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Design Guidelines for Horizontal Sightline Offsets (2019)

Chapter: Appendix C - Case Studies of Existing Roadways with Sight Obstructions

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Suggested Citation:"Appendix C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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 C - Case Studies of Existing Roadways with Sight Obstructions." 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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82 A P P E N D I X C Case Studies of Existing Roadways with Sight Obstructions This appendix presents case studies of seven horizontal curve sites selected from among the 263 field sites reviewed by Potts et al. (2018). These case studies address horizontal curves with limited SSD for a range of roadway types, directions of curvature, curve radii, and sight obstruction types. Each case study addresses the characteristics of the site, available sight distance profiles, traffic operational performance, safety performance, mitigation challenges, and lessons learned. The case study sites are located in the states of Illinois, Kansas, Pennsylvania, and Washington. The case study sites were selected without reference to crash history data. The objective was to choose case study sites that represented a range typical of site conditions, not to select locations that were known to be low crash or high crash sites. The case studies include: • Case Study 1—Site IL009—Curve to the Right on a Rural Two-Lane Highway • Case Study 2—Site IL035—Curve to the Right on an Urban Interchange Ramp • Case Study 3—Site KS009—Curve to the Left on an Urban Interchange Ramp • Case Study 4—Site KS025—Curve to the Right on a Rural Two-Lane Highway • Case Study 5—Site PA002—Curve to the Left on a Rural Mainline Freeway • Case Study 6—Site WA082—Curve to the Left on an Urban Mainline Freeway • Case Study 7—Site WA091—Curve to the Right on an Urban Interchange Ramp Each case study is presented below. C.1 Case Study 1—Site IL009—Curve to the Right on a Rural Two- Lane Highway Site IL009 consists of a horizontal curve on a rural two-lane undivided highway. The primary study direction for this horizontal curve is eastbound because, in the eastbound direction of travel, the curve is to the right and traffic is immediately adjacent to the horizontal sight obstruction on the inside of the curve. The curve has a radius of 819 ft and is 0.15 mi in length. The sight obstruction on the inside of the horizontal curve consists of a continuous row of trees (essentially a forest) along the entire length of the curve. The line of trees extends beyond both ends of the curve by 0.08 mi. The trees begin at an offset of 7 ft from the inside edge of the traveled way. The rural two-lane highway has a speed limit of 55 mph; there is a signed advisory speed of 50 mph on the curve. The two-way AADT on the roadway is 6,750 veh/day.

Case Studies of Existing Roadways with Sight Obstructions 83 Figure C-1. Photograph of the rural two-lane highway curve for Case Study 1. Figure C-2. Two-lane highway alignment showing the position of the horizontal sight obstructions. C.1.1 Sight Distance Profile The minimum ASSD at this location is 292 ft, as compared to the DSSD of 495 ft for a 55-mph roadway, or a DSSD of 425 ft for a 50-mph roadway. There is a railroad bridge abutment with an offset from the edge of traveled way of 2 ft, located approximately 250 ft upstream of the PC of the curve. However, the limitation on sight distance imposed by the bridge abutment does not create an ASSD less than the DSSD of 425 ft. Figure C-3 shows a sight distance profile for the primary direction of travel (eastbound) at the site. The figure shows that ASSD becomes less than the DSSD approximately 250 ft upstream from the PC and decreases to the minimum of 292 ft at the PC. The sight distance continues at its minimum value until 275 ft from the PT. At this point ASSD rapidly increases. Figure C-1 shows a photograph of the curve, showing the trees on the inside of the horizontal curve. More details are presented in the sight distance profile discussion. Figure C-2 presents a drawing of the roadway curve showing the position of the trees.

