presented by nazir lalani p.e. traffex engineers inc. n ... distance.pdfpresented by nazir lalani...

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SIGHT DISTANCESIGHT DISTANCEPresented by Presented by

NazirNazir LalaniLalani P.E.P.E.TraffexTraffex Engineers Inc.Engineers [email protected][email protected]

WHY IS SIGHT DISTANCE SO WHY IS SIGHT DISTANCE SO IMPORTANT?IMPORTANT?

Stop for objects in the roadwayStop for objects in the roadway

Stop for stationary vehicles ahead Stop for stationary vehicles ahead

See cross traffic at intersections before enteringSee cross traffic at intersections before entering

See on coming vehicles when passingSee on coming vehicles when passing

See traffic control devices and react to themSee traffic control devices and react to them

See trains at Railroad Xings not controlled by gatesSee trains at Railroad Xings not controlled by gates

See pedestrians waiting to cross the streetSee pedestrians waiting to cross the street

Drivers must be able to: Drivers must be able to:

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AASHTO GEOMETRIC DESIGN BOOKAASHTO GEOMETRIC DESIGN BOOK

What is Stopping Sight What is Stopping Sight Distance?Distance?

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Stopping Sight Distance: The available sight distance on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path

Brake Reaction TimeBrake Reaction Time

Stopping sight distance is the sum of two distances:Stopping sight distance is the sum of two distances:Stopping sight distance is the sum of two distances:

The distance traversed by the vehicle from the instant The distance traversed by the vehicle from the instant the driver sights an object necessitating a stop to the the driver sights an object necessitating a stop to the instant the brakes are applied instant the brakes are applied (brake reaction (brake reaction distance)distance)..

The distance needed to stop the vehicle from the The distance needed to stop the vehicle from the instant brake application begins instant brake application begins ((braking distancebraking distance).).

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Brake Reaction TimeBrake Reaction TimeBraking DistanceBraking Distance

Braking DistanceBraking Distance

TThe approximate he approximate braking distancebraking distance of a vehicle on a level of a vehicle on a level roadway traveling :roadway traveling :

Where:Where:d = braking distance;d = braking distance;VV = design speed, mph;= design speed, mph;a = deceleration rate, ft/sa = deceleration rate, ft/s²²

VV²²d = 1.075 d = 1.075 aa

US CustomaryUS Customary

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Studies documented in the literature show that most drivers decelerate at a rate greater than 14.8 ft/s² when confronted with the need to stop for an unexpected object in the roadway

Approximately 90 percent of all drivers decelerate at rates greater than 11.2 ft/s²

Therefore, 11.2 ft/s² (a comfortable deceleration for most drivers) is recommended as the deceleration threshold for determining stopping sight distance

Where:Where:d = braking distance;d = braking distance;VV = design speed, mph;= design speed, mph;a = deceleration rate, ft/sa = deceleration rate, ft/s²²

VV²²d = 1.075 d = 1.075 aa

US CustomaryUS Customary

Brake Reaction TimeBrake Reaction TimeBrake Reaction Distance

Brake Reaction Distance

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Brake reaction time is the interval from the instant that the Brake reaction time is the interval from the instant that the driver recognizes the existence of an obstacle on the roadway driver recognizes the existence of an obstacle on the roadway ahead that necessitates braking to the instant that the driver ahead that necessitates braking to the instant that the driver actually applies the brakesactually applies the brakes

In addition, the driver must not only see the object but must In addition, the driver must not only see the object but must also recognize it as a stationary or slowly moving objectalso recognize it as a stationary or slowly moving object


Comprises of PIEV which includes speed and conditions.Comprises of PIEV which includes speed and conditions.

PPerceptionerceptionIIdentification (understanding)dentification (understanding)EEmotion (decision making)motion (decision making)VVolition (execution of decision)olition (execution of decision)

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Both recent research and the studies documented in the Both recent research and the studies documented in the literature show that a 2.5literature show that a 2.5--s brake reaction time for stopping sight s brake reaction time for stopping sight situations encompasses the capabilities of most drivers, situations encompasses the capabilities of most drivers, including those of older drivers including those of older drivers

The recommended design criterion of 2.5The recommended design criterion of 2.5--s for brake reaction s for brake reaction time exceeds the 90time exceeds the 90thth percentile of reaction time for all driverspercentile of reaction time for all drivers

Stopping Sight DistanceStopping Sight Distance

The sum of the distance traversed during the brake reaction timeThe sum of the distance traversed during the brake reaction time and and the braking distance is the stopping sight distancethe braking distance is the stopping sight distance

Where:Where:

VV = design speed, mph;= design speed, mph;a = deceleration rate, ft/sa = deceleration rate, ft/s²²

t = Brake reaction time in seconds

VV²²d = 1.47d = 1.47VVt + 1.075 t + 1.075 aa

US CustomaryUS Customary Driver’s eye is estimated to be 3.5 ft and the height of the object to be seen by the driver is 2 ft, equivalent to the tail light height of a passenger car.

