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Select Location Complete Study Prepare Document Sight Distance 1. Review ordinances/regulations, speed studies, crash analysis, and citizen input 1. Plan the data collection 2. Review historical data trends 2. Site visitations 1. Data collection 2. Determine alternatives 1. Finalize the report 3. Implement improvements 2. Communicate the results 3. Consult with other agencies 3. Review with neighborhood associations/ property owners Sight Distance 4.1 INTRODUCTIONINTRODUCTION 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. This handbook will not discuss passing sight distance because it primarily occurs in rural settings and this handbook generally addresses urban areas. (Information on passing sight distance can be found in Chapter 3 of the AASHTO Green Book and in the CTRE Iowa Traffic Control Devices and Pavement Markings manual.) INTERSECTION SIGHT DINTERSECTION SIGHT D ISTANCEISTANCE The driver of a vehicle approaching or departing from an intersection should have an unobstructed view of the intersection, including any traffic control devices, and sufficient lengths along the intersecting highway to permit the driver to anticipate and avoid potential collisions (Maze and Plazak 2000). These unobstructed views form triangular areas known as sight triangles. A typical intersection is divided into areas between each leg known as quadrants. There may be three quadrants, such as for a “T” intersection, or four, such as for a four-legged intersection. Sight triangles are the specified areas along an intersection’s approach legs and across the included corners (see Figures 4.1 and 4.2 for an illustration). These areas should be clear of obstructions that might block a driver’s view of conflicting vehicles or pedestrians. The two types of sight triangles are approach sight triangles and departure sight triangles (AASHTO, Green Book, 2001). ApproAppro ach Sight Tr ianglesach Sight Tr iangles Approach sight triangles provide the driver of a vehicle approaching an intersection an unobstructed view of any conflicting vehicles or pedestrians. These triangular areas should be large enough that drivers can see approaching vehicles and pedestrians in sufficient time to slow or stop and avoid a crash. Approach sight triangles are illustrated in Figure 4.1. 4.2 Sight Distance Major Street Major Street M in or S tre et M in or S tre et Decision Point Decision Point N Figure 4.1. Approach Sight Triangles Departure Sight Tr ianglesDeparture Sight Tr iangles Departure sight triangles provide adequate sight distance for a stopped driver on a minor roadway to depart from the intersection and enter or cross the major roadway. These sight triangles should be provided in each quadrant of a controlled intersection. Departure sight triangles are illustrated in Figure 4.2. Major Street Major Street M in or S tre et M in or S tre et Decision PointDecision Point N Figure 4.2. Departure Sight Triangles Sight Distance 4.3 Obstruct ions within Sight Tr ianglesObstruct ions within Sight Tr iangles To determine whether an object is a sight obstruction, consider both the horizontal and vertical alignment 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 an approaching vehicle is 4.25 feet above the roadway surface, as illustrated in Figure 4.3. At the decision point, as shown in Figure 4.3, the driver’s eye height is used for measurement. Major Street M in or S tre et Decision Point Driver Eye Height 3.5 feet Vehicle Height 4.25 feet Figure 4.3. Heights Pertaining to Sight Triangles Any object within the sight triangle that would obstruct the driver’s view of an approaching vehicle (4.25 feet in height) should be removed or modified or appropriate traffic control devices should be installed as per the Manual on Uniform Traffic Control Devices. Obstructions within sight triangles could be buildings, vehicles, hedges, trees, bushes, tall crops, walls, fences, etc. Figure 4.4 shows a clear sight triangle and an obstructed sight triangle. 4.4 Sight Distance Major Street Major Street M in or S tr ee t M in or S tr ee t Decision Point Decision Point Obstruction Clear Sight Triangle Obstructed Sight Triangle RowRow N Figure 4.4. Clear versus Obstructed Sight Triangles SIGHT DISTANCE STUDYSIGHT DISTANCE STUDY METHODS METHODS Different types of traffic control require different sight distances. For example, intersections with no control require adequate distance for the approaching vehicle to identify any conflicts in or approaching the 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 for approaching vehicles in the intersection, and then depart. A departure sight triangle is used for this analysis. Sight Distance Study Preparat ion Checkl istSight Distance Study Preparat ion Checkl ist When preparing for an intersection sight distance study, use the checklist in Table 4.1. The checklist may be modified or expanded as necessary. Sight Distance 4.5 Table 4.1. Sight Distance Study Preparation Checklist Step ÖÖ When Complete Notes Obtain target and sighting rods Obtain measuring wheel Obtain hardhat and safety vest Obtain sight distance diagram form Select time and day Determine availability of observers Contact corresponding jurisdiction(s) Other: If an agency does not possess the equipment necessary to complete a sight distance study, it may be obtained from the Iowa DOT, another jurisdiction, or a responsible consulting firm. A blank sight distance diagram form is located in Appendix C. Information on contracting for a sight distance study, including a project work order example, is provided near the end of this chapter. UNCONTROLLED