84 Design Guidelines for Horizontal Sightline Offsets The trees on the inside of the curve (adjacent to the eastbound direction of travel) also obstruct the view of westbound vehicles slightly. Figure C-4 shows a sight distance profile for the westbound direction of travel. The minimum ASSD in the westbound direction of travel is 406 ft, which is just below the DSSD of 425 ft. In fact, if the line of trees were 3 ft further from the traveled way on the inside of the curve, there would be no limited sight distance in the westbound direction. Figure C-3. Sight distance profile for the primary (eastbound) direction of travel at the rural two-lane highway site for Case Study 1. The reliability analysis model presented in Chapter 5 was applied to consider the effect of alternative sight distance measurement assumptions. For the eastbound or primary direction of travel, if the driver’s eye is positioned 3 ft from the left edge of the travel lane, the ASSD increases to 324 ft. However, the ASSD for the westbound direction would decrease to 380 ft if the driver’s eye were positioned 3 ft from the left edge of the westbound travel lane. These alternative assumptions are more realistic in that the driver’s eye is positioned a quarter of the way across the lane, rather than at the center of the lane. However, even with these alternative assumptions, the ASSD is less than the DSSD for speeds of 50 and 55 mph in both directions of travel. C.1.2 Traffic Operational Performance The AADT of the rural two-lane highway at this site is 6,750 vehicles per day. This site operates well below capacity. 0 100 200 300 400 500 600 700 800 -500 -300 -100 100 300 500 700 900 Av ai la bl e Si gh t D ist an ce (ft ) Longitudinal Distance Along Roadway Available SD EB (ft) Design SSD at 55 mi/h Design SSD at 50 mi/h PC PT

Case Studies of Existing Roadways with Sight Obstructions 85 Figure C-4. Sight distance profile for the secondary (westbound) direction of travel at the rural two-lane highway site for Case Study 1. C.1.3 Safety Performance There was just one sight-distance-related crash that occurred at this site during the 5-year study period in the primary direction of travel. The crash was a rear-end collision that resulted in property damage only. Thus, there is no indication that the sight distance limitation at this site results in any pattern of sight-distance-related crashes. C.1.4 Mitigation Challenges The only logical mitigation measure for application at this site is the clearing of trees on the inside of the curve. The tree line would need to be pushed back by an additional 3 ft to remove the sight distance limitation for the westbound direction of travel or by 15 ft to remove the sight distance limitation for the primary or eastbound direction of travel. Given the density of trees at this site, this would involve a relatively high cost. The trees appear to be outside the right-of- way, so either additional right-of-way or a sight easement would need to be acquired. The limited crash experience provides no indication that a sight distance improvement is needed. C.1.5 Lessons Learned There are several lessons that can be learned from Case Study 1. These include: 1. A sight obstruction on the inside of the horizontal curve on a two-lane undivided highway may limit the ASSD in both directions of travel on some curves. The ASSD for traffic in 0 100 200 300 400 500 600 700 800 -500 -300 -100 100 300 500 700 900 Av ai la bl e Si gh t D ist an ce (ft ) Longitudinal Distance Along Roadway Available SD WB (ft) Design SSD at 55 mi/h Design SSD at 50 mi/h PC PT

86 Design Guidelines for Horizontal Sightline Offsets 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. 2. Where crash experience is limited, high-cost sight distance improvements are not likely to be practical. C.2 Case Study 2—Site IL035—Curve to the Right on an Urban Interchange Ramp Site IL035 consists of a curve to the right on an urban interchange ramp. The ramp in question is an outer connection off-ramp in a full cloverleaf interchange on an urban freeway. The curve is located approximately 0.02 mi downstream from the gore area at which point the ramp departs from the mainline freeway. The ramp consists of a single travel lane with paved shoulders. The curve has a radius of 275 ft and is 0.03 mi in length. The width of the ramp traveled way is 16 ft. The sight obstruction on the inside of the horizontal curve consists of trees and a concrete traffic barrier that begin 0.014 mi downstream of the PC of the curve and continue to the PT. The sight obstruction is located 12 ft from the inside edge of the ramp traveled way. The mainline freeway has a speed limit of 55 mph, while the ramp curve has a signed advisory speed of 25 mph. No speed studies are available, but traffic frequently exceeds the 25 mph advisory speed. The mainline freeway carries a two-way AADT of 149,700 veh/day. The AADT of the ramp is 3,700 veh/day. Figure C-5 shows a photograph of the ramp, illustrating that the trees and the roadside barrier substantially limit the driver’s view of the roadway ahead. More details are presented in the sight distance profile discussion. Figure C-6 presents a drawing of the ramp curve showing the position of the horizontal sight obstruction. C.2.1 Sight Distance Profile The minimum ASSD on the ramp is 220 ft, as determined with the AASHTO Green Book sight distance measurement assumptions. The minimum ASSD is substantially less than the AASHTO DSSD for the mainline posted speed limit of 55 mph, but slightly exceeds the DSSD for the signed advisory speed of 25 mph. The minimum ASSD is between the AASHTO DSSD values for 30 and 35 mph. However, this indicates that vehicles traversing the ramp curve at speeds above 35 mph may not have sufficient SSD. Figure C-7 shows the sight distance profile for the ramp curve. The figure indicates that the ASSD becomes less than the DSSD for the posted speed limit on the freeway approximately 240 ft upstream of the gore area and continues to decrease until reaching the minimum ASSD value of 220 ft approximately 35 ft upstream of the PC. The ASSD continues at approximately its minimum value past the PC and opens up to essentially unlimited sight distance by 35 ft downstream of the PC. The point at which the driver’s view opens up to essentially unlimited sight distance is approximately 40 ft upstream of the beginning of the sight obstruction. At this point, the trees no longer limit the driver’s view and the driver can see over the roadside barrier. Thus, the area with limited ASSD is entirely upstream of the sight obstruction.

