway (6 lanes) - Urban freeway (8 lanes) - Rural one-lane entrance ramp - Rural one-lane exit ramp - Urban one-lane entrance ramp - Urban one-lane exit ramp - Urban two-lane entrance ramp - Urban two-lane exit ramp • Number of lanes in analysis direction: This value is automatically updated based on the facility type selection and displayed in the tabular format shown in Figure B-1. The number of lanes is for both directions of travel combined, except for ramp sites which are, by definition, one-way roadways. However, on two-way roads, only the lanes in the analysis direction (assumed to be half of the total number of lanes specified here) are analyzed. • Average lane width: Enter the average lane width of the roadway, in feet. • AADT (one direction): Enter the annual average daily traffic volume (veh/day) in the analysis direction. NOTE: If only two-way AADT data are available, enter half of the two-way AADT. B.2.2 Obstruction Type and Vertical Alignment Figure B-2 shows a screenshot of the form used on the UserInput worksheet to specify the obstruction type, the presence of a vertical curve, and the direction of curve selection. Each of these items is discussed. B.2.1 Facility Type The site analyzed by the spreadsheet tool is a roadway of user-specified type and number of lanes containing a horizontal curve. The roadway is analyzed for one direction of travel only. The analysis direction is selected by specifying the direction of curve and the offset distance from the edge of the traveled way to the horizontal sight obstruction. General roadway

Users Guide for Reliability Analysis Tool 73 Figure B-2. Selecting obstruction type and curve type. • Point obstruction: Select this option if a point obstruction, with no longitudinal extent, is present. The analysis of point obstructions is a simplified case that assumes there is no grade over 3 percent present, no vertical curve is present, and that the horizontal sight obstruction is sufficiently tall that a driver cannot see over it. If any of these assumptions is not met for a point obstruction, then analyze the site as a longitudinal obstruction with a very short longitudinal extent. • Continuous obstruction, straight grade: Select this option if a continuous horizontal sight obstruction is present, but no vertical curve is present. • Continuous obstruction, vertical curve present: Select this option if a continuous horizontal sight obstruction is present, and a vertical curve is also present. • Direction of curve: Select the direction of the curve (to the left or to the right) using a drop-down menu. B.2.3 Horizontal Curve and Obstruction Length and Location Input The input data for the horizontal curve and obstruction length and location vary slightly depending on what options were chosen in Figure B-2. B.2.3.1 Point Obstruction Figure B-3 shows a screenshot of the table used on the UserInput worksheet to enter horizontal curve and obstruction length and location data when a point obstruction is being considered. Any point obstruction between Station PC-DSSD and Station PT+DSSD should be entered. Figure B-3. Horizontal curve and obstruction length and location data input for point obstructions.

74 Design Guidelines for Horizontal Sightline Offsets • Curve radius: Enter the horizontal curve radius, in feet. • Curve length: Enter the horizontal curve length, in miles. • Design or operating speed: Enter the design speed or operating speed of the analysis site, in mph. The design speed of the horizontal curve may be used, if available. If the design speed used when the curve was designed is not known or if speed conditions have changed since the curve was designed, enter an operating speed based on the posted speed limit or the measured or estimated 85th percentile speed of traffic. • Longitudinal distance from PC to point obstruction: Enter the distance along the roadway from the PC of the horizontal curve to the point obstruction in miles. Enter a negative value if the point obstruction is located before the PC, zero if the point obstruction is located at the PC, or a positive value if the point obstruction is located after the PC. • Offset from edge of traveled way to sight obstruction: Enter the distance from the point obstruction to the edge of the traveled way on the inside of the horizontal curve, in feet. On a divided highway with a median, the horizontal sight obstruction may be on the roadside on the inside of the curve for a curve to the right or in the median for a curve to the left. On an undivided roadway, either the curve to the right in the primary direction of travel (adjacent to the horizontal sight obstruction) or the curve to the left in the opposing direction of travel (separated from the horizontal sight obstruction by the travel lanes in the primary direction) may be analyzed. In the case of a curve to the left in the opposing direction of travel on an undivided roadway, the offset to the horizontal sight obstruction should be equal to the offset for the primary direction of travel plus the total width of all lanes in the primary direction of travel. B.2.3.2 Continuous Obstruction Figure B-4 shows a screenshot of the table used on the UserInput worksheet to enter horizontal curve and obstruction length and location data when a continuous obstruction is being considered. Any continuous sight obstruction located between Station PC-DSSD and Station PT+DSSD should be entered.

