Sight DistanceCTC 440

ObjectivesUnderstand the meanings of sight distanceand stopping sight distanceUnderstand how to determine minimum SSDsUnderstand how to calculate SSD and HSD for vertical alignments

Sight DistanceLength of roadway ahead visible to the driverNote: 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 DistanceTwo components:Distance traveled while reacting(2.5 seconds assumed reaction time)Distance traveled while brakingAssumes wet road (decel rate of 3.4 m/sec2 or 11.2 ft/sec2)Can be calculated; however, minimum is usually obtained by HDM, chapter 2 or AASHTO book

Minimum Design SSD; 2001 AASHTO

During DesignDetermine minimum SSDCalculate actual SSD/HSD and check that it meets the minimum

SSD-actual stopping sight distance (crest)HSD-headlight sight distance (sag)

Vertical CurvesCrest Curves (3 types)Sag Curves (3 types)

Careful with signs of G1 and G2!!

Crest Vertical CurveHeight of Eye: 1070 mm; 3.5 ftHeight of Object: 600 mm; 2.0 ft(for passing HO=1070 mm; 3.5 ft)G1 and G2-grades (%)L=length of vertical curve (ft or m)S=sight distance (ft or m)

Metric Equations-Crest CurvesS>L L=2S-[658/(G1-G2)]S

English Equations-Crest CurvesS>L L=2S-[2158/(G1-G2)]S

Crest Curve ExampleEnglish, Solve for LG1=+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

2001 AASHTO

Crest Curve ExampleMinimum SSD is 425 (see previous slide)Assume SL; yes)

Sag Vertical CurveHeadlight Height: 600 mm; 2 ftHeadlight Divergence of 1 degree upwards G1 and G2-grades (%)L=length of vertical curve (ft or m)S=sight distance (ft or m)

Metric Equations-Sag CurvesS>L L=2S-[(120+3.5*S)/[(G2-G1)]S

English Equations-Sag CurvesS>L L=2S-[(400+3.5*S)/[(G2-G1)]S

Sag Curve ExampleMetric; Solve for LG1=+1.86% and G2=+5.04% L=300mFind HSDAssume S>LS=375m (S>L; ok)Note: S

Sight Distance on Horizontal CurvesSight 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 Curveshttp://www.docstoc.com/docs/53404877/Sight-Distance-on-Horizontal-Curves

Passing Sight DistanceDistance required for a moving vehicle to overtake and pass another vehicle in the same traffic laneThree distances:Distance traveled by the passing vehicle during perception, reaction and accelerationDistance traveled by the vehicle being passedDistance traveled by an oncoming vehicle during the passing maneuver

Intersection Sight DistanceIntersection 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 Distancehttp://www.ite.org/css/online/DWUT10.html

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