1. Sight Distance CTC 440

2. Objectives Understand the meanings of “sight distance”and “stopping sight distance” Understand how to determine minimum SSD’s Understand how to calculate SSD and HSD for vertical alignments

3. Sight Distance Length of roadway ahead visible to the driver Note: The minimum designed stopping sight distance should be long enough for a driver going at design speed to see an object (potential hazard) and stop before hitting the object

4. Minimum Required Stopping Sight Distance Two components: Distance traveled while reacting (2.5 seconds assumed reaction time) Distance traveled while braking Assumes wet road (decel rate of 3.4 m/sec 2 or 11.2 ft/sec 2 ) Can be calculated; however, minimum is usually obtained by HDM, chapter 2 or AASHTO book

5. Minimum Design SSD; 2001 AASHTO

6. During Design Determine minimum SSD Calculate actual SSD/HSD and check that it meets the minimum SSD-actual stopping sight distance (crest) HSD-headlight sight distance (sag)

7. Vertical Curves Crest Curves (3 types) Sag Curves (3 types) Careful with signs of G1 and G2!!

8. Crest Vertical Curve Height of Eye: 1070 mm; 3.5 ft Height of Object: 600 mm; 2.0 ft (for passing HO=1070 mm; 3.5 ft) G 1 and G 2 -grades (%) L=length of vertical curve (ft or m) S=sight distance (ft or m)

9. Metric Equations-Crest Curves S>L L=2S-[658/(G 1 -G 2 )] S<L L=[(G 1 -G 2 )*S 2 ]/658 Whether S is greater or less than L is often not known; must assume, calculate, and then recheck that assumption is correct

10. English Equations-Crest Curves S>L L=2S-[2158/(G 1 -G 2 )] S<L L=[(G 1 -G 2 )*S 2 ]/2158 Whether S is greater or less than L is often not known; must assume, calculate, and then recheck that assumption is correct

11. Crest Curve Example English, Solve for L G1=+3.9% and G2=+1.1% PVI Sta=20+50; Elev=1005.00’ Determine the minimum length of crest vertical curve for a design speed of 50 mph

12. 2001 AASHTO

13. Crest Curve Example Minimum SSD is 425’ (see previous slide) Assume S<=L G1-G2=2.8 L=234’ (Check S<L; no) Assume S>L L=80’ (Check S>L; yes)

14. Sag Vertical Curve Headlight Height: 600 mm; 2 ft Headlight Divergence of 1 degree upwards G 1 and G 2 -grades (%) L=length of vertical curve (ft or m) S=sight distance (ft or m)

15. Metric Equations-Sag Curves S>L L=2S-[(120+3.5*S)/[(G 2 -G 1 )] S<L L=[(G 2 -G 1 )*S 2 ]/[120+3.5*S)] Whether S is greater or less than L is often not known; must assume, calculate, and then recheck that assumption is correct

16. English Equations-Sag Curves S>L L=2S-[(400+3.5*S)/[(G 2 -G 1 )] S<L L=[(G 2 -G 1 )*S 2 ]/[400+3.5*S)] Whether S is greater or less than L is often not known; must assume, calculate, and then recheck that assumption is correct

17. Sag Curve Example Metric; Solve for L G1=+1.86% and G2=+5.04% L=300m Find HSD Assume S>L S=375m (S>L; ok) Note: S<L; quadratic equation

18. Sight Distance on Horizontal Curves Sight distance can also be a problem on horizontal curves (buildings, embankments, tree growth, etc.) The line of sight is a chord of the curve. The sight distance should be measured along the centerline of the inside lane of the curve (not the centerline of the roadway)

19. Sight Distance on Horizontal Curves

20. Passing Sight Distance Distance required for a moving vehicle to overtake and pass another vehicle in the same traffic lane Three distances: Distance traveled by the passing vehicle during perception, reaction and acceleration Distance traveled by the vehicle being passed Distance traveled by an oncoming vehicle during the passing maneuver

21. Intersection Sight Distance Intersection sight distances should also be looked at. Can someone turning onto a major road see far enough ahead to safely pull out? Usual culprits: guide railing, signs, embankments, plantings

22. Intersection Sight Distance http://www.ite.org/css/online/DWUT10.html

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