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44 This chapter presents recommended procedures to assess removal or mitigation of hori- zontal sight obstructions. The primary procedure is for application to existing sight obstruc- tions. The chapter then discusses adaptation of the procedure to projects under design for new construction. 6.1 Step-by-Step Procedure for Assessing Whether to Remove or Mitigate Existing Horizontal Sight Obstructions This section presents a step-by-step procedure to review and analyze existing horizontal curves with roadside sight obstructions on the inside of the curve. The procedure assumes the site being studied has at most one horizontal and one vertical curve. If this is the case, the site may be evaluated with the spreadsheet tool described in Chapter 5 and Appendix B. If there is more than one horizontal curve or more than one vertical curve present, the spreadsheet tool is not applicable. In that case, perform a comparable review to that described here using a CADD system or another 3D tool. The steps in the analysis process presented here are suggestions. Any step may be skipped if the planner or designer concludes that it would not be productive or that the likely finding is evident from the information already gathered. 1. Locations to be reviewed for potential horizontal sight distance limitations may be identi- fied by many methods including: â Sites that are noted in the planning or design process for a particular project â Sites noted in a field review or a review of street level photography â Sites reported by motorists â Sites identified through network screening to identify high-crash locations â Sites of potential interest identified by any other method 2. Data should be assembled on each identified location including: â Roadway type, number of lanes, and other roadway characteristics â Horizontal curve radius and length â Direction of curve (right/left) â Vertical alignment (percent grade or, if a vertical curve is present, approach grade, departure grade, vertical curve length and location relative to the horizontal curve) â Type of horizontal sight obstruction â Location of sight obstruction relative to the horizontal curve â Height of sight obstruction above the inside edge of the traveled way (ft) â Distance to sight obstruction from the inside edge of the traveled way (ft) â Directional traffic volume (veh/day) C H A P T E R 6 Assessing Removal or Mitigation of Horizontal Sight Obstructions
Assessing Removal or Mitigation of Horizontal Sight Obstructions 45 â Design speed of horizontal curve from as-built plans (mph), if available and if still applicable to current conditions â Posted speed limit (mph) upstream of the horizontal curve (in the case of exit ramps, use the posted speed on the mainline roadway) â Signed advisory speed (mph), if any â Average lane width (ft) 3. Apply the spreadsheet tool described in Chapter 5 and Appendix B to generate the ASSD profile for the site and compare the minimum ASSD to the DSSD for applicable speed(s), including the design speed and/or posted speed limit for the roadway upstream of the horizontal curve and the signed advisory speed, if any. If there is more than one applicable speed to consider, the spreadsheet tool should be applied for each applicable speed, keeping all input data except the speed unchanged. The same ASSD profile will be generated for each application of the tool, but the minimum ASSD will be compared in each run to a different DSSD value. 4. For each applicable speed, consider not only the default sight distance measurement rules based on AASHTO Green Book criteria, but also appropriate variations in the sight distance measurement rules. These might include: â Consideration of an alternative object height of 3.5 or 4.0 ft if it appears likely that an approaching driver could see over the horizontal sight obstruction. â Consideration of an alternative placement of the vehicle within its lane with the driverâs eye 3 ft from the left edge of the curve. â Consideration of an alternative driver eye height of 8.0 ft if there is a substantial volume of large trucks at the site. 5. Review the sight distance profile(s) to determine whether the minimum ASSD is less than the DSSD for any of the applicable speed(s). If the minimum ASSD is greater than the DSSD for all speeds of interest, there is no horizontal sight restriction at the site and there is no need to continue the investigation further. Any crash patterns or other concerns at the site likely have a cause other than lack of horizontal sight distance and other appropriate engineering studies should be conducted. 6. Determine the location and length of the sight-restricted area for each applicable speed. The length of the sight-restricted area is given in the output results portion of the spread- sheet. The location of the sight-restricted area can be determined by noting all ASSD values in each sight distance profile that are less than the applicable DSSD value. 