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910910908.3908.3614.3614.32942948080

820820815.5815.5539.9539.9275.6275.67575

730730727.6727.6470.3470.3257.3257.37070

645645644.4644.4405.5405.5238.9238.96565

570570566566345.5345.5220.5220.56060

495495492.4492.4290.3290.3202.1202.15555

425425423.8423.8240240183.8183.85050

360360359.8359.8194.4194.4165.4165.44545

305305300.6300.6153.6153.61471474040

250250246.2246.2117.6117.6128.6128.63535

200200196.7196.786.486.4110.3110.33030

155155151.9151.9606091.991.92525

115115111.9111.938.438.473.573.52020

808076.776.721.621.655.155.11515

(ft)(ft)(ft)(ft)(ft)(ft)(ft)(ft)(mph)(mph)

DesignDesignCalculatedCalculatedon levelon leveldistancedistancespeedspeed

Braking distanceBraking distanceBrake reactionBrake reactionDesignDesign

Stopping sight distanceStopping sight distance

US US CustomaryCustomary

Source: Geometric Design of Highways and Streets 2004

Why is it important on Why is it important on horizontal and vertical horizontal and vertical

curves?curves?

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Vertical Curve Crest Vertical Curve Crest ConditionCondition

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Design Speed (mph)Design Speed (mph)

Alg

ebra

ic D

iffer

ence

in G

rade

s (%

)A

lgeb

raic

Diff

eren

ce in

Gra

des

(%)

Exhibit 3-71: Length of Crest Vertical Curve (feet)


Exhibit 3Exhibit 3--72 Design Controls for Crest Vertical 72 Design Controls for Crest Vertical Curves Based on Curves Based on Stopping Stopping Sight DistanceSight Distance


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Exhibit 3Exhibit 3--73 Design Controls for Crest Vertical 73 Design Controls for Crest Vertical Curves Based on Curves Based on Passing Passing Sight DistanceSight Distance


Not designed for either stopping or passing sight Not designed for either stopping or passing sight distance!distance!

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Vertical Curve Sag Vertical Curve Sag ConditionCondition

Stopping Sight Distance on Sag Vertical Stopping Sight Distance on Sag Vertical CurvesCurves

The minimum length of vertical curve which provides headlight siThe minimum length of vertical curve which provides headlight sight ght distance in grade sags for a given design speed can be obtained.distance in grade sags for a given design speed can be obtained.

Source: Caltrans Highway Design Manual

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Design Speed (mph)Design Speed (mph)

Alg

ebra

ic D

iffer

ence

in G

rade

s (%

)A

lgeb

raic

Diff

eren

ce in

Gra

des

(%)

Exhibit 3-74 Length of Sag Vertical Curve (feet)



Exhibit 3Exhibit 3--75 Design Controls for Sag Vertical Curves 75 Design Controls for Sag Vertical Curves Based on Based on Stopping Stopping Sight DistanceSight Distance

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Formula for length of sag vertical curve using Formula for length of sag vertical curve using comfort factor ( length is 50% less than based on comfort factor ( length is 50% less than based on

headlight distance)headlight distance)

Exhibit 3Exhibit 3--76 Sight Distance at Under 76 Sight Distance at Under CrossingsCrossings


Note: AASHTO provide differentformulas for calculating curves for under Crossings

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Bridge structure limits Bridge structure limits sight distancesight distance

General Controls for Vertical AlignmentsGeneral Controls for Vertical Alignments

Provide smooth grade line with gradual changesProvide smooth grade line with gradual changes

Avoid Avoid ““roller coasterroller coaster”” or sudden dip type profilesor sudden dip type profiles

Avoid Avoid ““broken backbroken back”” curves ( two vertical curves in curves ( two vertical curves in the same direction separated by short tangentthe same direction separated by short tangent

On long grades, the steepest at the bottom with On long grades, the steepest at the bottom with flattening of the grades near the topflattening of the grades near the top

Sag vertical curves must have adequate drainageSag vertical curves must have adequate drainage

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Sight Distance on Sight Distance on Horizontal CurvesHorizontal Curves

Stopping Sight Distance on Horizontal CurvesStopping Sight Distance on Horizontal CurvesWhere an object off the pavement such as a bridge pier, buildingWhere an object off the pavement such as a bridge pier, building, ,

cut slope, or natural growth restricts sight distance, the minimcut slope, or natural growth restricts sight distance, the minimum um radius of curvature is determined by the stopping sight distanceradius of curvature is determined by the stopping sight distance..