Case Studies of Existing Roadways with Sight Obstructions 87 Figure C-5. Photograph of the ramp curve for Case Study 2. Figure C-6. Ramp alignment showing the position of the horizontal sight obstructions. The reliability analysis model presented in Chapter 5 was applied to consider the effect of alternative sight distance measurement assumptions. If the driver’s eye were positioned 4 ft from the left edge of the ramp traveled way (i.e., one quarter of the way across the traveled way) and

88 Design Guidelines for Horizontal Sightline Offsets the object height to be seen in the roadway was increased to 3.5 ft, the ASSD would increase to 248 ft, which is still substantially lower than the DSSD for the mainline freeway speed. Figure C-7. Sight distance profile for the existing condition at the ramp site for Case Study 2. C.2.2 Traffic Operational Performance The mainline freeway adjacent to the ramp is an urban 6-lane freeway with an AADT upstream of the ramp diverge of 149,700 veh/day. The mainline freeway operates above capacity for part of the day. The off-ramp has an AADT of 3,700 veh/day. Congestion on the off ramp is unlikely unless a crash occurs or a bottleneck is located downstream. C.2.3 Safety Performance Seven crashes of types that are potentially related to sight distance occurred on or near the curve of interest during a 5-year period. Based on the reported latitude and longitude coordinates for the crashes, only two of these seven crashes occurred in the area with limited sight distance. One of these crashes was a severe-injury overturning crash that occurred at the gore area, while the other was a property-damage-only crash just downstream of the PC of the curve that involved running off the road and striking a fixed object. There is no definitive evidence that either of these crashes was attributable to limited sight distance, but this is a possibility. The reported locations for the other five crashes were in the downstream portion of the ramp curve where sight distance is not limited. 0 100 200 300 400 500 600 700 800 -500 -400 -300 -200 -100 0 100 200 Av ai la bl e St op pi ng S ig ht D ist an ce (ft ) Longitudinal Distance Along Ramp (ft) Available SSD Design SSD at 25 mi/h Design SSD at 55 mi/h Gore Point PC Begin Obstruction PT

Case Studies of Existing Roadways with Sight Obstructions 89 C.2.4 Mitigation Challenges The simplest and lowest cost mitigation measure for this site has already been implemented— signing of a 25-mph advisory speed. While signing of an advisory speed warns drivers of the need to slow down, it does not necessarily reduce travel speeds to 25 mph. Another possible low-cost mitigation measure would be to post a flashing beacon activated when a vehicle speed over 25 or 30 mph is detected. However, this mitigation measure may not be appropriate at a location this close to a key decision point, the gore area of a freeway off-ramp. There are two sight obstructions at this site: a line of trees and a 2.5-ft tall concrete barrier that is situated at the base of the trees. An obvious mitigation measure would be to remove the trees. This would increase the minimum ASSD from 220 to 252 ft. As shown in Figure C-8, the available sight distance at a point about 10 ft upstream of the gore point immediately becomes unlimited because drivers can see over the concrete barrier. This mitigation measure would have low to medium cost depending on the difficulties involved in tree removal at the site. If the alternative assumptions of the object height to be seen in the roadway is 3.5 ft, ASSD is not affected by the concrete barrier. Figure C-8. Sight distance profile with trees removed at the ramp site for Case Study 2. A high-cost mitigation measure would be realignment of the ramp to increase the radius of curvature and/or move the traveled way further from the sight obstructions. The crash history of the site does not suggest the need for any high-cost mitigation measure of this type. 0 100 200 300 400 500 600 700 800 -500 -400 -300 -200 -100 0 100 200 Av ai la bl e St op pi ng S ig ht D ist an ce (ft ) Longitudinal Distance Along Ramp (ft) Available SSD Design SSD at 25 mi/h Design SSD at 55 mi/h Gore Point PC Begin Obstruction PT