Users Guide for Reliability Analysis Tool 75 Figure B-4. Horizontal curve and continuous obstruction data input. • Curve radius: Enter the horizontal curve radius, in feet. • Curve length: Enter the horizontal curve length, in miles. • Design or operating speed: Enter the design speed or operating speed of the analysis site, in mph. The design speed of the horizontal curve may be used, if available. If the design speed used when the curve was designed is not known or if speed conditions have changed since the curve was designed, enter an operating speed based on the posted speed limit or the measured or estimated 85th percentile speed of traffic. • Longitudinal distance from PC to beginning of continuous obstruction: Enter the distance along the roadway from the PC of the horizontal curve to the beginning of the continuous obstruction, in miles. Enter a negative value if the continuous obstruction begins before the PC, zero if the point obstruction begins at the PC, or a positive value if the point obstruction begins after the PC. If a continuous sight obstruction that extends along the entire roadway, such as a median barrier or a long retaining wall, is present, enter the value of B1 as a negative value equal to -1*DSSD (i.e., DSSD with a leading minus sign). • Longitudinal distance from PC to end of continuous obstruction: Enter the distance along the roadway from the PC of the horizontal curve to the end of the continuous obstruction, in miles. Enter a negative value if the continuous obstruction ends before the PC, zero if the continuous obstruction ends at the PC, or a positive value if the continuous obstruction ends after the PC. If a continuous sight obstruction extends along the entire roadway, such as a median barrier or retaining wall, enter the value of B2 as a positive value equal to the curve length (L) plus the DSSD (i.e., L+DSSD). This is equivalent to a location at a distance DSSD beyond the PT. • Offset from edge of traveled way to sight obstruction: Enter the distance from the continuous obstruction to the edge of the traveled way on the inside of the horizontal curve, in feet. On a divided highway with a median, the horizontal sight obstruction may be on the roadside on the inside of the curve for a curve to the right or in the median for a curve to the left. On an undivided roadway, either the curve to the right in the primary

76 Design Guidelines for Horizontal Sightline Offsets direction of travel (adjacent to the horizontal sight obstruction) or the curve to the left in the opposing direction of travel (separated from the horizontal sight obstruction by the travel lanes in the primary direction) may be analyzed. In the case of a curve to the left in the opposing direction of travel on an undivided roadway, the offset to the horizontal sight obstruction should be equal to the offset for the primary direction of travel plus the total width of all lanes in the primary direction of travel. • Height of obstruction above edge of traveled way: Enter the height of the horizontal sight obstruction as the difference in elevation between the top of the obstruction and the edge of the traveled way on the inside of the horizontal curve, in feet. B.2.4 Vertical Alignment Data Vertical alignment data only needs to be entered for sites that have a continuous horizontal sight obstruction. B.2.4.1 Continuous Obstruction, Straight Grade Figure B-5 shows a screenshot of the table used on the UserInput worksheet to enter vertical alignment data for a site where no vertical curve is present. In this case, the only value to be entered is the percent grade of the roadway. Figure B-5. Vertical alignment data input, no vertical curve present. • Roadway grade: Enter the roadway grade as a percent grade. Enter a negative value for a downgrade or a positive value for an upgrade. For a level roadway, enter 0.00%. B.2.4.2 Continuous Obstruction, Vertical Curve Present Figure B-6 shows a screenshot of the table used on the UserInput worksheet to enter vertical alignment data for a site where a vertical curve is present. Figure B-6. Vertical alignment data input, vertical curve present. • Approach grade: Enter the roadway grade approaching the vertical curve as a percent grade. Enter a negative value for a downgrade or a positive value for an upgrade. For a level roadway, enter 0.00%.