HSO: Horizontal HSO: Horizontal Sightline OffsetSightline Offset

Available stopping Available stopping sight distance on sight distance on horizontal curves is horizontal curves is obtained from Exhibit obtained from Exhibit 33--5353

It is assumed It is assumed that the driverthat the driver’’s s eye is 3.5 feet eye is 3.5 feet above the center of above the center of the inside lane the inside lane (inside with (inside with respect to curve) respect to curve) and the object is 2 and the object is 2 feet highfeet high

Insert Exhibit 3-54


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General Controls for Horizontal AlignmentsGeneral Controls for Horizontal AlignmentsProvide passing distance on 2Provide passing distance on 2--lane roadslane roads

Provide greater radius of curvature than the Provide greater radius of curvature than the minimum where possibleminimum where possible

Avoid sharp horizontal curves at the ends of long Avoid sharp horizontal curves at the ends of long tangent sections and back to back reverse curvestangent sections and back to back reverse curves

Curves should be at least 500 feet for a central Curves should be at least 500 feet for a central angle of 5 degrees angle of 5 degrees –– 100 feet per degree100 feet per degree

Minimum length of horizontal curves should be 15 Minimum length of horizontal curves should be 15 times the design speed in mphtimes the design speed in mph

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Stopping Sight Distance for Bicyclists

Stopping Sight Distance for Stopping Sight Distance for BicyclistsBicyclists

Source: Caltrans Highway Design Manual Chapter 1000Source: Caltrans Highway Design Manual Chapter 1000

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Source: Caltrans Highway Design Manual Chapter 1000Source: Caltrans Highway Design Manual Chapter 1000

Minimum Length of Crest Vertical CurveMinimum Length of Crest Vertical CurveSource: Caltrans Highway Design Manual Chapter 1000Source: Caltrans Highway Design Manual Chapter 1000

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Horizontal Lateral Clearance FormulaHorizontal Lateral Clearance FormulaSource: Caltrans Highway Design Manual Chapter 1000Source: Caltrans Highway Design Manual Chapter 1000

Stopping Sight DistanceStopping Sight DistanceSource: Caltrans Highway Design Manual Chapter 1000Source: Caltrans Highway Design Manual Chapter 1000

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Intersection Sight Distance Intersection Sight Distance

Uncontrolled IntersectionsUncontrolled Intersections

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Sight TrianglesSight Triangles

Specified areas along intersection approach legs and across their included corners should be clear of obstructions that might

block a driver’s view of potentially conflicting vehicles

These specified areas are known as clear sight triangles

The dimensions of the legs of the sight triangles depend on the The dimensions of the legs of the sight triangles depend on the design speeds of the intersecting roadways and the type of traffdesign speeds of the intersecting roadways and the type of traffic ic control used at the intersection. control used at the intersection.

Two types of clear sight triangles are considered in intersection design:

Approach Sight Approach Sight TrianglesTriangles

Departure Sight Departure Sight TrianglesTriangles

AND

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Approach Sight Approach Sight Triangles for Triangles for Uncontrolled Uncontrolled

LocationsLocations

Each quadrant of an intersection should contain a triangular areEach quadrant of an intersection should contain a triangular area a free of obstructions that might block an approaching driverfree of obstructions that might block an approaching driver’’s view s view of potentially conflicting vehiclesof potentially conflicting vehicles -- drivers eye height and object drivers eye height and object height are 3.5 feet (AASTO) height are 3.5 feet (AASTO)

The length of the legs of this triangular area, along both The length of the legs of this triangular area, along both intersecting roadways, should be such that the drivers can see aintersecting roadways, should be such that the drivers can see any ny potentially conflicting vehicles in sufficient time to slow or spotentially conflicting vehicles in sufficient time to slow or stop top before colliding within the intersectionbefore colliding within the intersection

Major Road

Minor R

oad

a

b

Decision PointClear Sight Triangle

A- Approach Sight Triangles

(uncontrolled)

Exhibit 9Exhibit 9--50: Intersection Sight Triangles50: Intersection Sight Triangles

This decision point is the location at This decision point is the location at which the minorwhich the minor--road driver should begin road driver should begin to brake to a stop if another vehicle is to brake to a stop if another vehicle is present on an intersecting approach.present on an intersecting approach.