90 Design Guidelines for Horizontal Sightline Offsets C.2.5 Lessons Learned There are several lessons that can be learned from Case Study 2. These include: 1. 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. 2. 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. 3. Where multiple horizontal sight obstructions are present, removing even one of those sight obstructions (in this case the sight obstruction with greater height) may substantially reduce the resulting sight distance restriction. At the case study site, the removal of the trees would substantially reduce the area of restricted sight distance, because drivers can more readily see over a 2.5-ft concrete barrier. C.3 Case Study 3—Site KS009—Curve to the Left on an Urban Interchange Ramp Site KS009 consists of a curve to the left on an urban interchange ramp. The ramp in question is a parclo loop off ramp within a partial cloverleaf (folded diamond) interchange on an urban freeway. The curve begins approximately 0.21 mi downstream from the gore area, at which point the ramp departs from the mainline freeway and ends approximately 0.08 mi upstream from the at-grade crossroad ramp terminal. The ramp consists of a single travel lane with paved shoulders. The curve has a radius of 495 ft and is 0.05 mi in length. The sight obstruction on the inside of the horizontal curve consists of a continuous concrete median barrier, with a height of 4 ft, separating the off-ramp in question from the adjacent on-ramp on which traffic operates in the opposite direction. The sight obstruction is located 5 ft from the inside edge of the ramp traveled way. The mainline freeway has a speed limit of 65 mph, while the ramp curve has a signed advisory speed of 30 mph. No speed studies are available, but traffic substantially exceeds the 30-mph advisory speed. The mainline freeway carries a two-way AADT of 77,000 veh/day. The AADT of the ramp is 1,930 veh/day. Figure C-9 shows a photograph of the ramp curve and the concrete barrier. Figure C-10 presents a drawing of the ramp curve in question showing the position of the horizontal sight obstruction on the inside of the curve. C.3.1 Sight Distance Profile The minimum ASSD at this site is 214 ft. The DSSD for the mainline speed limit of 65 mph is 645 ft. The DSSD for the ramp advisory speed of 30 mph is 200 ft. Thus, the ASSD on the ramp curve slightly exceeds the DSSD for the ramp advisory speed, but is substantially below the DSSD for the mainline freeway speed. Figure C-11 shows the sight distance profile for the ramp curve. The figure indicates that the minimum ASSD begins at the PC and opens up to essentially unlimited sight distance near the midpoint of the curve. At this point, the barrier no longer limits the driver’s view.

Case Studies of Existing Roadways with Sight Obstructions 91 Figure C-9. Photograph of the ramp curve for Case Study 3 Figure C-10. Ramp alignment showing the position of the horizontal sight obstructions.

92 Design Guidelines for Horizontal Sightline Offsets Figure C-11. Sight distance profile for the existing condition at the ramp site for Case Study 3 C.3.2 Traffic Operational Performance The AADT of this exit ramp is 1,930 veh/day. The volume on this off-ramp is much lower than capacity. A stop sign is located at the ramp terminal 440 ft downstream of the horizontal curve. However, significant queueing does not occur at this ramp terminal, so spillback of traffic into the study curve is unlikely. The mainline freeway is an urban six-lane freeway carrying approximately 77,000 veh/day. The freeway segment adjacent to the ramp does not experience any capacity limitation likely to result in recurring congestion. C.3.3 Safety Performance No crashes of types potentially related to sight distance occurred during the five-year study period at this site. C.3.4 Mitigation Challenges If the barrier height cannot be lowered, increasing the ASSD on the ramp curve would need an increase in the lateral distance between the barrier and the edge of the traveled way. Any increase in the barrier offset would require either a sharper curve upstream, just downstream of the gore area or reconstruction of the opposing ramp on the other side of the median barrier. Such an improvement would have a relatively high cost. Since there is no history of sight- distance-related crashes at this site, there is no apparent need for an improvement that would make an upstream curve sharper or require the opposing ramp to be reconstructed. 0 50 100 150 200 250 300 350 -300 -200 -100 0 100 200 300 Av ai la bl e St op pi ng S ig ht D ist an ce (ft ) Longitudinal Distance Along Ramp (ft) Design SSD 30mph Available SSD PC PT

