CTC 440 Sight Distance Objectives n n n

CTC 440 Sight Distance

Objectives n n n 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

Sight Distance n 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

Minimum Required Stopping Sight Distance Two components: n Distance traveled while reacting n n Distance traveled while braking n n (2. 5 seconds assumed reaction time) 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

Minimum Design SSD; 2001 AASHTO

During Design n n 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)

Vertical Curves n Crest Curves (3 types) Sag Curves (3 types) n Careful with signs of G 1 and G 2!! n

Crest Vertical Curve n n Height of Eye: 1070 mm; 3. 5 ft Height of Object: 600 mm; 2. 0 ft n n (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)

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

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

Crest Curve Example English, Solve for L n n n G 1=+3. 9% and G 2=+1. 1% PVI Sta=20+50; Elev=1005. 00’ Determine the minimum length of crest vertical curve for a design speed of 50 mph

2001 AASHTO

Crest Curve Example n n n Minimum SSD is 425’ (see previous slide) Assume S<=L G 1 -G 2=2. 8 L=234’ (Check S<L; no) Assume S>L L=80’ (Check S>L; yes)

Sag Vertical Curve n n n 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)

Metric Equations-Sag Curves S>L n L=2 S-[(120+3. 5*S)/[(G 2 -G 1)] n S<L n L=[(G 2 -G 1)*S 2]/[120+3. 5*S)] n Whether S is greater or less than L is often not known; must assume, calculate, and then recheck that assumption is correct

English Equations-Sag Curves S>L n L=2 S-[(400+3. 5*S)/[(G 2 -G 1)] n S<L n L=[(G 2 -G 1)*S 2]/[400+3. 5*S)] n Whether S is greater or less than L is often not known; must assume, calculate, and then recheck that assumption is correct

Sag Curve Example Metric; Solve for L G 1=+1. 86% and G 2=+5. 04% n L=300 m Find HSD n Assume S>L n S=375 m (S>L; ok) n Note: S<L; quadratic equation n

Sight Distance on Horizontal Curves n n 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)

Sight Distance on Horizontal Curves

Passing Sight Distance n n Distance required for a moving vehicle to overtake and pass another vehicle in the same traffic lane Three distances: n n n 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

Intersection Sight Distance n n 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

Intersection Sight Distance n http: //www. ite. org/css/online/DWUT 10. html

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