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Human Factors Guidelines for Road Systems: Second Edition (2012)

Chapter: Chapter 5 - Sight Distance Guidelines

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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 5 - Sight Distance Guidelines." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Key Components of Sight Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-2 Determining Stopping Sight Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-4 Determining Intersection Sight Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-6 Determining When to Use Decision Sight Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-8 Determining Passing Sight Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-10 Influence of Speed on Sight Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-12 Key References for Sight Distance Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-14 Where to Find Sight Distance Information for Specific Roadway Features . . . . . . . . . . . . . .5-16 Where to Find Sight Distance Information for Intersections . . . . . . . . . . . . . . . . . . . . . . . . .5-18 5-1 C H A P T E R 5 Sight Distance Guidelines

KEY COMPONENTS OF SIGHT DISTANCE Introduction Sight distance (SD) is the distance that a vehicle travels before completing a maneuver in response to some roadway element, hazard, or condition that necessitates a change of speed and/or path. Sight distance is based on two key components: The perception-reaction time (PRT) required to initiate a maneuver (pre-maneuver phase) The time required to safely complete a maneuver (MT). The PRT component includes the time needed to see/perceive the roadway element, time needed to complete relevant cognitive operations (e.g., recognize hazard, read sign, decide how to respond, etc.), and time needed to initiate a maneuver (e.g., take foot off accelerator and step on brake pedal). MT includes actions and time required to safely coordinate and complete a required driving maneuver (e.g., stop at intersection, pass a vehicle, etc.). Typically, a vehicle maintains its current speed and trajectory during the PRT phase, while changing its speed and/or path during the MT phase. Design Guidelines Sight Distance = Distance traveled while driver perceives, makes decisions about, and initiates action in response to roadway element (PRT) + Distance traveled while the driver completes an appropriate maneuver (MT) Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data SCHEMATIC SHOWING THE PERCEPTION-REACTION TIME AND MANEUVER TIME COMPONENTS OF SIGHT DISTANCE Insufficient Sight DistanceB Perception Reaction Time Maneuver Time Sufficient Sight DistanceA Perception Reaction Time Maneuver Time Hazard (2ft high) Driver’s Eye (3.5 ft High) Line of Sight HFG SIGHT DISTANCE Version 2.0 5-2

Discussion Before drivers can execute a maneuver, they must first recognize that some action is required and decide what that action should be. Therefore, this mental activity—perception, cognition, and action planning—precedes an overt vehicle control action and takes some amount of time. The PRT is typically defined as the period from the time the object or condition requiring a response becomes visible in the driver’s field of view to the moment of initiation of the vehicle maneuver (e.g., first contact with the brake pedal). Although a particular PRT value (e.g., 2.5 s (1)) is used in deriving sight distance requirements for a given design situation, this PRT value should not be viewed as a fixed human attribute, because it is influenced by many factors. Some of the key factors that influence PRT are shown in the table below. FACTORS THAT AFFECT THE DIFFERENT COMPONENTS OF PERCEPTION-REACTION TIME Low contrast (e.g., night) Drivers take longer to perceive low-contrast objects. Visual glare Objects are perceived less quickly in the presence of glare. Older age Older drivers are less sensitive to visual contrast and are more impaired by visual glare (e.g., oncoming headlights). Object size/height Smaller objects/text require drivers to be closer to see them. Driver expectations Drivers take substantially longer to perceive unexpected objects. Visual complexity Drivers take longer to perceive objects “buried” in visual clutter. Seeing/ Perceiving Driver experience/familiarity PRT to objects and situations will generally be faster with increased experience and/or familiarity. Older age Older drivers require more time to make decisions. Cognitive Elements Complexity Drivers require more time to comprehend complex information or situations and to initiate more complex or calibrated maneuvers. Initiating Actions Older age Older drivers require more time to make vehicle control movements and their range of motion may be limited. In contrast to the PRT, the MT is primarily affected by the physics of the situation, including vehicle performance capabilities. In particular, tire-pavement friction, road-surface conditions (e.g., ice), and downgrades can increase MT or make some maneuvers unsafe at higher speeds. MT is also affected to a lesser extent by driver-related factors (e.g., deceleration profile), but these factors are highly situation specific because the maneuvers are very different (e.g., emergency stop, passing, left turn through traffic, etc.). Design Issues Although most design requirements are expressed as a design distance, from the driver’s perspective, the critical aspect is time. Time is required to recognize a situation, understand its implications, decide on a reaction, and initiate the maneuver. While this process may seem almost instantaneous to us when driving, it can translate into hundreds of feet at highway speeds before a maneuver is even initiated. Speed selection is also critical, because the relative speed between the driver and the hazard determines how much distance is traversed in the time required for the driver to initiate and complete the maneuver. Cross References Determining Intersection Sight Distance, 5-6 Determining When to Use Decision Sight Distance, 5-8 Determining Passing Sight Distance, 5-10 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington DC. Activity Factor Explanation 5-3 HFG SIGHT DISTANCE Version 2.0

