Sight Distance Studies - National Association Of City .

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Sight DistanceSelect LocationPrepareComplete StudyDocument1. Review ordinances/regulations,speed studies, crash analysis, andcitizen input2. Site visitations3. Consult with other agencies1. Plan the data collection2. Review historical data trends1. Data collection2. Determine alternatives3. Review with neighborhood associations/property owners1. Finalize the report2. Communicate the results3. Implement improvements

INTRODUCTIONSight distance is the length of roadway visible to a driver. The three types of sight distance common inroadway design are intersection sight distance, stopping sight distance, and passing sight distance. Thishandbook will not discuss passing sight distance because it primarily occurs in rural settings and thishandbook generally addresses urban areas. (Information on passing sight distance can be found inChapter 3 of the AASHTO Green Book and in the CTRE Iowa Traffic Control Devices andPavement Markings manual.)INTERSECTION SIGHT D ISTANCEThe driver of a vehicle approaching or departing from an intersection should have an unobstructed viewof the intersection, including any traffic control devices, and sufficient lengths along the intersectinghighway to permit the driver to anticipate and avoid potential collisions (Maze and Plazak 2000). Theseunobstructed views form triangular areas known as sight triangles.A typical intersection is divided into areas between each leg known as quadrants. There may be threequadrants, such as for a “T” intersection, or four, such as for a four-legged intersection. Sight trianglesare the specified areas along an intersection’s approach legs and across the included corners (seeFigures 4.1 and 4.2 for an illustration). These areas should be clear of obstructions that might block adriver’s view of conflicting vehicles or pedestrians. The two types of sight triangles are approach sighttriangles and departure sight triangles (AASHTO, Green Book, 2001).Approach Sight TrianglesApproach sight triangles provide the driver of a vehicle approaching an intersection an unobstructedview of any conflicting vehicles or pedestrians. These triangular areas should be large enough thatdrivers can see approaching vehicles and pedestrians in sufficient time to slow or stop and avoid acrash. Approach sight triangles are illustrated in Figure 4.1.Sight Distance4.1

Minor StreetMinor StreetMajor StreetNMajor StreetDecision PointDecision PointFigure 4.1. Approach Sight TrianglesDeparture Sight TrianglesDeparture sight triangles provide adequate sight distance for a stopped driver on a minor roadway todepart from the intersection and enter or cross the major roadway. These sight triangles should beprovided in each quadrant of a controlled intersection. Departure sight triangles are illustrated in FigureMinor StreetMinor Street4.2.Major StreetDecision PointNMajor StreetDecision PointFigure 4.2. Departure Sight Triangles4.2Sight Distance

Obstructions within Sight TrianglesTo determine whether an object is a sight obstruction, consider both the horizontal and verticalalignment of both roadways, as well as the height and position of the object (AASHTO, Green Book).For passenger vehicles, it is assumed that the driver’s eye height is 3.5 feet and the height of anapproaching vehicle is 4.25 feet above the roadway surface, as illustrated in Figure 4.3. At the decisionMinor Streetpoint, as shown in Figure 4.3, the driver’s eye height is used for measurement.Vehicle Height4.25 feetMajor StreetDecision PointDriver Eye Height3.5 feetFigure 4.3. Heights Pertaining to Sight TrianglesAny object within the sight triangle that would obstruct the driver’s view of an approaching vehicle (4.25feet in height) should be removed or modified or appropriate traffic control devices should be installedas per the Manual on Uniform Traffic Control Devices. Obstructions within sight triangles could bebuildings, vehicles, hedges, trees, bushes, tall crops, walls, fences, etc. Figure 4.4 shows a clear sighttriangle and an obstructed sight triangle.Sight Distance4.3

Clear Sight TriangleObstructed Sight TriangleMinor StreetMinor StreetNMajor StreetMajor StreetDecision PointObstructionDecision PointRowRowFigure 4.4. Clear versus Obstructed Sight TrianglesSIGHT DISTANCE STUDY METHODSDifferent types of traffic control require different sight distances. For example, intersections with nocontrol require adequate distance for the approaching vehicle to identify any conflicts in or approachingthe intersection before entering. An approach sight triangle is used for this analysis. However,intersections with stop or yield control require drivers to stop or yield at the intersection, check forapproaching vehicles in the intersection, and then depart. A departure sight triangle is used for thisanalysis.Sight Distance Study Preparation ChecklistWhen preparing for an intersection sight distance study, use the checklist in Table 4.1. The checklistmay be modified or expanded as necessary.4.4Sight Distance