Case Studies of Existing Roadways with Sight Obstructions 93 C.3.5 Lessons Learned There are several lessons that can be learned from Case Study 3. These include: 1. At ramp sites where stopping sight distance is sufficient for the advisory speed posted on a ramp, traffic on the ramp is likely to be traveling at speeds substantially above the advisory speed. 2. Improvement of sight distance on a ramp is likely to be impractical where the appropriate mitigation strategy would involve realignment of the ramp. 3. 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. C.4 Case Study 4—Site KS025—Curve to the Right on a Rural Two- Lane Highway The rural two-lane undivided highway on which Site KS025 is located connects a city of approximately 5,000 population with an Interstate highway. The study site is a horizontal curve with a radius of 1,400 ft and a length of 0.17 mi. The primary direction of travel studied is the northbound direction. A tall embankment covered with vegetation is located on the inside of the curve adjacent to the northbound lane and obstructs the view of northbound drivers. The sight obstruction has an offset of 14 ft from the inside edge of the traveled way. The obstruction begins approximately 0.09 mi upstream of the PC and ends approximately 0.08 mi downstream of the PC. A very low-volume roadway (likely carrying 100 veh/day or less) intersects the two- lane highway from the west on the curve near the end of the obstruction. The posted speed limit of the highway is 65 mph, however, the horizontal curve has an advisory speed of 50 mph. Observation of the site indicates that vehicles regularly travel at speeds greater than 50 mph on this curve. The two-way AADT of the two-lane highway is 3,540 veh/day. Figure C-12 presents a photograph of the horizontal curve and the sight obstruction looking to the north. Figure C-13 shows a diagram of the curve showing the location of the sight obstruction.

94 Design Guidelines for Horizontal Sightline Offsets Figure C-12. Photograph of the rural two-lane highway curve for Case Study 4. Figure C-13. Two-lane highway alignment showing the position of the horizontal sight obstructions.

Case Studies of Existing Roadways with Sight Obstructions 95 C.4.1 Sight Distance Profile The minimum ASSD for northbound drivers due to the obstruction is 474 ft. This available sight distance is greater than the DSSD of 425 ft for the advisory speed of 50 mph, but is substantially less than the DSSD of 645 ft for the posted speed limit of 65 mph. Figure C-14 shows a sight distance profile for the horizontal curve site for traffic in the northbound direction. The figure shows that ASSD becomes less than the DSSD approximately 320 ft upstream from the PC and decreases to the minimum of 474 ft at the PC. The sight distance continues at its minimum value until 215 ft downstream of the PC. At this point ASSD rapidly increases. The obstruction on the inside of the curve (adjacent to the northbound direction of travel) also obstructs the view of southbound vehicles slightly. Figure C-15 shows a sight distance profile for the southbound direction of travel. The minimum ASSD in the southbound direction of travel is 600 ft, which is below the DSSD of 645 ft. Figure C-14. Sight distance profile for the primary (northbound) direction of travel at the rural two-lane highway site for Case Study 4. 0 100 200 300 400 500 600 700 800 900 1000 -700 -500 -300 -100 100 300 500 700 900 Av ai la bl e St op pi ng S ig ht D ist an ce (ft ) Longitudinal Distance Along Roadway (ft) Available SSD (NB) Design SSD 65mph Design SSD 50mph PC PT

96 Design Guidelines for Horizontal Sightline Offsets Figure C-15. Sight distance profile for the primary (eastbound) direction of travel at the rural two-lane highway site for Case Study 4 The reliability analysis model presented in Chapter 5 was applied to consider the effect of alternative sight distance measurement assumptions. If the driver’s eye is positioned 3 ft from the left edge of the travel lane, the ASSD for the northbound direction increases to 508 ft. However, the ASSD for the opposing southbound direction decreases to 571 ft. C.4.2 Traffic Operational Performance The AADT of the rural two-lane highway is 3,540 veh/day. Roadway capacity is not an issue on this facility. C.4.3 Safety Performance During the 5-year study period, no crashes of types potentially related to sight distance occurred at this site. C.4.4 Mitigation Challenges The embankment is a hill that extends upwards away from the roadway. In order to push back the embankment, vegetation would need to be removed and a retaining wall would most likely need to be built. To achieve a minimum ASSD greater than the DSSD for the posted speed limit, the embankment would need to be pushed back 17 ft from its current position. This would involve a relatively high cost. Given that there is no history of sight-distance-related crashes at this location, there appears to be little need to implement any mitigation measures. 0 100 200 300 400 500 600 700 800 900 1000 -700 -500 -300 -100 100 300 500 700 900 Av ai la bl e St op pi ng S ig ht D ist an ce (ft ) Longitudinal Distance Along Roadway (ft) Available SSD (SB) Design SSD 65mph Design SSD 50mph PC PT