INTERSEUNCONTROLLED INTERSECTIONSCTIONS For uncontrolled intersections, the drivers of both approaching vehicles should be able to see conflicting vehicles in adequate time to stop or slow to avoid a crash. The required sight distance for safe operation at an uncontrolled intersection is directly related to the vehicle speeds and the distances traveled during perception, reaction, and braking time. Table 4.2 lists the minimum recommended sight distances for specific design speeds. For example, if a vehic le is traveling 20 mph, a sight distance of 90 feet is the minimum recommended stopping sight distance. Table 4.2. Minimum Recommended Sight Distances Vehicle Speed (mph) Stopping Sight Distance (feet) 15 70 20 90 25 115 30 140 35 165 40 195 45 220 50 245 55 285 Note: Distances are from the 2001 AASHTO Green Book and 2001 AASHTO Little Green Book . Distances may change in future versions. 4.6 Sight Distance Key Steps to a S ight Distance Study at an Uncontro l led Key Steps to a S ight Distance Study at an Uncontro l led Intersect ionIntersect ion A 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 Distance Determine the minimum sight distance for the posted or operating speed at the intersecting roadway (see Table 4.2). Obtain or Construct Sighting and Target Rods Sighting and target rods are illustrated in Figure 4.5. The target rod can be constructed out of 2-inch by 0.75-inch wood. The target rod should be 4.25 feet tall to represent the vehicle height and be painted fluorescent orange on both the top portion and bottom 2 feet of the rod. The bottom 2-foot portion represents the object height for measuring stopping sight distance. (This will be further explained later in the stopping sight distance section.) The sighting rod should be 3.5 feet tall to represent the driver’s eye height. 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) Sight Distance 4.7 Measure Current Sight Distances and Record Observations Sight distance measurements should be gathered for all legs of the uncontrolled intersection. Traffic approaching from both the left and right should be considered for measurements. On the sight distance intersection diagram (a blank diagram form is provided in Appendix C), the observer records the date and time, posted or operating speed, site location, and weather conditions. The measuring process is represented in Figure 4.6 and described below. Observer Assistant Decision Point Obstruction Si gh t L in e X Y Figure 4.6. Sight Distance Measurement at Uncontrolled Intersection The observer holds the sighting rod, and the assistant holds the target rod. They position themselves on two intersecting approaches at the appropriate stopping sight distances taken from Table 4.2. These are the X and Y dimensions. The observer represents the approaching vehicle and is located at the decision point. The observer uses the 3.5-foot sighting rod, which represents the driver’s eye height. The assistant represents the intersecting vehicle. The assistant uses the 4.25-foot target rod, which represents the height of the approaching vehicle. The observer sights from the top of the sighting rod to the target rod. 4.8 Sight Distance If the target rod is visible, the approach sight triangle for the intersection is appropriate. If the top of the target rod is not visible, the assistant holding the target rod should walk toward the intersection along the centerline 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 the centerline of the roadway. This is the X dimension. Perform Sight Distance Analysis The analysis of intersection sight distance consists of comparing the recommended sight distance to the measured sight distance. The measured sight distance should be equal to or greater than the recommended stopping sight distance. If the measured sight distance is less than the recommended sight distance, 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 Uncontrol led Example Sight Distance Study at an Uncontrol led Intersect ionIntersect ion The city of Cottonwood Glen noticed an increase of crashes at the intersection of 6th Street and Phoenix Avenue. The city suspected that the crash problem may be related to sight distance. The problem seemed to be centered around vehicles traveling northbound at the intersection. Cottonwood Glen decided to conduct a sight distance study at the intersection to see whether that was a contributing factor. The intersection of 6th Street and Phoenix Avenue has no traffic control. The posted speed limit for both of the roadways is 25 mph. Cottonwood Glen referred to Table 4.2 for the recommended sight distance for this situation: 115 feet for both roadways. Cottonwood Glen conducted the study on a Tuesday at 2:00 p.m. under clear weather conditions. The study was conducted early afternoon to Sight Distance 4.9 avoid heavy traffic volumes. City staff measured the sight distance for the eastbound and westbound approaches. Figure 4.7 shows that the measured sight distance on the west approach is 140 feet. The recommended stopping sight distance for this approach is 115 feet. This tells us that the measured sight distance satisfies the minimum recommended. No sight distance related improvements need to be considered on the west approach. Figure 4.8 shows that the measured sight distance on the east approach is 100 feet. The recommended stopping sight distance for this approach is 115 feet. This tells us that the measured sight distance does not satisfy the minimum recommended. The stopping sight distance diagram shows that there is an obstruction limiting the sight distance, located outside of the right-of-way. In this situation, the property owner should be contacted for cooperation in eliminating, modifying, or moving the obstruction. If they are unwilling to cooperate, other mitigation measures should be considered. 