Table 4.1. Sight Distance Study Preparation ChecklistStepObtain target and sighting rodsObtain measuring wheelObtain hardhat and safety vestObtain sight distance diagram formSelect time and dayDetermine availability of observersContact corresponding jurisdiction(s)Other: When CompleteNotesIf an agency does not possess the equipment necessary to complete a sight distance study, it may beobtained from the Iowa DOT, another jurisdiction, or a responsible consulting firm. A blank sight distancediagram form is located in Appendix C. Information on contracting for a sight distance study, including aproject work order example, is provided near the end of this chapter.UNCONTROLLED INTERSECTIONSFor uncontrolled intersections, the drivers of both approaching vehicles should be able to see conflictingvehicles in adequate time to stop or slow to avoid a crash. The required sight distance for safe operation atan uncontrolled intersection is directly related to the vehicle speeds and the distances traveled duringperception, reaction, and braking time. Table 4.2 lists the minimum recommended sight distances forspecific design speeds. For example, if a vehic le is traveling 20 mph, a sight distance of 90 feet is theminimum recommended stopping sight distance.Table 4.2. Minimum Recommended Sight DistancesVehicle Speed(mph)152025303540455055Stopping Sight Distance(feet)7090115140165195220245285Note: Distances are from the 2001 AASHTO Green Book and 2001AASHTO Little Green Book. Distances may change in future versions.Sight Distance4.5

Key Steps to a Sight Distance Study at an UncontrolledIntersectionA sight distance study at an uncontrolled intersection includes four key steps:1. Determine the minimum recommended sight distance.2. Obtain or construct sighting and target rods.3. Measure current sight distances and record observations.4. Perform sight distance analysis.Determine the Minimum Recommended Stopping Sight DistanceDetermine the minimum sight distance for the posted or operating speed at the intersecting roadway (seeTable 4.2).Obtain or Construct Sighting and Target RodsSighting and target rods are illustrated in Figure 4.5. The target rod can be constructed out of 2-inch by0.75-inch wood. The target rod should be 4.25 feet tall to represent the vehicle height and be paintedfluorescent orange on both the top portion and bottom 2 feet of the rod. The bottom 2-foot portionrepresents the object height for measuring stopping sight distance. (This will be further explained later inthe stopping sight distance section.) The sighting rod should be 3.5 feet tall to represent the driver’s eyeheight. The sighting rod can be constructed out of the same type of wood but should be painted flat black.The sighting rod and target rod are used in measuring sight distance.Figure 4.5. Sighting Rod (left) and Target Rod (right)4.6Sight Distance

Measure Current Sight Distances and Record ObservationsSight distance measurements should be gathered for all legs of the uncontrolled intersection. Trafficapproaching from both the left and right should be considered for measurements. On the sight distanceintersection diagram (a blank diagram form is provided in Appendix C), the observer records the dateand time, posted or operating speed, site location, and weather conditions. The measuring process isrepresented in Figure 4.6 and described ObserverFigure 4.6. Sight Distance Measurement at Uncontrolled IntersectionThe observer holds the sighting rod, and the assistant holds the target rod. They position themselves ontwo intersecting approaches at the appropriate stopping sight distances taken from Table 4.2. These arethe X and Y dimensions. The observer represents the approaching vehicle and is located at the decisionpoint. The observer uses the 3.5-foot sighting rod, which represents the driver’s eye height. Theassistant represents the intersecting vehicle. The assistant uses the 4.25-foot target rod, whichrepresents the height of the approaching vehicle. The observer sights from the top of the sighting rod tothe target rod.Sight Distance4.7

If the target rod is visible, the approach sight triangle for the intersection is appropriate. If the top of thetarget rod is not visible, the assistant holding the target rod should walk toward the intersection along thecenterline of the intersecting lane until the observer can see the target rod. When the target rod is visible,the position should be marked and the distance to the intersection should be measured along thecenterline of the roadway. This is the X dimension.Perform Sight Distance AnalysisThe analysis of intersection sight distance consists of comparing the recommended sight distance to themeasured sight distance. The measured sight distance should be equal to or greater than therecommended stopping sight distance. If the measured sight distance is less than the recommended sightdistance, some mitigation may be required. Some mitigation measures are as follows: Remove/modify obstruction.Reduce speeds.Install traffic control devices (if warranted by the MUTCD).Example Sight Distance Study at an UncontrolledIntersectionThe city of Cottonwood Glen noticed an increase of crashes at the intersection of 6th Street andPhoenix Avenue. The city suspected that the crash problem may be related to sight distance. Theproblem seemed to be centered around vehicles traveling northbound at the intersection. CottonwoodGlen decided to conduct a sight distance study at the intersection to see whether that was a contributingfactor.The intersection of 6th Street and Phoenix Avenue has no traffic control. The posted speed limit forboth of the roadways is 25 mph. Cottonwood Glen referred to Table 4.2 for the recommended sightdistance for this situation: 115 feet for both roadways. Cottonwood Glen conducted the study on aTuesday at 2:00 p.m. under clear weather conditions. The study was conducted early afternoon to4.8Sight Distance