7. If the minimum ASSD value is less than the applicable DSSD value for some applicable speeds but not for others, conduct a further investigation of actual operating speeds at the site. The 85th percentile speed of traffic is typically used as a representative value. Either make an estimate of operating speed or conduct a speed study at the site. If there is a signed advisory speed on the curve, do not assume that the operating speed is equal to the signed advisory speed unless field observation or field studies indicate that this is the case. 8. Determine whether critical roadway features are located in or near the sight-restricted area. Critical roadway features could include at-grade intersections, driveways, additional horizontal curves, ramp terminals, or pedestrian crossings. In the case where an inter- section or a stop- or signal-controlled ramp terminal is present, check whether the Green Book design value for ISD, which is greater than DSSD, is available. If a free-flow ramp terminal or a midblock pedestrian crossing is present, check whether the Green Book design value for decision sight distance, which is also greater than DSSD, is available. 9. Review the crash history of the site, focusing on crashes in or near the sight-restricted area. Identify crashes of types that may possibly be related to limited sight distance including rear-end, same-direction sideswipe, and run-off-road crashes. Where practical, review hard-copy police crash reports and note crashes for which there is no evident explanation of
46 Design Guidelines for Horizontal Sightline Offsets the causation of the crash other than limited sight distance. Crashes of the types considered possibly related to sight distance should be eliminated from consideration if: â The crash location was misreported, and the crash did not occur at the site in question. â The crash occurred in a lane, which the spreadsheet tool indicates does not have a sight distance limitation. â The crash can be verified as occurring at a location not within or immediately adjacent to the sight-restricted area. â The crash was clearly caused by other factors such as mechanical failure of the at-fault vehicle, loss of control due to a wet or snow-and-ice-covered roadway, limited visibility due to weather, or an animal entering the roadway. â The at-fault driver stated explicitly that the crash was caused by their inattention to the driving task. The remaining crashes not eliminated by the above criteria are considered potentially, but not definitely, related to limited sight distance. If the hard-copy police crash report states explicitly that the crash was caused by limited sight distance, which is rare because law enforcement officers are not trained to use engineering terms, the crash should not be eliminated from consideration, even if one of the listed factors is present. Considering the crashes not eliminated in this step, determine whether there is a pattern of crashes potentially related to sight distance. The definition of a crash pattern is left to each indi- vidual agency to determine based on traffic volumes and other factors, but clearly a pattern consists of multiple crashes. 10. In the results from the spreadsheet tool analysis, note the number and percentage of approaching vehicles per year potentially affected by the sight distance restriction. If the number of vehicles potentially affected is high, this may indicate that removing or mitigat- ing the sight distance restriction may be desirable. If the percentage of approach vehicles potentially affected by the sight distance restriction is also high (e.g., 0.5 percent or more), removing or mitigating the sight distance restriction may be desirable. 11. If it appears that removing or mitigating the sight distance restriction would be desirable, estimate the cost to remove the sight obstruction(s). First, identify whether all obstacles present on the inside of the curve are, in fact, obstructions that may need to be removed. The proce- dures developed by Mauga (2014, 2015b) and presented in Appendix A can be used to define the area that should be clear of sight obstructions. The reliability analysis model, presented in Chapter 5, and the spreadsheet tool to implement the reliability analysis model, presented in Appendix B, can be used to determine whether the driver will be able to see over an obstruction. 12. Perform an economic analysis to determine whether it is likely to be cost effective to remove the sight obstruction. Use Equation (11) to determine the maximum implementation cost that would be cost effective. To be conservative, the benefit should be the average number of crashes per year found to be potentially related to sight distance in Step 9 or an alternative estimate that the planner or designer considers more realistic. If the cost to remove the sight obstruction is less than the maximum implementation cost that would be cost-effective [maxICij in Equation (11)], this indicates that removal of the sight obstruction is an eco- nomically viable alternative. 13. Considering all of the data and findings assembled in Steps 2 through 12, develop a recom- mendation as to whether the sight obstruction should be removed. Removal of the sight obstruction is indicated if ASSD is less than the applicable value of DSSD for the speed determined in Step 7, if removal of the sight obstruction was found to be cost-effective in Step 12, if the funds to remove the sight obstruction are available and are approved by agency management, and if one or more of the following were also found: â There are critical features such as intersections, driveways, additional horizontal curves, ramp terminals, or pedestrian crossings, located in or near the sight-restricted area. Where applicable, as explained in Step 8, consider ISD or decision sight distance.