The distance from the major road, The distance from the major road, along the minor road, is illustrated by the along the minor road, is illustrated by the dimension dimension ““aa”” in Exhibit 9in Exhibit 9--50 A.50 A.

Dimension Dimension ““bb”” illustrates the length of illustrates the length of this leg of the sight triangle along the this leg of the sight triangle along the major road A. major road A.

Source: AASHTO A Policy on Design of Highways and Streets

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Corner sight triangle for uncontrolled locationsCorner sight triangle for uncontrolled locations


Stop Sign Controlled Intersections

Stop Sign Controlled Intersections

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Major Road

Minor R

oad

a

b

Decision PointClear Sight Triangle

A- Approach Sight Triangles

(uncontrolled or yield-controlled)

Exhibit 9-50: Intersection Sight TrianglesAlthough desirable at higher volume

intersections, approach sight triangles like those shown in exhibit 9-50A are not needed for intersection approaches controlled by stop signs or traffic signals.

In that case, the need for approaching vehicles to stop at the intersection is determined by the traffic control devices and not by the presence or absence of vehicles on the intersecting approaches.


Case B-Intersections with Stop Control on the Minor Road

Case B-Intersections with Stop Control on the Minor Road

Departure sight triangles for intersections with stop control onthe minor road should be considered for three situations:

Departure sight triangles for intersections with stop control onthe minor road should be considered for three situations:

• Case B1---Left turns from the minor road;• Case B1---Left turns from the minor road;

Intersection sight distance criteria for stop-controlled intersections are longer than stopping sight distance to ensure that the intersection operates smoothly

Intersection sight distance criteria for stop-controlled intersections are longer than stopping sight distance to ensure that the intersection operates smoothly

Minor-road vehicle operators have to wait for a gap in cross traffic until they can proceed safely without forcing a major-road vehicle to stop

Minor-road vehicle operators have to wait for a gap in cross traffic until they can proceed safely without forcing a major-road vehicle to stop

• Case B2---Right turn from the minor road; and• Case B2---Right turn from the minor road; and• Case B3---Crossing the major road from a minor-road approach• Case B3---Crossing the major road from a minor-road approach

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Major Road

Minor R

oad

a

b

Decision Point

Clear Sight Triangle

B- Departure Sight Triangles

(Stop control)

Exhibit 9Exhibit 9--50. Intersection 50. Intersection Sight TrianglesSight Triangles

A second type of clear sight triangle provides sight distance sufficient for a stopped driver on a minor-road approach to depart from the intersection and enter or cross the major road.

Departure sight triangles should be provided in each quadrant of each intersection approach controlled by stop or yield signs.


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Major Road

Minor R

oad

a

b

Decision Point


B- Departure Sight Triangles

(Stop control)

Exhibit 9Exhibit 9--50. Intersection Sight 50. Intersection Sight TrianglesTriangles

Departure sight triangles should also be provided for some signalized intersection approaches where right turns on red are permitted.

The recommended dimensions of the clear sight triangle for desirable traffic operations where stopped vehicles enter or cross a major road are based on assumptions derived from field observations of driver gap acceptance behavior.


Case B1---Left Turn from the Minor RoadCase B1---Left Turn from the Minor RoadThe vertex (decision point) of the departure sight triangle on

the minor road should be 4.4 m [14.5 ft] from the edge of the major-road traveled way

The vertex (decision point) of the departure sight triangle on the minor road should be 4.4 m [14.5 ft] from the edge of the major-road traveled way

This represents the typical position of the minor-road driver’s eye when a vehicle is stopped relatively close to the major road

This represents the typical position of the minor-road driver’s eye when a vehicle is stopped relatively close to the major road

Minor R

oad

a

b


Departure Sight Triangles

(Stop control)

Major Road

Decision Point


Exhibit 9Exhibit 9--50B: 50B: Intersection Sight Intersection Sight

TrianglesTriangles

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The intersection sight distance along The intersection sight distance along the major road (dimension the major road (dimension ““bb”” in Exhibit in Exhibit 99--50B) is determined by:50B) is determined by:

where:

US Customary

ISD = 1.47 V major t g (9-1)

intersection sight distance (length of the leg of sight

road) (ft)

ISD =

design speed of major road (mph)

V major =

triangle along the major

time gap for minor road vehicle to enter the major road (s)

t g =


Exhibit 9-54. Time Gap for Case B1---Left Turn from StopExhibit 9-54. Time Gap for Case B1---Left Turn from Stop