DETERMINING STOPPING SIGHT DISTANCE Introduction Stopping sight distance (SSD) is the distance from a stopping requirement (such as a hazard) that is required for a vehicle traveling at or near design speed to be able to stop before reaching that stopping requirement. Stopping sight distance depends on (1) the time required for a driver to perceive and respond to the stopping requirement (PRT) and (2) how aggressively the driver decelerates (MT). Design Guidelines EQUATIONS FOR STOPPING SIGHT DISTANCE DESIGN VALUES Metric US Customary a V VtSSD RT 2 039.0278.0 a V VtSSD RT 2 075.147.1 Where: tRT = perception-reaction time V = design speed, km/h a = deceleration level, m/s2 (see discussion) Where: tRT = perception-reaction time V = design speed, mi/h a = deceleration level, ft/s2 (see discussion) Based Primarily on Expert Judgment Based Equally on Expert Judgment Based Primarily on Empirical Dataand Empirical Data The following table presents driver PRTs and mean deceleration levels under favorable and unfavorable conditions calculated from driver responses to unexpected roadway hazards (1, 2). The mean deceleration rates and 85th percentile values (3.7 m/s2) are higher than those recommended in Lerner, Huey, McGee and Sullivan (1), but are shown to provide an indication of driver performance capabilities under emergency conditions. Good Traction Conditions Poor Traction Condition Mean Deceleration Level (a)Mean Deceleration Level (a)Visibility PRT Metric US Customary PRT Metric US Customary Good 1.6 s 5.4 m/s2 17.7 ft/s2 1.6 s 4.2 m/s2 13.8 ft/s2 Poor 5+ s 5.4 m/s2 17.7 ft/s2 5+ s 4.2 m/s2 13.8 ft/s2 Although the mean deceleration level differs for good (5.4 m/s2) and poor (4.2 m/s2) traction conditions, the 85th percentile values are the same (3.7 m/s2). Component Favorable Conditions Unfavorable Conditions PRT Daytime Hazard clearly visible and directly in driver’s line of sight Nighttime Self-illuminated or retro-reflectorized hazard, with a lighting configuration that is immediately recognizable, near driver’s line of sight Daytime Hazard camouflaged by background and initially off line of sight Nighttime Hazard unreflectorized and not self-illuminated Lighting configuration is unfamiliar to the driver Low beams with or without street lighting Glare from oncoming vehicles MT Tangent with no grade Dry or wet pavement Passenger vehicles; tires in good condition Unexpected object Curve Downgrade HFG SIGHT DISTANCE Version 2.0 5-4

