# SIGHT DISTANCE. Presented by Nazir Lalani P.E. Traffex Engineers Inc. WHY IS SIGHT DISTANCE SO IMPORTANT?

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1 SIGHT DISTANCE Presented by Nazir Lalani P.E. Traffex Engineers Inc. WHY IS SIGHT DISTANCE SO IMPORTANT? Drivers must be able to: Stop for objects in the roadway Stop for stationary vehicles ahead See cross traffic at intersections before entering See on coming vehicles when passing See traffic control devices and react to them See trains at Railroad Xings not controlled by gates See pedestrians waiting to cross the street 1

2 AASHTO GEOMETRIC DESIGN BOOK What is Stopping Sight Distance? 2

3 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 Stopping sight distance is the sum of two distances: The distance traversed by the vehicle from the instant the driver Brake sights an Reaction object necessitating Time a stop to the instant the brakes are applied (brake reaction distance). The distance needed to stop the vehicle from the instant brake application begins (braking distance). 3

4 Brake Braking Reaction Distance Time Braking Distance The approximate braking distance of a vehicle on a level roadway traveling : US Customary V² d = a Where: d = braking distance; V = design speed, mph; a = deceleration rate, ft/s² 4

5 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 US Customary V² d = a Where: d = braking distance; V = design speed, mph; a = deceleration rate, ft/s² Therefore, 11.2 ft/s² (a comfortable deceleration for most drivers) is recommended as the deceleration threshold for determining stopping sight distance Brake Brake Reaction Time Distance 5

6 Brake Reaction Time Brake reaction time is the interval from the instant that the driver recognizes the existence of an obstacle on the roadway ahead that necessitates braking to the instant that the driver actually applies the brakes In addition, the driver must not only see the object but must also recognize it as a stationary or slowly moving object Brake Reaction Time Comprises of PIEV which includes speed and conditions. Perception Identification (understanding) Emotion (decision making) Volition (execution of decision) 6

7 Both recent research and the studies documented in the literature show that a 2.5-s s brake reaction time for stopping sight situations encompasses the capabilities of most drivers, including those of older drivers The recommended design criterion of 2.5-s s for brake reaction time exceeds the 90 th percentile of reaction time for all drivers Stopping Sight Distance The sum of the distance traversed during the brake reaction time and the braking distance is the stopping sight distance US Customary V² d = 1.47Vt t a 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. Where: V = design speed, mph; a = deceleration rate, ft/s² t = Brake reaction time in seconds 7

8 US Customary Stopping sight distance Design Brake reaction Braking distance speed distance on level Calculated Design (mph) (ft) (ft) (ft) (ft) Source: Geometric Design of Highways and Streets 2004 Why is it important on horizontal and vertical curves? 8

9 Vertical Curve Crest Condition 9

10 Design Speed (mph) Algebraic Difference in Grades (%) Exhibit 3-71: Length of Crest Vertical Curve (feet) Source: Geometric Design of Highways and Streets 2004 Exhibit Design Controls for Crest Vertical Curves Based on Stopping Sight Distance Source: Geometric Design of Highways and Streets

11 Exhibit Design Controls for Crest Vertical Curves Based on Passing Sight Distance Source: Geometric Design of Highways and Streets 2004 Not designed for either stopping or passing sight distance! 11

12 Vertical Curve Sag Condition Stopping Sight Distance on Sag Vertical Curves The minimum length of vertical curve which provides headlight sight distance in grade sags for a given design speed can be obtained. Source: Caltrans Highway Design Manual 12

13 Design Speed (mph) Algebraic Difference in Grades (%) Exhibit 3-74 Length of Sag Vertical Curve (feet) Source: Geometric Design of Highways and Streets 2004 Exhibit Design Controls for Sag Vertical Curves Based on Stopping Sight Distance Source: Geometric Design of Highways and Streets

14 Formula for length of sag vertical curve using comfort factor ( length is 50% less than based on headlight distance) Exhibit Sight Distance at Under Crossings Note: AASHTO provide different formulas for calculating curves for under Crossings Source: Geometric Design of Highways and Streets

15 Bridge structure limits sight distance General Controls for Vertical Alignments Provide smooth grade line with gradual changes Avoid roller coaster or sudden dip type profiles Avoid broken back curves ( two vertical curves in the same direction separated by short tangent On long grades, the steepest at the bottom with flattening of the grades near the top Sag vertical curves must have adequate drainage 15

16 Sight Distance on Horizontal Curves Stopping Sight Distance on Horizontal Curves Where an object off the pavement such as a bridge pier, building, cut slope, or natural growth restricts sight distance, the minimum radius of curvature is determined by the stopping sight distance. HSO: Horizontal Sightline Offset Insert Exhibit 3-54 Available stopping sight distance on horizontal curves is obtained from Exhibit 3-53 It is assumed that the driver s eye is 3.5 feet above the center of the inside lane (inside with respect to curve) and the object is 2 feet high Source: Geometric Design of Highways and Streets