Passenger car Passenger car 7.57.5SingleSingle‐‐unit truck unit truck 9.59.5Combination truck Combination truck 11.511.5

Time gap (tTime gap (tgg) (seconds) at design speed) (seconds) at design speed

Design vehicleDesign vehicle of major roadof major road

Note: Time gaps are for a stopped vehicle to turn left onto a twNote: Time gaps are for a stopped vehicle to turn left onto a twoo--lane highway with no median and lane highway with no median and grades 3 percent or less. The table values require adjustment asgrades 3 percent or less. The table values require adjustment as follows:follows:

For multilane highwaysFor multilane highways::For left turns onto twoFor left turns onto two--way highways with more than two lanes, add 0.5 seconds for way highways with more than two lanes, add 0.5 seconds for passenger cars or 0.7 seconds for trucks for each additional lanpassenger cars or 0.7 seconds for trucks for each additional lane, from the left, in excess of e, from the left, in excess of one, to be crossed by the turning vehicle one, to be crossed by the turning vehicle

For minor road approach grades:For minor road approach grades:

If the approach grade is an upgrade that exceeds 3 percent; add If the approach grade is an upgrade that exceeds 3 percent; add 0.2 seconds for each 0.2 seconds for each percent grade for left turns percent grade for left turns


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Where substantial volumes of heavy vehicles enter the major Where substantial volumes of heavy vehicles enter the major road, such as from a ramp terminal, the use of tabulated values road, such as from a ramp terminal, the use of tabulated values for singlefor single--unit or combination trucks should be considered.unit or combination trucks should be considered.

Adjustment for the grade of the minorAdjustment for the grade of the minor--road approach is needed road approach is needed only if the rear wheels of the design vehicle would be on an only if the rear wheels of the design vehicle would be on an upgrade that exceeds 3 percent when the vehicle is at the stop upgrade that exceeds 3 percent when the vehicle is at the stop line of the minorline of the minor--road approach.road approach.

Use the tabulated values shown in Exhibit 9Use the tabulated values shown in Exhibit 9--55 from AASTHO if 55 from AASTHO if no adjustments are needed.no adjustments are needed.

NOTE:NOTE:

Stopping Design sightSpeed distance Calculated Design(mph) (ft) (ft) (ft)

15 80 165.4 17020 115 220.5 22525 155 275.6 28030 200 330.8 33535 250 385.9 39040 305 441.0 44545 360 496.1 50050 425 551.3 55555 495 606.4 61060 570 661.5 66565 645 716.6 72070 730 771.8 77575 820 826.9 83080 910 882.0 885

passenger cars

US CustomaryIntersection sight

distance for

Note: Intersection sight distance shown is for a stopped passenNote: Intersection sight distance shown is for a stopped passenger ger car to turn left onto a twocar to turn left onto a two‐‐lane highway with no median and grades lane highway with no median and grades 3 percent or less..3 percent or less..

Exhibit 9Exhibit 9‐‐55. Design Intersection Sight Distance55. Design Intersection Sight Distance——Case B1Case B1——Left Left Turn StopTurn Stop


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MotorcyclistMotorcyclist-- LeftLeft--turning Vehicle crashturning Vehicle crash

Motorcyclist

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The intersection sight distance for right turns is determined inthe same manner as for Case B1, except that the time gaps (tg) in Exhibit 9-54 should be adjusted

The intersection sight distance for right turns is determined inthe same manner as for Case B1, except that the time gaps (tg) in Exhibit 9-54 should be adjusted

Field observations indicate that, in making right turns, driversgenerally accept gaps that are slightly shorter than those accepted in making left turns

Field observations indicate that, in making right turns, driversgenerally accept gaps that are slightly shorter than those accepted in making left turns

The time gaps in Exhibit 9-54 can be decreased by 1.0 s for right-turn maneuvers without undue interference with major-road traffic

The time gaps in Exhibit 9-54 can be decreased by 1.0 s for right-turn maneuvers without undue interference with major-road traffic

Case B2Case B2——Right turn from the Minor RoadRight turn from the Minor Road

The intersection sight distance along The intersection sight distance along the major road (dimension the major road (dimension ““bb”” in Exhibit in Exhibit 99‐‐50B) is determined by:50B) is determined by:

where:

US Customary

ISD = 1.47 V major t g (9-1)

intersection sight distance (length of the leg of sight

road) (ft)

ISD =

design speed of major road (mph)