avoid heavy traffic volumes. City staff measured the sight distance for the eastbound and westboundapproaches.Figure 4.7 shows that the measured sight distance on the west approach is 140 feet. The recommendedstopping sight distance for this approach is 115 feet. This tells us that the measured sight distancesatisfies the minimum recommended. No sight distance related improvements need to be considered onthe west approach.Figure 4.8 shows that the measured sight distance on the east approach is 100 feet. The recommendedstopping sight distance for this approach is 115 feet. This tells us that the measured sight distance doesnot satisfy the minimum recommended. The stopping sight distance diagram shows that there is anobstruction limiting the sight distance, located outside of the right-of-way. In this situation, the propertyowner should be contacted for cooperation in eliminating, modifying, or moving the obstruction. If theyare unwilling to cooperate, other mitigation measures should be considered.Sight Distance4.9

DateTime of DayPosted Speed Limit or 85% forMajor Roadway (X(R))Posted Speed Limit or 85% forMinor Roadway (Y)Traffic Controls PresentIntersection ManeuverWeatherHorizontal CurveVertical CurveMM/DD/YY1400Major Roadway WidthNo. of Lanes40 feet225 mphMinor Roadway Width30 feet25 mphNo. of LanesNo ControlN.A.ClearNNY Stopping DistanceX(R) RecommendedX(M) Measured2115 feet115 feet140 feet30ftX(M)X(R)coShrubsNorthPhoenix Avenue40ftDi m mstan e nce d eMe(X d S(R i gaDisu)) h tsta renc d Se(X ight(M))6th StreetReYROWBuildingDecision PointConclusion: X(M) X(R). The measured sight distance was 140 feet, which is more than therecommended sight distance of 115 feet. Sight distance on the west approach is adequate.Figure 4.7. 6th Street and Phoenix Avenue, West Approach4.10Sight Distance

DateTime of DayPosted Speed Limit or 85% forMajor Roadway (X(R))Posted Speed Limit or 85% forMinor Roadway (Y)Traffic Controls PresentIntersection ManeuverWeatherHorizontal CurveVertical CurveMM/DD/YY1400Major Roadway WidthNo. of Lanes40 feet225 mphMinor Roadway Width30 feet25 mphNo. of LanesNo ControlN.A.ClearNNY Stopping DistanceX(R) RecommendedX(M) Measured2115 feet115 feet100 feet30ft6th StreetShrubstghhtSi )g)Si M) d e d (R)ed (X( e n (Xrsu ce m m nceo aea nM ista e c istR DDNorthX(R)40ftX(M)Phoenix AvenueObstructionYROWBuildingDecision PointConclusion: X(M) X(R). The measured sight distance was 100 feet, which is less than therecommended sight distance of 115 feet. There is an obstruction limiting sight distance andit is outside of the right-of-way.Figure 4.8. 6th Street and Phoenix Avenue, East ApproachSight Distance4.11

I N T E R S E C T I O N S W I T H ST O P S I G N C O N T R O LVehicles stopped at an at-grade intersection must have sufficient sight distance to permit a safe departure.At intersections with stop sign or yield control, close attention should be given to departure sight triangles.Vehicle Maneuvers at Intersections with Stop Sign ControlThree maneuvers can be completed for vehicles stopped at an intersection: crossing maneuver, left-turnMinor StreetCrossover Maneuvermaneuver, and right-turn maneuver. See Figure 4.9.Major StreetLeft Turn ManeuverRight Turn ManeuverStopFigure 4.9. Three Maneuvers at an Intersection with Stop Sign ControlCrossing Maneuver from the Minor RoadwayWhen a driver is completing a crossing maneuver, there must be sufficient sight distance in both directionsavailable to cross the intersecting roadway and avoid approaching traffic. The sight distance required forthis maneuver is based on the distance approaching vehicles will travel on the major road during the timeperiod it takes a stopped vehicle to clear the intersection. Table 4.3 lists the recommended sight distancesfor this maneuver based on design speeds.4.12Sight Distance

