Sight Distance

CTC 440

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

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

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/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 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)

Vertical Curves

Crest Curves (3 types) Sag Curves (3 types)

Careful with signs of G1 and G2!!

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) G1 and G2-grades (%) L=length of vertical curve (ft or m) S=sight distance (ft or m)

Metric Equations-Crest Curves S>L L=2S-[658/(G1-G2)] S<L L=[(G1-G2)*S2]/658

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 L=2S-[2158/(G1-G2)] S<L L=[(G1-G2)*S2]/2158

Whether S is greater or less than L is often not known; must assume, calculate, and then recheck that assumption is correct

Crest Curve ExampleEnglish, 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

2001 AASHTO

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)

Sag Vertical Curve

Headlight Height: 600 mm; 2 ft Headlight 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 Curves S>L L=2S-[(120+3.5*S)/[(G2-G1)] S<L L=[(G2-G1)*S2]/[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

English Equations-Sag Curves S>L L=2S-[(400+3.5*S)/[(G2-G1)] S<L L=[(G2-G1)*S2]/[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

Sag Curve ExampleMetric; Solve for L

G1=+1.86% and G2=+5.04% L=300mFind HSD Assume S>L S=375m (S>L; ok) Note: S<L; quadratic equation

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)

Sight Distance on Horizontal Curves

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

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

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

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