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From page 82... ... 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. Read the entire page →
From page 83... ... Case Studies of Existing Roadways with Sight Obstructions 83 Figure C-1. Photograph of the rural two-lane highway curve for Case Study 1. Read the entire page →
From page 84... ... 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. Read the entire page →
From page 85... ... Case Studies of Existing Roadways with Sight Obstructions 85 Figure C-4. Sight distance profile for the secondary (westbound) Read the entire page →
From page 86... ... 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. Read the entire page →
From page 87... ... Case Studies of Existing Roadways with Sight Obstructions 87 Figure C-5. Photograph of the ramp curve for Case Study 2. Read the entire page →
From page 88... ... 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. Read the entire page →
From page 89... ... 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. Read the entire page →
From page 90... ... 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. Read the entire page →
From page 91... ... Case Studies of Existing Roadways with Sight Obstructions 91 Figure C-9. Photograph of the ramp curve for Case Study 3 Figure C-10. Read the entire page →
From page 92... ... 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. Read the entire page →
From page 93... ... 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. Read the entire page →
From page 94... ... 94 Design Guidelines for Horizontal Sightline Offsets Figure C-12. Photograph of the rural two-lane highway curve for Case Study 4. Read the entire page →
From page 95... ... 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. Read the entire page →
From page 96... ... 96 Design Guidelines for Horizontal Sightline Offsets Figure C-15. Sight distance profile for the primary (eastbound) Read the entire page →
From page 97... ... 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. Read the entire page →
From page 98... ... 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. Read the entire page →
From page 99... ... 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. Read the entire page →
From page 100... ... 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. Read the entire page →
From page 101... ... 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. Read the entire page →
From page 102... ... 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. Read the entire page →
From page 103... ... 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. Read the entire page →

From page 82...

... 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.