Assessing Removal or Mitigation of Horizontal Sight Obstructions 47 â There is a documented pattern of crashes potentially related to sight distance, as defined in Step 9. â The values from the spreadsheet tool for both the number and percentage of approach- ing vehicles potentially affected by the sight distance restriction are relatively high, as discussed in Step 10. 14. If the recommendation reached in Step 13 was not to remove the sight obstruction, con- sider whether specific mitigation strategies should be implemented. Review the table in Section 7.2 and identify candidate mitigation strategies to determine which strategies can be implemented for less than the maximum implementation cost determined in Step 12. For each of the candidate strategies identified, review the description of the strategy in Section 7.2 to assess its appropriateness for the site in question. Develop a recommendation as to which candidate strategy (or strategies), if any, should be implemented. 6.2 Assessment of Horizontal Sight Obstructions Under Design for New Construction The step-by-step procedure for existing sight obstructions presented in Section 6.1 can be adapted to consider sight obstructions under design for new construction. The procedure should be adapted as follows: â¢ In Step 1, the only method to identify horizontal sight limitations of interest may be from project planners and designers. â¢ There will be no crash history to review in Step 9. â¢ No economic analysis can be performed in Step 12 unless the designer is able to estimate potential future crash reduction benefits of removing a sight obstruction using some method other than crash history review. Alternative methods might include past experience with similar sites. Many potential horizontal sight obstructions may be less expensive to remove at the design stage than on an existing roadway. However, where the horizontal sight obstruction is created by a bridge structure or retaining wall, removal of the horizontal sight obstruction may be very expensive, even at the design stage, and consideration of mitigation measures may be more appropriate. 6.3 Case Studies of Existing Roadways with Horizontal Sight Obstructions Appendix C presents seven case studies for existing roadways with horizontal sight obstruc- tions. The seven horizontal curve sites are located in the states of Illinois, Kansas, Pennsylvania, and Washington, and were selected from among 263 horizontal curve sites reviewed in research by Potts et al. (2018). The case studies include two rural two-lane highway sites, two urban main- line freeway sites, one rural mainline freeway site, and two interchange ramp sites. The lessons learned from the case studies include: â¢ Where crash experience is limited, high-cost sight distance improvements are not likely to be practical. â¢ In many cases, only the inside lane on a horizontal curve will experience a horizontal sight distance limitation. â¢ On some curves, a sight obstruction on the inside of a horizontal curve may limit the ASSD in both directions of travel. The ASSD for traffic in the direction of travel on the outside of the curve is likely to be only slightly limited unless the curve radius is very small and/or the sight obstruction on the inside of the curve is very close to the traveled way.
48 Design Guidelines for Horizontal Sightline Offsets â¢ Depending on the location and offset of the horizontal sight obstructions, sight distance limitations may extend upstream of a horizontal curve and may end before the PT of the curve is reached. â¢ Advisory speed signing is useful for warning drivers of the need to slow down, but does not necessarily result in drivers traveling at or below the advisory speed. â¢ Where multiple horizontal sight obstructions are present, removing even one of those sight obstructions may substantially reduce the resulting sight distance restriction, without the need to remove both sight obstructions. This approach may be desirable where the sight distance with the greater height is less expensive to remove (e.g., trees or bushes behind a roadway structure or roadside barrier). â¢ Even where the conditions for sight-distance-related collisions appear to exist, crashes will not necessarily occur. Drivers are often very effective at adapting to conditions they encounter in the field. â¢ More realistic positioning for the driverâs eye closer to the outside of the travel lane, rather than along the centerline of the lane, on a horizontal curve to the right will increase the sight distance for that direction of travel. However, the opposite is true for the curve to the left, where the available sight distance may decrease if measured with more realistic positioning of the driverâs eye. â¢ Where the upper portion of vehicles can be seen over a barrier on the inside of a horizontal curve, the sight distance on the curve may be effectively adequate, even where sight distance measures using AASHTO criteria appears limited.