Case Studies of Existing Roadways with Sight Obstructions 97 C.4.5 Lessons Learned There are several lessons that can be learned from Case Study 4. These include: 1. A sight obstruction on the inside of the horizontal curve on a two-lane undivided highway may limit the ASSD in both directions of travel on some curves. 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 quite small. 2. More realistic positioning for the driver’s eye on a horizontal curve to the right on a two- lane undivided highway will increase the sight distance for that direction of travel. However, the opposite is true for the curve to the left in the opposing direction of travel, where the available sight distance may decrease if measured with more realistic positioning of the driver’s eye. 3. Where there is no history of potentially sight-distance-related crashes, high-cost sight distance improvements are not likely to be needed. C.5 Case Study 5—Site PA002—Curve to the Left on a Rural Mainline Freeway Site PA002 consists of a curve to the left on a rural mainline freeway. The curve has a radius of 1,432 ft and is 0.33 mi in length. The roadway has two basic lanes in each direction of travel, with passing lanes on grades where needed. There is a passing lane in the westbound direction at this location. Thus, there are three travel lanes in the primary direction of travel on a 3 to 3.5 percent upgrade. The sight obstruction on the inside of the horizontal curve consists of a 4.5-ft tall continuous concrete median barrier located 4 ft from the inside edge of the traveled way. The posted speed limit on the roadway is 55 mph. The roadway carries a two-way AADT of 14,572 veh/day. C.5.1 Sight Distance Profile The minimum ASSD for the inside (left) lane on the horizontal curve is 339 ft, as determined with the Green Book sight distance measurement assumptions. The minimum ASSD is substantially less than the AASHTO DSSD of 495 ft for the mainline posted speed limit of 55 mph. Figure C-16 shows the sight distance profile with the sight obstruction on the inside of the horizontal curve. The figure shows that only the inside (left) has an ASSD less than the DSSD. The ASSD in the left lane becomes less than the DSSD approximately 265 ft upstream of the PC. The ASSD reaches its minimum value of 339 ft at the PC. At a point approximately 330 ft upstream of the PT, ASSD starts to increase. The minimum ASSD for the center lane is 505 ft, slightly higher than the DSSD of 495 ft. The minimum ASSD for the right lane is greater than 600 ft. The reliability analysis model presented in Chapter 5 was applied to consider the effect of alternative sight distance measurement assumptions. If the driver’s eye for a vehicle in the left

98 Design Guidelines for Horizontal Sightline Offsets lane is positioned 3 ft from the left edge of the lane, the ASSD for the inside lane decreases to 283 ft, which is 56 ft less than the ASSD of 339 ft measured along the centerline of the lane. Using the alternative driver’s eye position, the ASSD for the center lane decreases to 469 ft, which is slightly lower than the DSSD. Thus, on a curve to the left these more realistic assumptions for the driver’s eye position decrease the ASSD. Figure C-16. Sight distance profile for rural multilane freeway curve in Case Study 5. C.5.2 Traffic Operational Performance The mainline freeway has an AADT of 14,572 veh/day. There is no recurrent daily congestion of this roadway. C.5.3 Safety Performance Two crashes of the types that are potentially related to sight distance occurred on the curve of interest during a 5-year period. Both were property-damage-only rear-end crashes. These crashes may or may not be related to limited sight distance, but there is clearly no definitive pattern of sight-distance-related crashes. C.5.4 Mitigation Challenges If the height of the median barrier cannot be decreased, mitigation of the limited sight distance at this location would involve widening the median and/or realigning the roadways. Given the lack of any consistent pattern of sight-distance related crashes, there appears to be little justification for such a high-cost improvement. 0 200 400 600 800 1000 -600 -400 -200 0 200 400 600 800 1000 1200 1400 1600 1800 Av ai la bl e St op pi ng S ig ht D ist an ce (ft ) Longitudinal Distance Along Freeway (ft) Design SSD Available SSD Left Lane Available SSD Center Lane Available SSD Right Lane PC PT

