Chapter 1260 Sight Distance

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Chapter t DistanceGeneralReferencesStopping Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft)Passing Sight Distance (Eye height – 3.5 ft, Object height – 3.5 ft)Decision Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft)Documentation1260.01 GeneralSight distance allows the driver to assess developing situations and take actions appropriatefor the conditions. Sight distance relies on drivers being aware of and paying attention to theirsurroundings and driving appropriately for conditions presented. For the purposes of design,sight distance is considered in terms of stopping sight distance, passing sight distance, anddecision sight distance.For additional information, see the following chapters:Chapter12501310132013401515SubjectSight distance at railroad crossingsSight distance at intersections at gradeSight distance at roundaboutsSight distance at drivewaysSight distance for shared-use paths1260.02 References1260.02(1)Design GuidanceManual on Uniform Traffic Control Devices for Streets and Highways, USDOT, FHWA; asadopted and modified by Chapter 468-95 WAC “Manual on uniform traffic control devicesfor streets and highways” (MUTCD)1260.02(2) Supporting InformationA Policy on Geometric Design of Highways and Streets (Green Book), AASHTOPassing Sight Distance Criteria, NCHRP 6051260.031260.03(1)Stopping Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft)Design CriteriaStopping sight distance is provided when the sight distance available to a driver equals or exceedsthe stopping distance for a passenger car traveling at the design speed.Stopping distance for design is very conservatively calculated, with lower deceleration andslower perception reaction time than normally expected from the driver. Provide design stoppingsight distance at all points on all highways and on all intersecting roadways, unless a designanalysis is deemed appropriate.WSDOT Design Manual M 22-01.13July 2016Page 1260-1

Sight Distance1260.03(1)(a)Chapter 1260Stopping Sight DistanceStopping sight distance isthe sum of two distances:the distance traveled duringperception and reaction timeand the distance to stop thevehicle. The perception andreaction distance used indesign is the distance traveledin 2.5 seconds at the designspeed.The design stopping sightdistance is calculated using thedesign speed and a constantdeceleration rate of 11.2feet/second2. For stoppingsight distances on grades lessthan 3%, see Exhibit 1260-1;for grades 3% or greater, seeExhibit 1260-2.1260.03(1)(b)Design Stopping Sight DistanceExhibit 1260-1 gives the design stopping sight distances for grades less than 3%, the minimumcurve length for a 1% grade change to provide the stopping sight distance for a crest (Kc) andsag (Ks) vertical curve, and the minimum length of vertical curve for the design speed (VCLm).For stopping sight distances when the grade is 3% or greater, see Exhibit 1260-2.Design Speed(mph)Design StoppingSight Distance (ft)KcKsVCLm 10225240Design Stopping Sight DistanceExhibit 1260-1Page 1260-2WSDOT Design Manual M 22-01.10July 2013

Chapter 1260Sight Distance1260.03(2) Effects of GradeThe grade of the highway has an effect on the stopping sight distance. The stopping distance isincreased on downgrades and decreased on upgrades. Exhibit 1260-2 gives the stopping sightdistances for grades of 3% and steeper. When evaluating sight distance with a changing grade,use the grade for which the longest sight distance is needed.Design Speed(mph)253035404550556065707580Stopping Sight Distance 8736817Design Stopping Sight Distance on GradesExhibit 1260-2For stopping sight distances on grades between those listed, interpolate between the valuesgiven or use the equation in Exhibit 1260-3.S 1.47V (2.5) V2 G 30 0.347826 100 Where:S Stopping sight distance on grade (ft)V Design speed (mph)G Grade (%)Stopping Sight Distance on GradesExhibit 1260-31260.03(3)Crest Vertical CurvesWhen evaluating an existing roadway, refer to 1260.03(7).Use Exhibit 1260-4 or the equations in Exhibit 1260-5 to find the minimum crest vertical curvelength to provide stopping sight distance when given the algebraic difference in grades. Exhibit1260-4 does not show the sight distance greater than the length of curve equation.WSDOT Design Manual M 22-01.10July 2013Page 1260-3