17 Source: Geometric Design of Highways and Streets 2004 General Controls for Horizontal Alignments Provide passing distance on 2-lane 2 roads Provide greater radius of curvature than the minimum where possible Avoid sharp horizontal curves at the ends of long tangent sections and back to back reverse curves Curves should be at least 500 feet for a central angle of 5 degrees 100 feet per degree Minimum length of horizontal curves should be 15 times the design speed in mph 17

18 Stopping Sight Distance for Bicyclists Source: Caltrans Highway Design Manual Chapter

19 Source: Caltrans Highway Design Manual Chapter 1000 Minimum Length of Crest Vertical Curve Source: Caltrans Highway Design Manual Chapter

20 Horizontal Lateral Clearance Formula Source: Caltrans Highway Design Manual Chapter 1000 Stopping Sight Distance Source: Caltrans Highway Design Manual Chapter

21 Intersection Sight Distance Uncontrolled Intersections 21

22 Sight 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 design speeds of the intersecting roadways and the type of traffic control used at the intersection. Two types of clear sight triangles are considered in intersection design: Approach Sight Triangles A N D Departure Sight Triangles 22

23 Approach Sight Triangles for Uncontrolled Locations Each quadrant of an intersection should contain a triangular area free of obstructions that might block an approaching driver s s view of potentially conflicting vehicles - drivers eye height and object height are 3.5 feet (AASTO) The length of the legs of this triangular area, along both intersecting roadways, should be such that the drivers can see any a potentially conflicting vehicles in sufficient time to slow or stop s before colliding within the intersection Exhibit 9-50: 9 Intersection Sight Triangles b Minor Road Major Road This decision point is the location at which the minor-road road driver should begin to brake to a stop if another vehicle is present on an intersecting approach. The distance from the major road, along the minor road, is illustrated by the dimension a in Exhibit A. Clear Sight Triangle a Decision Point Dimension b illustrates the length of this leg of the sight triangle along the major road A. A- Approach Sight Triangles (uncontrolled) Source: AASHTO A Policy on Design of Highways and Streets 23

24 Corner sight triangle for uncontrolled locations Source: AASHTO A Policy on Design of Highways and Streets Stop Sign Controlled Intersections 24

25 Exhibit 9-50: Intersection Sight Triangles b Minor Road Although 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. Clear Sight Triangle Major Road a Decision Point 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. A- Approach Sight Triangles (uncontrolled or yieldcontrolled) Source: AASHTO A Policy on Design of Highways and Streets Case B-Intersections with Stop Control on the Minor Road Departure sight triangles for intersections with stop control on the minor road should be considered for three situations: Case B1---Left turns from the minor road; Case B2---Right turn from the minor road; and Case B3---Crossing the major road from a minor-road approach 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 25

26 Exhibit Intersection Sight Triangles b Minor Road Major Road 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. Clear Sight Triangle a Decision Point Departure sight triangles should be provided in each quadrant of each intersection approach controlled by stop or yield signs. B- Departure Sight Triangles (Stop control) Source: AASHTO A Policy on Design of Highways and Streets 26

27 Exhibit Intersection Sight Triangles b Minor Road Departure sight triangles should also be provided for some signalized intersection approaches where right turns on red are permitted. Major Road a Decision Point Clear Sight Triangle B- Departure Sight Triangles (Stop control) 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. Source: AASHTO A Policy on Design of Highways and Streets Case B1---Left Turn from the Minor Road 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 b Minor Road Major Road a Decision Point Exhibit 9-50B: 9 Intersection Sight Triangles Clear Sight Triangle Departure Sight Triangles (Stop control) Source: AASHTO A Policy on Design of Highways and Streets 27

30 Motorcyclist Motorcyclist- Left-turning turning Vehicle crash 30

31 Case B2 Right turn from the Minor Road The intersection sight distance for right turns is determined in the 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, drivers generally 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 rightturn maneuvers without undue interference with major-road traffic The intersection sight distance along the major road (dimension b in Exhibit 9 50B) is determined by: US Customary ISD = 1.47 V major t g (9-1) where: ISD = V major = t g = intersection sight distance (length of the leg of sight triangle along the major road) (ft) design speed of major road (mph) time gap for minor road vehicle to enter the major road (s) Source: AASHTO A Policy on Design of Highways and Streets 31

33 Case B3 Crossing the Major Road from a Minor-road road approach 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: 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 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 Yield Controlled Intersections 33