V major =

triangle along the major

time gap for minor road vehicle to enter the major road (s)

t g =


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Exhibit 9-57. Time Gap for Case B2---Right Turn from Stopand Case B3---Crossing Maneuver

Exhibit 9-57. Time Gap for Case B2---Right Turn from Stopand Case B3---Crossing Maneuver




For multilane highwaysFor multilane highways::

For minor road approach grades:For minor road approach grades:

For crossing a major road with more than two lanes, add 0.5 secoFor crossing a major road with more than two lanes, add 0.5 seconds for passenger cars and nds for passenger cars and 0.7 seconds for trucks for each additional lane to be crossed an0.7 seconds for trucks for each additional lane to be crossed and for narrow medians that d for narrow medians that cannot store the design vehiclecannot store the design vehicle

If the approach grade is an upgrade that exceeds 3 percent; add If the approach grade is an upgrade that exceeds 3 percent; add 0.1 seconds for each 0.1 seconds for each percent gradepercent grade

Note: Time gaps are for a stopped vehicle to turn right onto or Note: Time gaps are for a stopped vehicle to turn right onto or cross a twocross a two--lane highway with no lane highway with no median and grades 3 percent or less. The table values require admedian and grades 3 percent or less. The table values require adjustment as follows:justment as follows:


Exhibit 9Exhibit 9--58. Design Intersection Sight Distance58. Design Intersection Sight Distance——Case B2Case B2——Right Turn fromRight Turn from Stop and Stop and Case B3Case B3——Crossing ManeuverCrossing Maneuver

Stopping Design sightSpeed distance Calculated Design(mph) (ft) (ft) (ft)

15 80 143.3 14520 115 191.1 19525 155 238.9 24030 200 286.7 29035 250 334.4 33540 305 382.2 38545 360 430.0 43050 425 477.8 48055 495 525.5 53060 570 573.3 57565 645 621.1 62570 730 668.9 67075 820 716.6 72080 910 764.4 765

passenger cars

US CustomaryIntersection sight

distance for

Note: Intersection sight distance shown is for a stopped passenger car to turn right onto or cross a two-lane highway with no median and grades 3 percent or less. For other conditions, the time gap must be adjusted and required sight distance recalculated.


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In most cases, the departure sight triangles for left and right turns onto the major road, as described for cases B1 and B2, will also provide more than adequate sight distance for minor-road vehicles to cross the major road

In most cases, the departure sight triangles for left and right turns onto the major road, as described for cases B1 and B2, will also provide more than adequate sight distance for minor-road vehicles to cross the major road

However, in the following situations, it is advisable to check the availability of sight distance for crossing maneuvers:

However, in the following situations, it is advisable to check the availability of sight distance for crossing maneuvers:

Where left and/or right turns are not permitted from a particular approach and the crossing maneuver is the only legal maneuver;

Where left and/or right turns are not permitted from a particular approach and the crossing maneuver is the only legal maneuver;

Where the crossing vehicle would cross the equivalent width of more than six lanes; or

Where the crossing vehicle would cross the equivalent width of more than six lanes; or

Where substantial volumes of heavy vehicles cross the highway and steep grades that might slow the vehicle while its back portion is still in the intersection are present on the departure roadway on the far side of the intersection

Where substantial volumes of heavy vehicles cross the highway and steep grades that might slow the vehicle while its back portion is still in the intersection are present on the departure roadway on the far side of the intersection

Case B3Case B3——Crossing the Major Road Crossing the Major Road from a Minorfrom a Minor--road approachroad approach

Yield Controlled IntersectionsYield Controlled Intersections

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Exhibits 9Exhibits 9--60 to 960 to 9--64 in AASHTO address Yield 64 in AASHTO address Yield controlled intersectionscontrolled intersections

Assumes vehicle will slow but not stopAssumes vehicle will slow but not stop

Experience shows that drivers tend to treat them as Experience shows that drivers tend to treat them as uncontrolled intersections and do not slow for uncontrolled intersections and do not slow for through movementsthrough movements

AASHTO assumes drivers turning left or right will AASHTO assumes drivers turning left or right will slow to 10 mphslow to 10 mph

Yield Controlled IntersectionsYield Controlled Intersections

Exhibit 9Exhibit 9--61: Case C1 61: Case C1 –– Length of Sight Triangle for Length of Sight Triangle for Crossing Maneuvers at Yield Controlled Crossing Maneuvers at Yield Controlled

IntersectionsIntersections

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Exhibit 9Exhibit 9--64: Case C2 64: Case C2 –– Design Intersection Sight Design Intersection Sight Distance for Left/Right Turns Yield Controlled Distance for Left/Right Turns Yield Controlled