Sight DistanceChapter 1260When the sight distance is greater than the length of curve and the length of curve is critical,the S L equation given in Exhibit 1260-5 shall be used to find the minimum curve length.When a new crest vertical curve is built or an existing one is rebuilt with grades less than 3%,provide design stopping sight distance from Exhibit 1260-1. For grades 3% or greater, providestopping sight distance from 1260.03(2).SObject height h2 2.0'Eye height h1 3.5'0fS 30,ph5045mph,S h,mp40542365f50 fS 2ft.35 mph, 200.ph, SS 155 ft1230 m25 mph,Algebraic Difference in Grade, A (%)14t.t.16t.The minimum length can also be determined by multiplying the algebraic difference in grades by theKC value from Exhibit 1260-1 (L KC*A). Both the exhibit and the equation give approximately thesame length of curve. Neither use the S L equation.ft.549ft. S ,Sphm55t.0fmh,p0m7 5S6.5 ftph,64S 10m65870.0 ft73, S hmp.20 ft, S 8hp.75 m10 ft, S 9hp80 m64200500150010002000Length of Vertical Curve, L (ft)Stopping Sight Distance: Crest Vertical CurvesExhibit 1260-4Page 1260-4WSDOT Design Manual M 22-01.10July 2013

Chapter 1260Sight DistanceWhen S L 2158 L A S 22158L 2S AWhen S LL AS 22158S 2158LAWhere:L Length of vertical curve (ft)S Sight distance (ft)A Algebraic difference in grades (%)Sight Distance: Crest Vertical CurveExhibit 1260-51260.03(4) Sag Vertical CurvesWhen evaluating an existing roadway, refer to 1260.03(7).Sight distance is not restricted by sag vertical curves during the hours of daylight. Therefore,headlight sight distance is used for the sight distance design criteria at sag vertical curves.In some cases, a lesser length may be allowed. For guidance, see Chapter 1220.Refer to Exhibit 1260-6 or the equations in Exhibit 1260-7 to find the minimum length fora sag vertical curve to provide the headlight stopping sight distance when given the algebraicdifference in grades. The value for S is shown as the distance between the vehicle and thepoint where a 1-degree angle upward of the headlight beam intersects with the roadway.The sight distance greater than the length of curve equation is not used in Exhibit 1260-6. Whenthe sight distance is greater than the length of curve and the length of curve is critical, the S Lequation given in Exhibit 1260-7 shall be used to find the minimum length of curve.When a new sag vertical curve is built or an existing one is rebuilt with grades less than 3%,provide design stopping sight distance from Exhibit 1260-1. For grades 3% or greater, providestopping sight distance from 1260.03(2).WSDOT Design Manual M 22-01.10July 2013Page 1260-5

Sight DistanceChapter 1260S1 2.0'ft 305phm45ft95,S,S 3,Sphm4052 460ftt0f25S h,mp35mp3012h, SS 125 mph,14Algebraic Difference in Grade, A (%) 2055 ft160 ftThe minimum length can also be determined by multiplying the algebraic difference in grades by the KSvalue from Exhibit 1260-1 (L KS*A). Both the exhibit and equation give approximately the same lengthof curve. Neither use the S L equation.ph50m,phft4S m55t0f57 ,Sh60mph,65mp5 ft64S ft30S 7,fthmp S 82070,htmp10 f75S 9,hp80 h of Curve, L (ft)Stopping Sight Distance for Sag Vertical CurvesExhibit 1260-6Page 1260-6WSDOT Design Manual M 22-01.10July 2013

Chapter 1260Sight DistanceWhen S LL 2S 400 3.5SAS LA 4002 A 3.5When S LL AS 2400 3.5SS 3.5 L (3.5L )2 1600 AL2AWhere:L Curve length (ft)A Algebraic grade difference (%)S Sight distance (ft)Note:Values for A less than 1.75 are within the 1-degree divergeof the headlight beam and therefore do not need to beevaluated for SSD on sag curves.Sight Distance: Sag Vertical CurveExhibit 1260-71260.03(5)Horizontal CurvesWhen evaluating an existing roadway, see 1260.03(7).Use Exhibit 1260-8 or the equation in Exhibit 1260-9 to check stopping sight distance wheresightline obstructions are on the inside of a curve. A stopping sight distance sightline obstructionis any roadside object within thehorizontal sightline offset (M)distance (such as median barrier,guardrail, bridges, walls, cutslopes, buildings or woodedareas), 2.0 feet or greater abovethe roadway surface at thecenterline of the lane on the insideof the curve (h0). Exhibit 1260-8and the equation in Exhibit 1260-9are for use when the length ofcurve is greater than the sightdistance and the sight restriction ismore than half the sight distancefrom the end of the curve. Wherethe length of curve is less than thestopping sight distance or the sightrestriction is near either end of the curve, the desired sight distance may be available with a lesserM distance. When this occurs, the sight distance can be checked graphically.WSDOT Design Manual M 22-01.10July 2013Page 1260-7