Case Studies of Existing Roadways with Sight Obstructions 99 C.5.5 Lessons Learned There are several lessons that can be learned from Case Study 5. These include: 1. The assumption of more realistic positioning for the driver’s eye on a horizontal curve with a sight obstruction on the inside of the curve can reduce the minimum ASSD for the curve. In this case, the minimum ASSD for the left lane was reduced from a value below applicable DSSD to a lower value. The minimum ASSD for the right lane was reduced from above the applicable DSSD to below the applicable DSSD. 2. Even where ASSD is below DSSD on a high-speed highway, substantial patterns of sight-distance-related crashes do not necessarily occur. Sight-distance-related crashes occur only where the sight distance is limited and a stopped or slowing vehicle, or another unexpected object, is present in the sight-restricted area. C.6 Case Study 6—Site WA082—Curve to the Left on an Urban Mainline Freeway Site WA082 consists of a curve to the left on an urban mainline freeway. The curve has a radius of 1,975 ft and is 0.25 mi in length. The roadway has three travel lanes in both directions of travel. The sight obstruction on the inside of the horizontal curve consists of a 6-ft tall continuous concrete median barrier located 6 ft from the inside edge of the traveled way. The freeway has a posted speed limit of 60 mph and carries a two-way AADT of 143,684 veh/day. Figure C-17 presents a photograph of the freeway showing the concrete median barrier that limits the driver’s view of the roadway ahead. Figure C-17. Photograph of the urban mainline freeway for Case Study 6 C.6.1 Sight Distance Profile The minimum ASSD for this horizontal curve is 436 ft, compared to the DSSD of 570 ft for the posted speed limit of 60 mph. Figure C-18 shows the sight distance profile for the primary direction of travel with the sight obstruction on the inside of the horizontal curve. The figure shows that only the inside (left) lane has an ASSD less than the DSSD. The ASSD in the inside lane becomes less than the DSSD approximately 270 ft upstream of the PC. The ASSD reaches

100 Design Guidelines for Horizontal Sightline Offsets its minimum value at the PC. At a point approximately 400 ft upstream of the PT, ASSD in the inside lane starts to increase. The minimum ASSD for the center lane is 618 ft, slightly higher than the DSSD of 570 ft. The reliability analysis model presented in Chapter 5 was applied to consider the effect of alternative sight distance measurement assumptions. If the driver’s eye is positioned at a more realistic position 3 ft from the left edge of the inside travel lane and the object height to be seen in the roadway is 3.5 ft, the ASSD for the inside lane decreases to 377 ft, which is a substantial decrease of 59 ft. The ASSD for the center lane decreases to 578 ft, which is only 8 ft greater than the DSSD. Figure C-18. Sight distance profile for northbound direction of travel at the urban six-lane freeway site for Case Study 6 C.6.2 Traffic Operational Performance The two-way AADT of the freeway is 143,684 veh/day. There is recurrent congestion with slowing or stopped traffic during portions of each day. A substantial number of crashes at sites such as this one are likely to be related to the presence of congestion, rather than sight distance. C.6.3 Safety Performance The basic safety analysis for this site identified 10 crashes of types that are possibly related to sight distance during the 5-year study period, including six rear-end crashes, three sideswipe same-direction crashes, and one run-off-road crash. A more detailed review found that the run- off-road crash involved running off the outside (right side) of the road, which seems unlikely to be related to sight distance. Only four of the crashes were found to involve an impact in the 0 200 400 600 800 1000 -600 -400 -200 0 200 400 600 800 1000 1200 1400 Av ai la bl e St op pi ng S ig ht D ist an ce (ft ) Longitudinal Distance Along Freeway (ft) Design SSD Available SSD Left Lane Available SSD Center Lane Available SSD Right Lane PC PT