4.10 Sight Distance Date MM/DD/YY Major Roadway Width 40 feet Time of Day 1400 No. of Lanes 2 Posted Speed Limit or 85% for Major Roadway (X(R)) 25 mph Minor Roadway Width 30 feet Posted Speed Limit or 85% for Minor Roadway (Y) 25 mph No. of Lanes 2 Traffic Controls Present No Control Intersection Maneuver N.A. Y Stopping Distance 115 feet Weather Clear X(R) Recommended 115 feet Horizontal Curve N X(M) Measured 140 feet Vertical Curve N Shrubs Building Phoenix Avenue 6t h St re et Decision Point R O W 30ft 40ft North X(M) Y Recommended Sight Distance (X(R)) Measured Sight Distance (X(M)) X(R) Conclusion: X(M) > X(R). The measured sight distance was 140 feet, which is more than the recommended sight distance of 115 feet. Sight distance on the west approach is adequate. Figure 4.7. 6th Street and Phoenix Avenue, West Approach Sight Distance 4.11 Date MM/DD/YY Major Roadway Width 40 feet Time of Day 1400 No. of Lanes 2 Posted Speed Limit or 85% for Major Roadway (X(R)) 25 mph Minor Roadway Width 30 feet Posted Speed Limit or 85% for Minor Roadway (Y) 25 mph No. of Lanes 2 Traffic Controls Present No Control Intersection Maneuver N.A. Y Stopping Distance 115 feet Weather Clear X(R) Recommended 115 feet Horizontal Curve N X(M) Measured 100 feet Vertical Curve N Shrubs Building Phoenix Avenue 6t h St re et Decision Point R O W 30ft 40 ft Re co mm en de d S igh t Di sta nc e ( X( R) ) Me as ur ed S igh t Di sta nc e ( X( M) ) X(M) Y North Obstruction X(R) Conclusion: X(M) < X(R). The measured sight distance was 100 feet, which is less than the recommended sight distance of 115 feet. There is an obstruction limiting sight distance and it is outside of the right-of-way. Figure 4.8. 6th Street and Phoenix Avenue, East Approach 4.12 Sight Distance INTERSECTIONS WITH SINTERSECTIONS WITH STOP SIGN CONTROLTOP SIGN CONTROL Vehicles 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 Intersect ions with Stop Sign Contro lVehic le Maneuvers at Intersect ions with Stop Sign Contro l Three maneuvers can be completed for vehicles stopped at an intersection: crossing maneuver, left-turn maneuver, and right-turn maneuver. See Figure 4.9. Major Street M in or S tre et Stop Left Turn Maneuver Right Turn Maneuver C ro ss ov er M an eu ve r Figure 4.9. Three Maneuvers at an Intersection with Stop Sign Control Crossing Maneuver from the Minor Roadway When a driver is completing a crossing maneuver, there must be sufficient sight distance in both directions available to cross the intersecting roadway and avoid approaching traffic. The sight distance required for this maneuver is based on the distance approaching vehicles will travel on the major road during the time period it takes a stopped vehicle to clear the intersection. Table 4.3 lists the recommended sight distances for this maneuver based on design speeds. Sight Distance 4.13 Turning Left from the Minor Roadway The left-turn maneuver requires first clearing the traffic on the left, then entering the traffic stream on the right. The required sight distance for this maneuver is affected by the amount of time it takes the stopped vehicle to turn left clearing traffic and reach average running speed without affecting the speed of the approaching vehicle. Table 4.3 lists the recommended sight distances for this maneuver based on design speeds. Turning Right from the Minor Roadway The right turn maneuver must have sufficient sight distance to permit entrance onto the intersecting roadway 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 Maneuver Vehicle Speed (mph) Stopping Sight Distance for Left-Turn Maneuver (feet) Stopping Sight Distance for Crossover and Right-Turn Maneuvers (feet) 15 170 145 20 225 195 25 280 240 30 335 290 35 390 335 40 445 385 45 500 430 50 555 480 55 610 530 Note: Distances are from the 2001 AASTHO Green Book and are for two-lane roadways. Distances may change in future versions. Key Steps to a S ight Distance Study at an Intersect ion with Key Steps to a S ight Distance Study at an Intersect ion with Stop Contro lStop Contro l A 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. 4.14 Sight Distance Determine the Minimum Recommended Sight Distances Determine the minimum sight distance for each maneuver and speed (see Table 4.3). Obtain or Construct Sighting and Target Rods Sighting and target rods are illustrated in Figure 4.5. The target rod can be constructed from 2-inch by 0.75-inch wood. The target rod should be 4.25 feet tall to represent the vehicle height and be painted fluorescent orange on both the top portion and bottom 2 feet of the rod. The bottom 2-foot portion represents the object height for measuring stopping sight distance. The sighting rod should be 3.5 feet tall to represent the driver’s eye height. The sighting rod can be constructed from the same type of wood but should be painted flat black. The sighting rod and target rod are used in measuring sight distance. Measure Current Sight Distances and Record Observations On the sight distance intersection diagram (a blank diagram form is provided in Appendix C), the observer 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 from the 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 the observer. This process should continue until the top of the target rod can no longer be seen. The point where the target rod disappears is the maximum sight distance along that leg and should be recorded from the observer’s sight. Perform Sight Dista