Sight DistanceChapter 1260Sight distance (S)A sightline obstruction is anyroadside object within thehorizontal sightline offset (M)distance, 2.0 feet or greaterabove the roadway surface atthe centerline of the lane onthe inside of the curve.MLine ofsightObstruction orbackslopeRadiusCenter of lane506580mpft20 8,Sphftm07375 ,Sphftm45 670Sh,ft70mp 5h, S60Lateral Clearance to Obstruction, M (ft)6040mph,S30 910ft201025 mph, 30 mph, 35 mph,S 155 ft S 200 ft S 250 ft001,00040 mph,S 305 ft2,00045 mph,S 360 ft3,00050 mph,S 425 ft4,00055 mph,S 495 ft5,0006,000Curve Radius, R (ft)Horizontal Stopping Sight DistanceExhibit 1260-8Page 1260-8WSDOT Design Manual M 22-01.10July 2013

Chapter 1260Sight DistanceWhen the road grade is less than 3%, provide design stopping sight distance from Exhibit 1260-1.When the grade is 3% or greater, provide stopping sight distance from 1260.03(2).Roadside objects with a height (h0) between 2.0 feet and 2.75 feet might not be a stopping sightdistance sightline obstruction. Objects with an h0 between 2.0 feet and 2.75 feet can be checkedgraphically to determine whether they are stopping sight distance sightline obstructions.Where a sightline obstruction exists and site characteristics preclude design modifications to meetcriteria, consult with the region Traffic Engineer and Assistant State Design Engineer for adetermination of appropriate action. 28.65S M R 1 cos R S R 1 R M cos 28.65 R Where:M Horizontal sightline offset measured from the centerline ofthe inside lane of the curve to the sightline obstruction (ft)R Radius of the curve (ft)S Sight distance (ft)Sight Distance: Horizontal CurvesExhibit 1260-91260.03(6)Overlapping Horizontal and Vertical CurvesVertical curves on a horizontal curve have an effect on which roadside objects are sightlineobstructions. Crest vertical curves make roadside objects more likely to become sightlineobstructions. Sag vertical curves make roadside objects less likely to be sightline obstructions.Exhibit 1260-10 can be used to determine the sight distance for crest vertical curves on horizontalcurves with: Sightlineobstructionsinside theM distance.CL of laneSightlineobstructionheight (h0)of 2.0 feetor less.For otherlocations, thesight distancecan be checkedgraphically.IMILine ofSightRoadside sightlineobstruction 2.0' orless above CL of laneEdge of roadwayhOh1 3.5'WSDOT Design Manual M 22-01.10July 2013h2 2.0'Page 1260-9

Sight DistanceChapter 1260The following equation may be used to determine the sight distance for roadside sightlineobstructions inside the horizontal sightline offset (M) distance (see Exhibit 1260-9) witha height of 2.0 feet or less above the centerline of the lane on the inside of the curve onoverlapping horizontal and crest vertical curves.S [100 L 2(h1 h0 ) 2(h2 h0 )]2AWhere:L Length of vertical curve (ft)S Sight distance (ft)A Algebraic difference in grades (%)h1 Eye height (3.5 ft)h2 Object height (2.0 ft)h0 Height of roadside sightline obstructions above thecenterline of the inside curve lane (2.0 ft or less)Note:The above equation cannot be used for sightline obstructionheight (h0) more than 2.0 ft above the centerline of the lane onthe inside of the curve. The available sight distance must bechecked graphically for these sightline obstructions.Sight Distance: Overlapping Horizontal and Crest Vertical CurvesExhibit 1260-101260.03(7) Existing Stopping Sight DistanceExisting stopping sight distance values from Exhibit 1260-11 may be used at all horizontaland vertical curves where all of the following are met at the curve: There is no identified collision trend.The existing vertical and horizontal alignment is retained.The existing roadway pavement is not reconstructed.The roadway will not be widened, except for minor shoulder widening requiring no workpast the bottom of the ditch.The sightline obstruction is existing.Roadside improvements to sight distance are within existing right of way.Crest Vertical Curves – The minimum length of an existing crest vertical curve may be foundusing the equations in Exhibit 1260-5 or using the KC values from Exhibit 1260-11.Sag Vertical Curves – The minimum length of an existing sag vertical curve may be found usingthe equations in Exhibit 1260-7 or using the KS values from Exhibit 1260-11. In some cases,when continuous illumination is provided, a lesser length may be allowed. For guidance, seeChapter 1220.Page 1260-10WSDOT Design Manual M 22-01.10July 2013