Users Guide for Reliability Analysis Tool 77 • Departure grade: Enter the roadway grade departing the vertical curve as a percent grade. Enter a negative value for a downgrade or a positive value for an upgrade. For a level roadway, enter 0.00%. • Distance from PC to PVC: Enter the distance from the PC of the horizontal curve to the PVC of the vertical curve, in miles. Enter a negative value if the PVC is located prior to the PC, zero if the PVC is located at the PC, or a positive value if the PVC is located after the PC. • Vertical curve length: Enter the vertical curve length, in miles. B.2.5 Advanced Options Several advanced options are available in the spreadsheet tool. Figure B-7 shows a screenshot of the table on the UserInput worksheet to utilize these advanced options. Figure B-7. Advanced options data entry. • Analysis increment: Enter the incremental distance along the roadway for which ASSD will be calculated, in feet. The recommended value is 10 ft. • Use AASHTO values: Select this option to use sight distance default values from the AASHTO Green Book. These dimensions and their AASHTO-specified values are: - Driver eye height: The height of the driver’s eye above the roadway, in feet. This value is set by default to 3.5 ft. - Object in road height: The height of the object in the roadway which the driver needs to see, in feet. This value is set by default to 2.0 ft. - Distance from left edge of lane to driver: The lateral distance from the edge of the lane to the driver’s position in the lane. This value is set by default to half of the lane width. • Use alternative values: Select this option to use values other than the default values from the Green Book. The alternative values may be set by the user in the column labeled “Alternative Values” as follows: - Driver eye height: The height of the driver’s eye above the roadway, in feet. In most cases, this value should not be varied from the default value of 3.5 ft for a passenger car driver. An alternative value of 3.0 ft might be considered in special cases for a driver of a low-profile passenger car, such as a sports car. An alternative value of 8.0 ft might be considered in special cases for a truck driver. - Object in road height: The height of the object in the roadway which the driver needs to see, in feet. AASHTO uses a default value of 2.0 ft, equivalent to the taillight height of a passenger car. Where the object to be seen is definitely a passenger car, with a typical vehicle height of 4.5 ft, consideration should be given to using an

78 Design Guidelines for Horizontal Sightline Offsets object height of 3.5 ft, equivalent to driver eye height, or 4.0 ft. Either of these choices allows for some of the upper portion of the vehicle to be visible to an approaching driver. - Distance from left edge of lane to driver: The lateral distance from the edge of the lane to the driver’s position in the lane. This value is set by default to half of the lane width. A more appropriate alternative value is one-quarter of the lane width. For a site with 12-ft lanes, this results in a driver’s eye position 3 ft from the inside edge of the traveled way for a curve to the left and 9 ft from the edge of the traveled way for a curve to the right. B.2.6 Traffic Data Figure B-8 shows a screenshot of the k-factor selection and k-factor distribution options. Figure B-8. K-factor selection and distributions. • Percentage of traffic in peak hour: Select the k-max value for the analysis site from the dropdown menu. The k-max value is the proportion of AADT in the peak hour of the day. Available options include 0.08, 0.09, 0.10, 0.11, and 0.12. • Default k-factor distributions: Select this option to use the default k-factor distribution for the selected k-max value. • Custom k-factor distributions: Select this option to use a user-supplied k-factor distribution for the selected k-max value. Values for the k factors must be entered for all 24 hours of the day and the k values must sum to 1.00. If the k values in any column do not sum to 1.00, a warning message will be displayed. Enter the k factors in the “Custom” column under the appropriate k-max value, as shown in Figure B-9. Figure B-9. K-factor default values and custom k-factor data entry. Figure B-10 shows a screenshot of the lane utilization data entry. Enter the percentage of directional traffic that is present in each lane. The sum of all lane percentages must equal 100%. If the lane utilization factors do not sum to 100%, a warning message will be displayed.