Case Studies of Existing Roadways with Sight Obstructions 101 inside travel lane. Furthermore, two of those four crashes occurred during peak-hour traffic conditions and could be explained by congestion rather than limited sight distance. The other two crashes that may be sight-distance-related included two rear-end injury crashes. C.6.4 Mitigation Costs If the height of the traffic barrier cannot be lowered, mitigation of the limited sight distance at this location would involve widening the median and/or realigning the roadways. Given the lack of any consistent pattern of sight-distance related crashes, there is little justification for such a high-cost improvement. C.6.5 Lessons Learned There are several lessons that can be learned from Case Study 6. These include: 1. At this site, as at many multilane sites with horizontal sight obstructions, only the inside lane experiences minimum ASSD less than DSSD. 2. The assumption of more realistic positioning for the driver’s eye on a horizontal curve with a sight obstruction on the inside of the curve can reduce the minimum ASSD for the curve. In this case, the minimum ASSD for the left lane was reduced from a value below applicable DSSD to a lower value. C.7 Case Study 7—Site WA091—Curve to the Right on an Urban Interchange Ramp Site WA091 is a curve to the right on a directional ramp within a freeway-to-freeway interchange. The ramp allows traffic to make a right-turn movement from the southbound roadway on one freeway to the westbound roadway on another. The ramp has a radius of 1,200 ft and a length of 0.24 mi. The ramp has a signed advisory speed limit of 50 mph. The upstream mainline freeway has a posted speed limit of 60 mph. The ramp has two travel lanes and is located on a bridge structure. The ramp AADT is 40,000 veh/day. The sight obstruction on this ramp is a continuous bridge rail on the right side of the structure. The bridge rail is 4 ft in height and is offset 10 ft from the inside edge of the traveled way.

102 Design Guidelines for Horizontal Sightline Offsets The minimum ASSD available on this horizontal curve is 392 ft in the inside lane, compared to the DSSD of 425 ft for the advisory speed limit of 50 mph and 570 ft for the mainline freeway speed limit of 60 mph. Figure C-19 shows the sight distance profile with the sight obstruction on the inside of the horizontal curve. The figure shows that only the inside (right) lane has a minimum ASSD less than the DSSD. The ASSD becomes less than the DSSD approximately 115 ft upstream of the PC. The ASSD reaches its minimum value at the PC. At a point approximately 370 ft upstream of the PT, ASSD starts to increase. The minimum ASSD for the left lane is 522 ft, substantially higher than the DSSD of 425 ft. The reliability analysis model presented in Chapter 5 was applied to consider the effect of alternative sight distance measurement assumptions. If the driver’s eye is assumed to be positioned at a more realistic 3 ft from the left edge of the inside lane and the object height to be seen in the roadway is 3.5 ft, the ASSD for the inside lane increases to 428 ft, which is 3 ft greater than the applicable AASHTO value of DSSD, but still less than the applicable DSSD for the mainline freeway speed limit of 60 mph. Figure C-19. Sight distance profile for the urban two-lane ramp site for Case Study 7. C.7.2 Traffic Operational Performance The mainline freeway upstream of the ramp has an AADT of 164,000 veh/day. This freeway experiences recurring daily congestion during some hours of the day. The ramp has an AADT of 40,000 veh/day. The ramp is unlikely to experience recurrent daily congestion generated by its own traffic volume. The freeway which the ramp traffic enters downstream of the ramp has an 0 200 400 600 800 1000 -500 -300 -100 100 300 500 700 900 1100 1300 Av ai la bl e St op pi ng S ig ht D ist an ce (ft ) Longitudinal Distance Along Ramp (ft) Design SSD 50mph Available SSD Right Lane Available SSD Left Lane PC PT C.7.1 Sight Distance Profile

Case Studies of Existing Roadways with Sight Obstructions 103 AADT of 133,000 veh/day. Congestion on this downstream freeway could create spillback congestion onto the ramp of interest. C.7.3 Safety Performance This ramp curve experienced no crashes of crash types potentially related to limited sight distance during the 5-year study period. C.7.4 Mitigation Challenges On this ramp, traffic ahead generally appears visible to drivers in the inside (right) lane, because the vehicles are generally higher than 3.5 ft. This observation, and the lack of any history of crash types potentially related to sight distance, does not suggest any need for high-cost mitigation strategies such as realigning the ramp. C.7.5 Lessons Learned There are several lessons that can be learned from Case Study 7. These include: 1. 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 appear limited. 2. On a curve to the right, a more realistic assumed lateral position for the driver’s eye will increase the estimated ASSD.

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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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