Chapter 1260Sight DistanceDesign Speed(mph)Existing StoppingSight Distance 058511413515911512714080630184152Existing Stopping Sight DistanceExhibit 1260-111260.041260.04(1)Passing Sight Distance (Eye height – 3.5 ft, Object height – 3.5 ft)Design CriteriaMinimum passing sight distanceis the distance (on a two-lanehighway) used for a driverto execute a normal passingmaneuver based on designconditions and design speed.The potential for passing maneuverconflicts is ultimately determinedby the judgments of the driver andthe conditions present at the timeof the maneuver. Exhibit 1260-12gives the passing sight distancesfor various design speeds.Design Speed (mph)Minimum Passing SightDistance 657075110012001300801400Passing Sight DistanceExhibit 1260-12WSDOT Design Manual M 22-01.10July 2013Page 1260-11

Sight DistanceChapter 1260On two-lane two-way highways, provide passing opportunities to meet traffic volume demands.This can be accomplished with roadway sections that provide passing sight distance or by addingpassing lanes at locations that would provide the greatest benefit to passing (see Chapter 1270).In the design stage, passing sight distance can be provided by adjusting the alignment eithervertically or horizontally to increase passing opportunities.These considerations also apply to multilane highways where staged construction includes atwo-lane two-way operation as an initial stage. Whether auxiliary lanes are provided, however,depends on the time lag proposed between the initial stage and the final stage of construction.1260.04(2) Passing Sight Distance Vertical CurvesExhibit 1260-14 gives the length of crest vertical curve used to provide passing sight distance fortwo lane highways. The distance from Exhibit 1260-12 and the equations in Exhibit 1260-13 mayalso be used to determine the minimum length of vertical curve to meet the passing sight distancecriteria.Sag vertical curves are not a restriction to passing sight distance.AEye height h1 3.5'Object height h2 3.5'SWhen S LL 2S 2800AS L 1400 2AS 2800 LAWhen S LL AS 22800Where:L Length of vertical curve (ft)A Algebraic grade difference (%)S Sight distance (ft)Passing Sight Distance: Crest Vertical Curve CalculationsExhibit 1260-13Page 1260-12WSDOT Design Manual M 22-01.10July 2013

Chapter 1260Sight DistancePassing Sight Distance: Crest Vertical CurvesExhibit 1260-141260.04(3) Passing Sight Distance Horizontal CurvesPassing sight distance can be restricted on the inside of a horizontal curve by sightlineobstructions that are 3.5 feet or more above the roadway surface. Use the distance fromExhibit 1260-12 and the equation in Exhibit 1260-9 to determine whether the object is closeenough to the roadway to be a restriction to passing sight distance. The equation assumes thatthe curve length is greater than the sight distance. Where the curve length is less than the sightdistance, the desired sight distance may be available with a lesser sightline offset (M) distance.WSDOT Design Manual M 22-01.10July 2013Page 1260-13

Sight Distance1260.05Chapter 1260Decision Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft)Decision sight distance values are greater than stopping sight distance values because theygive the driver an additional margin for error and afford sufficient length to maneuver at thesame or reduced speed rather than to just stop.Consider decision sight distances (see Exhibit 1260-15) at locations where there is highlikelihood for driver error in information reception, decision making, or control actions.If site characteristics and budget allow, locate these highway features where decision sightdistance can be provided. If this is not practicable, use suitable traffic control devices andpositive guidance to give advanced warning of the conditions.Decision Sight Distance for Maneuvers (ft)Design 118012601365145515451650Decision Sight DistanceExhibit 1260-15The maneuvers in Exhibit 1260-15 are as follows:A Rural stopD Suburban speed/path/direction changeB Urban stopE Urban speed/path/direction changeC Rural speed/path/direction changeUse the equations in Exhibits 1260-5, 1260-7, and 1260-9 to determine the available decisionsight distance for crest vertical curves, sag vertical curves, and horizontal curves.1260.06DocumentationIt is recognized that some designs do not allow all criteria and guidelines to be followed asoutlined in this chapter.Refer to Chapter 300 for design documentation requirements.Page 1260-14WSDOT Design Manual M 22-01.10July 2013

sight distance at all points on all highways and on all intersecting roadways, unless a design analysis is deemed appropriate. Sight Distance Chapter 1260 Page 1260-2 WSDOT Design Manual M 22-01.10 July 2013. 1260.03(1)(a) Stopping Sight Distance . Stopping sight distance is

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