Users Guide for Reliability Analysis Tool 79 Figure B-10. Lane utilization data entry. B.2.7 Crash Prediction Figure B-11 shows a screenshot of the crash prediction model options and custom data entry. Figure B-11. Crash prediction model options. • Use default SPF coefficients: Select this option to use SPF coefficients from the Highway Safety Manual. • Use custom SPF coefficients: Select this option to enter custom SPF coefficients for use in crash prediction modeling. • Calibration factor: Enter the SPF calibration factor. If no calibration factor for the local jurisdiction is available, use 1.00.

80 Design Guidelines for Horizontal Sightline Offsets • SPF coefficients: The default SPF coefficients are shown on four “Default” rows. The functional form for all SPFs are shown in the equation above the coefficients. Custom SPF coefficients can be entered on the four “Custom” rows, but the custom coefficients must be from SPFs following the same functional form of the equation shown. B.3 Perform Calculations Once all input data have been entered, click the “Calculate” button to perform all calculations for the reliability analysis. The location of the button is shown in Figure B-12. Figure B-12. Location of calculate button. B.4 Results Figure B-13 shows a screenshot of the results of the reliability analysis. Figure B-13. Reliability analysis results. • Minimum available sight distance: The minimum ASSD is shown for each lane, in feet. The column of output results for a particular lane will be green if ASSD is equal to or greater than the applicable DSSD or yellow if the ASSD is less than the applicable DSSD. If the analysis finds that the obstruction does not obstruct the driver’s view of the downstream roadway at any point (i.e., the driver can see over the obstruction), a value of 9999.0 will appear. • Length of sight-restricted roadway: The length of the area, in miles, which is not visible to approaching drivers for the full DSSD. The sight-restricted area is the portion of the roadway with ASSD less than DSSD. • Total number of potentially affected vehicles per year: The total number of vehicles per year that may be potentially affected by a crash-involved vehicle or a queue of stopped vehicles in the sight-restricted area is shown for each lane. • Total number of vehicles passing site per year: The total number of vehicles passing through a site per year is shown for each lane.

Users Guide for Reliability Analysis Tool 81 • Percent potentially affected: The percentage of total vehicles passing through a site that may be potentially affected by a crash-involved vehicle or a queue of stopped vehicles in the sight-restricted area is shown for each lane. • Time and date of last analysis: The date and time of the last analysis is shown and is updated every time the “Calculate” button is clicked. • AASHTO design stopping sight distance: The AASHTO DSSD for the applicable design speed is shown, in feet. B.5 View Calculations Intermediate calculations can be viewed in the Crash and Capacity worksheets. The Crash worksheet shows the computation of the total number of potentially affected vehicles per year per lane if stopped vehicles are present in the sight-restricted area due to a crash. The Capacity worksheet shows the computation of the total number of potentially affected vehicles per year per lane if stopped vehicles are present in the sight-restricted area due to congestion. B.6 View Intermediate Results ASSD values at incremental locations are shown for each lane in the Profile worksheet. The first column shows the driver’s location, in feet, relative to the PC of the horizontal curve.

Next: Appendix C - Case Studies of Existing Roadways with Sight Obstructions »
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The distance between the driver’s line of sight along the roadway ahead on a horizontal curve and a sight obstruction on the inside of the curve is known as the horizontal sightline offset (HSO). Highway agencies can use NCHRP Research Report 910: Design Guidelines for Horizontal Sightline Offsets as guidance to address the types of sight distance restrictions that are most likely to be encountered on specific roadway types.

The relationship between stopping sight distance (SSD) and the frequency and severity of crashes has been difficult to quantify because the role of SSD in reducing crashes is highly situational. The design criteria for the horizontal component of SSD in what is known as AASHTO's Green Book are based on the maximum sightline offset that may be needed at any point along a curve with a given radius, which doesn't cover all possible situations.

Designers compensate for the limitations on driver sight distance in various ways, including: accepting shorter sightlines, lowering design speed, increasing shoulder width, or providing additional signage. There are advantages and disadvantages to the trade-offs; as a result, many highway agencies have used the design exception process to address the trade-offs for sight distance in such situations.

This project conducted research to evaluate these situations and determine what criteria or mitigation will provide acceptable solutions when impaired horizontal sightline offsets are encountered. The project includes a tool (an Excel spreadsheet) that may be used to calculate sight distance.

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