IntersectionsIntersections

Decision Sight DistanceDecision Sight Distance

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Exhibit 3Exhibit 3--3: Decision Sight Distance3: Decision Sight Distance

Design ValuesDesign Values

The sum of the distance traversed during the brake reaction timeThe sum of the distance traversed during the brake reaction time and and the distance to brake the vehicle to a stop is the stopping sighthe distance to brake the vehicle to a stop is the stopping sight distancet distance

Where:Where:

VV = design speed, mph;= design speed, mph;a = deceleration rate, ft/sa = deceleration rate, ft/s²²

t = Brake reaction time in seconds

VV²²d = 1.47d = 1.47VVt + 1.075 t + 1.075 aa

US CustomaryUS Customary DriverDriver’’s eye is estimated to be 3.5 ft s eye is estimated to be 3.5 ft and the height of the object to be seen and the height of the object to be seen by the driver is 2 ft, equivalent to the by the driver is 2 ft, equivalent to the tail light height of a passenger car.tail light height of a passenger car.

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What Causes Corner Sight Distance Obstructions?

What Causes Corner Sight What Causes Corner Sight Distance Obstructions?Distance Obstructions?

Corner Sight Distance Corner Sight Distance ObstructionsObstructions

Parked VehiclesParked Vehicles

VegetationVegetation

Horizontal and vertical curvesHorizontal and vertical curves

SignsSigns

Offset curbsOffset curbs

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Vegetation Obstruction BeforeVegetation Obstruction Before

Vegetation Obstruction After TrimmingVegetation Obstruction After Trimming

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Vegetation Obstruction BeforeVegetation Obstruction Before

Vegetation Obstruction AfterVegetation Obstruction After

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Vertical Curve RestrictionVertical Curve Restriction

Corner sight distance blocked by signCorner sight distance blocked by sign

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Corner sight distance blocked by parkingCorner sight distance blocked by parking

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Sight distance obstruction causedSight distance obstruction causedby newspaper racksby newspaper racks

Now trees have been added to the mixNow trees have been added to the mix

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Sign and limit lineSign and limit lineplacement at intersectionsplacement at intersections

Source: MUTCDSource: MUTCD

Manual on Uniform Traffic Manual on Uniform Traffic Control Devices (MUTCD)Control Devices (MUTCD)

National MUTCD

National standard forall traffic controldevices installed onany street, highway orbicycle trail open to public travel.

Web site:mutcd.fhwa.dot.govWeb site:mutcd.fhwa.dot.govCompliance dates:http://mutcd.fhwa.dot.gov/kno-compliance.htm

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Source: Public Works Magazine, July 1981

Intersection looking left from a well Intersection looking left from a well position stop limit lineposition stop limit line

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Left-turn Sight Distance for Traffic Turning from Major

Street

LeftLeft--turn Sight Distance for turn Sight Distance for Traffic Turning from Major Traffic Turning from Major

StreetStreet

Sight distance triangles at driveways Sight distance triangles at driveways Reproduced with permission of the Transportation Research Board Reproduced with permission of the Transportation Research Board (TRB) (TRB) from the Access Management Manual, TRB, Washington DC, 2003from the Access Management Manual, TRB, Washington DC, 2003

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Transportation Research Board (TRB) Transportation Research Board (TRB) Access Management Manual, TRB, Access Management Manual, TRB,

Washington DC, 2003Washington DC, 2003

LeftLeft--turn sight distance at turn sight distance at driveway blocked by treesdriveway blocked by trees

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Source: AASHTO Policy on Design of Highways and Streets

Exhibit 9Exhibit 9--67 67 -- Intersection Sight Distance Intersection Sight Distance --LeftLeft--turn from the Major Road turn from the Major Road

Exhibit 9-54. Time Gap for Case F---Left Turns from Major RdExhibit 9Exhibit 9--54. Time Gap for Case F54. Time Gap for Case F------Left Turns from Major RdLeft Turns from Major Rd




For multilane highwaysFor multilane highways::For left turning vehicles that cross more than one opposing laneFor left turning vehicles that cross more than one opposing lane, add 0.5 seconds for , add 0.5 seconds for passenger cars or 0.7 seconds for trucks for each additional lanpassenger cars or 0.7 seconds for trucks for each additional lane to be crossed by the turning e to be crossed by the turning vehicle vehicle


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LeftLeft--turn sight distance blocked by trees turn sight distance blocked by trees Talk about protected permissive left turnsTalk about protected permissive left turns

Passing Sight DistancePassing Sight Distance

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No passing Zone on No passing Zone on Horizontal CurveHorizontal Curve

Passing Sight Passing Sight DistanceDistance

Passing sight distance is considered only on 2Passing sight distance is considered only on 2--lane lane roadsroads

At critical locations, a stretch of 3At critical locations, a stretch of 3-- or 4or 4--lane passing lane passing section with stopping sight distance is sometimes section with stopping sight distance is sometimes more economical than two lanes with passing sight more economical than two lanes with passing sight distance distance

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Dashed yellow center line on downhill Dashed yellow center line on downhill side of this up hill passing lane!side of this up hill passing lane!