34 Yield Controlled Intersections Exhibits to in AASHTO address Yield controlled intersections Assumes vehicle will slow but not stop Experience shows that drivers tend to treat them as uncontrolled intersections and do not slow for through movements AASHTO assumes drivers turning left or right will slow to 10 mph Exhibit 9-61: 9 Case C1 Length of Sight Triangle for Crossing Maneuvers at Yield Controlled Intersections 34

35 Exhibit 9-64: 9 Case C2 Design Intersection Sight Distance for Left/Right Turns Yield Controlled Intersections Decision Sight Distance 35

36 Exhibit 3-3: 3 3: Decision Sight Distance Design Values The sum of the distance traversed during the brake reaction time and the distance to brake the vehicle to a stop is the stopping sight t distance US Customary V² d = 1.47Vt t a Driver s 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. Where: V = design speed, mph; a = deceleration rate, ft/s² t = Brake reaction time in seconds 36

37 What Causes Corner Sight Distance Obstructions? Corner Sight Distance Obstructions Parked Vehicles Vegetation Horizontal and vertical curves Signs Offset curbs 37

38 Vegetation Obstruction Before Vegetation Obstruction After Trimming 38

39 Vegetation Obstruction Before Vegetation Obstruction After 39

40 Vertical Curve Restriction Corner sight distance blocked by sign 40

41 Corner sight distance blocked by parking 41

42 Sight distance obstruction caused by newspaper racks Now trees have been added to the mix 42

43 Source: MUTCD Sign and limit line placement at intersections Manual on Uniform Traffic Control Devices (MUTCD) National MUTCD National standard for all traffic control devices installed on any street, highway or bicycle trail open to public travel. Web site:mutcd.fhwa.dot.gov Compliance dates:http://mutcd.fhwa.dot.gov/kno-compliance.htm 43

44 Source: Public Works Magazine, July 1981 Intersection looking left from a well position stop limit line 44

45 Left-turn turn Sight Distance for Traffic Turning from Major Street Sight distance triangles at driveways Reproduced with permission of the Transportation Research Board (TRB) from the Access Management Manual, TRB, Washington DC,

46 Transportation Research Board (TRB) Access Management Manual, TRB, Washington DC, 2003 Left-turn turn sight distance at driveway blocked by trees 46

47 Exhibit Intersection Sight Distance - Left-turn turn from the Major Road Source: AASHTO Policy on Design of Highways and Streets Exhibit Time Gap for Case F---Left Turns from Major Rd Design vehicle Time gap (tg) ) (seconds) at design speed of major road Passenger car 5.5 Single unit truck 6.5 Combination truck 7.5 For multilane highways: For left turning vehicles that cross more than one opposing lane,, add 0.5 seconds for passenger cars or 0.7 seconds for trucks for each additional lane e to be crossed by the turning vehicle Source: AASHTO A Policy on Design of Highways and Streets 47

48 Left-turn turn sight distance blocked by trees Talk about protected permissive left turns Passing Sight Distance 48

49 No passing Zone on Horizontal Curve Passing Sight Distance Passing sight distance is considered only on 2-lane 2 roads At critical locations, a stretch of 3-3 or 4-lane 4 passing section with stopping sight distance is sometimes more economical than two lanes with passing sight distance 49

50 Dashed yellow center line on downhill side of this up hill passing lane! Passing Sight Distance - AASHTO The sight distance available for passing at any place is the longest distance at which a driver whose eyes are 3.5 feet above the pavement surface can see the top of an object 3.5 feet high on the road In general, 2-lane 2 highways should be designed to provide for passing where possible, especially those routes with high volumes of trucks or recreational vehicles Passing should be done on tangent horizontal alignments with constant grades or a slight sag vertical curve Minimum passing sight distance is about four times the minimum stopping sight distance at the same design speed 50

51 Passing Sight Distance - AASHTO Passing sight distance is the minimum sight distance required for the driver of one vehicle to pass another vehicle safely and comfortably at a 10 mph speed differential Passing must be accomplished assuming an oncoming vehicle comes into view and maintains the design speed, without reduction after the overtaking maneuver is started Sight Distance Standards Design Speed Stopping Passing (mph) (ft) (ft) Minimum Passing Sight Distances - MUTCD MUTCD Distances are shorter because the assumed difference in speed is greater 51

52 Pedestrians Clear sight distance should be provided on approaches to a crosswalk 52

53 Clear sight distance based on the gap time needed for a pedestrian to cross the street should be provided on the approaches to a crosswalk Traffic Signals 53

54 Cone of Vision - MUTCD Can you spot the signal? 54

56 56

57 Sight Distance for Roundabouts Stopping Sight Distance Source: FHWA Roundabout Guide 57

58 Source: Modern Roundabouts for Oregon (ODOT) Source: WA DOT Design Manual Chapter

61 QUESTIONS? 61

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