Turning Left from the Minor RoadwayThe left-turn maneuver requires first clearing the traffic on the left, then entering the traffic stream on theright. The required sight distance for this maneuver is affected by the amount of time it takes the stoppedvehicle to turn left clearing traffic and reach average running speed without affecting the speed of theapproaching vehicle. Table 4.3 lists the recommended sight distances for this maneuver based on designspeeds.Turning Right from the Minor RoadwayThe right turn maneuver must have sufficient sight distance to permit entrance onto the intersectingroadway and then accelerate to the posted speed limit without being overtaken by approaching vehicles.Table 4.3 lists the minimum recommended sight distances for this maneuver based on design speeds.Table 4.3. Minimum Recommended Sight Distances Based on Vehicle ManeuverVehicle Speed(mph)Stopping Sight Distance forLeft-Turn Maneuver (feet)Stopping Sight Distance for Crossoverand Right-Turn Maneuvers 0145195240290335385430480530Note: Distances are from the 2001 AASTHO Green Book and are for two-lane roadways. Distances maychange in future versions.Key Steps to a Sight Distance Study at an Intersection withStop ControlA sight distance study at an intersection with stop control includes four key steps:1. Determine the minimum recommended sight distance.2. Obtain or construct sighting and target rods.3. Measure current sight distances and record observations.4. Perform sight distance analysis.Sight Distance4.13

Determine the Minimum Recommended Sight DistancesDetermine the minimum sight distance for each maneuver and speed (see Table 4.3).Obtain or Construct Sighting and Target RodsSighting and target rods are illustrated in Figure 4.5. The target rod can be constructed from 2-inch by0.75-inch wood. The target rod should be 4.25 feet tall to represent the vehicle height and be paintedfluorescent orange on both the top portion and bottom 2 feet of the rod. The bottom 2-foot portionrepresents the object height for measuring stopping sight distance. The sighting rod should be 3.5 feettall to represent the driver’s eye height. The sighting rod can be constructed from the same type ofwood but should be painted flat black. The sighting rod and target rod are used in measuring sightdistance.Measure Current Sight Distances and Record ObservationsOn the sight distance intersection diagram (a blank diagram form is provided in Appendix C), theobserver records the date and time, posted or operating speed site location, and weather conditions.The observer with the sighting rod stands at the center of the approaching lane and 10 feet back fromthe stop bar or aligned with the stop sign. The observer’s eyes should be at the top of the sighting rod.The assistant walks away from the observer along the intersecting roadway toward approaching traffic.The assistant should stop periodically and place the target rod on the pavement for sighting by theobserver. This process should continue until the top of the target rod can no longer be seen. The pointwhere the target rod disappears is the maximum sight distance along that leg and should be recordedfrom the observer’s sight.Perform Sight Distance AnalysisThe analysis of intersection sight distance consists of comparing the recommended sight distance to themeasured available sight distance. The comparison of the actual distances should be performed with4.14Sight Distance

consideration to the greater of the 85th percentile of speed or the posted speed limit. If the measuredsight distance is less than the recommended sight distance some mitigation may be required. Somemitigation measures are as follows: Remove/modify obstruction.Reduce speed.Install traffic control devices (if warranted by the MUTCD).Example Sight Distance Study at an Intersection withStop Sign ControlThe city of Cottonwood Glen was changing the speed limit on one of their arterial streets from 25 to 30mph. This change required the city to conduct an intersection sight distance study. The only intersectionin question was the T-intersection of Ross Road and 13th Street. The east approach for both left- andright-turn maneuvers need to be studied. The city obtained recommended sight distances from Table4.3: 335 feet for a right-turn maneuver and 290 feet for a left-turn maneuver.The study was conducted on a Wednesday at 2:00 pm at the intersection of Ross Road and 13thStreet. Because of the need for adequate sight distance for both left- and right-turn maneuvers, twoseparate sight distance measurements were made.Figure 4.10 shows the measurement that was conducted for the right-turn maneuver. The measuredsight distance for this maneuver is 350 feet. The minimum recommended sight distance is 335 feet for 30mph. This shows that there is adequate sight distance for a right-turn maneuver from the side road, 13thStreet.Figure 4.11 shows the measurement that was conducted for a left-turn maneuver. The measured sightdistance for this maneuver is 300 feet. The minimum recommended sight distance is 290 for 30 mph.Th

Sight Distance 4.1 INTRODUCTION Sight distance is the length of roadway visible to a driver. The three types of sight distance common in roadway design are intersection sight distance, stopping sight distance, and passing sight distance.

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