Passing Sight Distance Passing Sight Distance -- AASHTOAASHTO

The sight distance available for passing at any place is the The sight distance available for passing at any place is the longest distance at which a driver whose eyes are 3.5 feet abovelongest distance at which a driver whose eyes are 3.5 feet abovethe pavement surface can see the top of an object 3.5 feet high the pavement surface can see the top of an object 3.5 feet high on the roadon the road

In general, 2In general, 2--lane highways should be designed to provide for lane highways should be designed to provide for passing where possible, especially those routes with high passing where possible, especially those routes with high volumes of trucks or recreational vehiclesvolumes of trucks or recreational vehicles

Passing should be done on tangent horizontal alignments with Passing should be done on tangent horizontal alignments with constant grades or a slight sag vertical curveconstant grades or a slight sag vertical curve

Minimum passing sight distance is about four times the Minimum passing sight distance is about four times the minimum stopping sight distance at the same design speedminimum stopping sight distance at the same design speed

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Sight Distance Standards

Design Speed Stopping Passing(mph) (ft) (ft)

20 125 80025 150 95030 200 110035 250 130040 300 150045 360 165050 430 180055 500 195060 580 210065 660 230070 750 250075 840 260080 930 2700

Passing Sight Distance Passing Sight Distance -- AASHTOAASHTO

Passing sight distance is Passing sight distance is the minimum sight distance the minimum sight distance required for the driver of one required for the driver of one vehicle to pass another vehicle vehicle to pass another vehicle safely and comfortably at a 10 safely and comfortably at a 10 mph speed differentialmph speed differential

Passing must be Passing must be accomplished assuming an accomplished assuming an oncoming vehicle comes into oncoming vehicle comes into view and maintains the design view and maintains the design speed, without reduction after speed, without reduction after the overtaking maneuver is the overtaking maneuver is startedstarted

Minimum Passing Sight Distances Minimum Passing Sight Distances -- MUTCDMUTCD

MUTCD Distances are shorter because the assumed MUTCD Distances are shorter because the assumed difference in speed is greaterdifference in speed is greater

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PedestriansPedestrians

Clear sight distance should be provided onClear sight distance should be provided onapproaches to a crosswalkapproaches to a crosswalk

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Clear sight distance based on the gap time needed for a Clear sight distance based on the gap time needed for a pedestrian to cross the street should be provided on the pedestrian to cross the street should be provided on the

approaches to a crosswalkapproaches to a crosswalk

Traffic SignalsTraffic Signals

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Cone of Vision Cone of Vision -- MUTCDMUTCD

Can you spot the signal?Can you spot the signal?

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Railroad XingsRailroad Xings

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http://www.ite.org/bookstore/gradecrossing/lo_res_RR_BOOK.pdf

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Sight Distance for RoundaboutsSight Distance for Roundabouts

Stopping Sight DistanceStopping Sight DistanceSource: FHWA Roundabout GuideSource: FHWA Roundabout Guide

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Source: Modern Roundabouts for Oregon (ODOT)Source: Modern Roundabouts for Oregon (ODOT)

Source: WA DOT Design Manual Source: WA DOT Design Manual –– Chapter 915Chapter 915

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Source: Caltrans Design Information Bulletin 80Source: Caltrans Design Information Bulletin 80--01:01:

Source: Caltrans Design Information Bulletin 80Source: Caltrans Design Information Bulletin 80--0101

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Roundabout stopping sight distanceRoundabout stopping sight distanceSource: Caltrans Design Information Bulletin 80Source: Caltrans Design Information Bulletin 80--01: Roundabouts01: Roundabouts

(6.5 seconds)(6.5 seconds)

Source: Modern Roundabouts for Oregon (ODOT)Source: Modern Roundabouts for Oregon (ODOT)

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QUESTIONS ?QUESTIONS ?

presented by nazir lalani p.e. traffex engineers inc. n ... distance.pdfpresented by nazir lalani...

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