Discussion The PRT stage is significantly influenced by visibility conditions. In particular, the distance at which drivers can see an unilluminated, unreflectorized hazard depends on their headlights, their sensitivity to contrast, and their expectation of seeing the hazard. When drivers are not expecting a particular low-contrast hazard, their seeing distance is one half of that which would pertain if the object were expected. A very-low-contrast hazard may not even be detected in time to start braking. At speeds of 60 km/h and greater, using low-beam headlights, most drivers will be too close to an unexpected, unreflectorized hazard at the point they can detect it in time to stop (e.g., pedestrian in a dark coat). Also, the PRT component can be further increased by high workload (e.g., traffic merging, reading signs), fatigue, and impairment. From an engineering perspective, the deceleration maneuver is significantly influenced by road surface conditions. From a human factors perspective, however, stopping is also influenced by the deceleration level that a driver adopts (which affects the braking efficiency). Under wet conditions, with standard brakes, the mean constant deceleration is about 0.43 g (54% of the pavement’s coefficient of friction), and the 85th percentile is 0.38 g (47%). On wet pavements with anti- locking brake systems (ABSs), the mean constant deceleration is about 0.53 g (66% of the pavement’s coefficient of friction), and the 85th percentile is about 0.45 g (56%). Under unfavorable conditions, slightly lower braking efficiencies (by 2% to 8%) are obtained on curves and tangents, but this information is based on physics because no human factors studies are available. Note also that downgrade MT can be increased by age and gender because older drivers and women will not apply as much braking force as younger drivers and males. Some research suggests that under most rushed braking situations, drivers stop rapidly, but not to the point of locked wheel braking (in locked wheel braking, which is typical in crashes, drivers are 100% efficient in making use of the available pavement friction) (2). The mean maximum deceleration in one comprehensive study was about 75% of the pavement’s coefficient of friction (2). Design Issues Stopping sight distance should always be provided because any road location can become a hazard. One study found that the most common objects hit on sight-restricted curves were large animals and parked cars (e.g., as provided by AASHTO (3)), the presence of which can create a hazard on any road section( 2). If SSD is below standard at a number of locations then priorities must be set. Examples of hazards and conditions that may be high priority with respect to the need for SSD are: • Change in lane width • Reduction in lateral clearance • Beginning of hazardous side slope • Crest vertical curve • Horizontal curve • Driveway • Narrow bridge • Roadside hazards (e.g., boulder markers at driveways) • Unmarked crossovers on high-speed rural arterials • Unlit pedestrian crosswalks • High-volume pedestrian crosswalks • Frequent presence of parked vehicles very near or intruding into through lane For design purposes, neither rapid nor locked wheel braking is a desirable driver response, because of the risk of skidding, or of a rear-end crash when there is a following vehicle. It should also be noted that the AASHTO model of driver deceleration assumes constant deceleration throughout the braking maneuver; however, empirical data suggest that maximum deceleration is generally not exhibited until the last part of the braking when the vehicle has slowed and come closer to the unexpected object (2). Under wet conditions, the 95th percentile value for equivalent constant deceleration without ABSs was 0.29 g (equivalent to 2.8 m/s2 [9.3 ft/s2]) and with ABSs, 0.41 g (equivalent to 4 m/s2 [13.2 ft/s2]). Many design references use the term “design speed” to characterize the expected driving speed on a roadway. However, as noted in “Influence of Speed on Sight Distance” (page 5-12 of this document), neither design speed nor posted speed is always the best determinant of actual driving speed. When available, actual operating speeds should be used instead of design speed to help determine needed sight distance. Cross References Key Components of Sight Distance, 5-2 Determining Intersection Sight Distance, 5-6 Determining When to Use Decision Sight Distance, 5-8 Key References 1. Lerner, N., Huey, R.W., McGee, H.W., and Sullivan, A. (1995). Older Driver Perception-Reaction Time for Intersection Sight Distance and Object Detection. Volume I, Final Report (FHWA-RD-93-168). Washington, DC: FHWA. 2. Fambro, D.B., Fitzpatrick, K., and Koppa, R.J. (1997). NCHRP Report 400: Determination of Stopping Sight Distances. Washington, DC: Transportation Research Board. 3. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. 5-5 HFG SIGHT DISTANCE Version 2.0

DETERMINING INTERSECTION SIGHT DISTANCE Introduction Providing stopping sight distance at intersections is fundamental to intersection operation. In addition, drivers also require an unobstructed view of the entire intersection, including any traffic control devices, and sufficient lengths along the intersecting highway to permit the driver to anticipate and avoid potential collisions with other vehicles. Thus, intersection sight distance (ISD) differs depending on the type of intersection and maneuver involved. The different types of ISD are summarized in the table below. Design Guidelines Case Intersection Type and/or Maneuver Sight Triangle Sight Distance Determinant Location in AASHTO (1) A Intersection with no control Approach Triangle Stopping sight distance with modified assumptions Exhibit 9-51 Pg 655 B Intersections with stop control on the minor road B1 Left turn from the minor road Departure Triangle Gap time equation Exhibit 9-54 Pg 559 B2 Right turn from the minor road Departure Triangle Gap time equation Exhibit 9-58 Pg 664 B3 Crossing maneuver from the minor road Departure Triangle Gap time equation Exhibit 9-58 Pg 664 C Intersections with yield control on the minor road C1 Crossing maneuver from the minor road Approach Triangle Stopping sight distance with modified assumptions Exhibit 9-60 Pg 667 C2 Left turn from the minor road Departure Triangle Gap time equation Exhibit 9-64 Pg 672 D Intersections with traffic signal control Both* See Case D Guideline (1) Pg 671 E Intersections with all-way stop control None None required Pg 674 F Left turns from major road Departure Triangle Gap time equation Exhibit 9-67 Pg 675 * First vehicle stopped on one approach should be visible to the driver of the first vehicle stopped on each of the other approaches and left-turning vehicles should have sufficient sight distance to select safe gaps in oncoming traffic. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The figure below shows the approach and departure triangles for different intersections/maneuvers. Major Road M in or R oa d Clear Sight Triangle Decision Point A B a b Approach sight triangle for viewing traffic approaching the minor road from the right Major Road M in or R oa d Clear Sight Triangle Decision Point B a b Approach sight triangle for viewing traffic approaching the minor road from the left A Major Road M in or R oa d Clear Sight Triangle Decision Point A B a b Departure sight triangle for viewing traffic approaching the minor road from the right Major Road M in or R oa d Clear Sight Triangle a b Departure sight triangle for viewing traffic approaching the minor road from the left Decision Point B A Approach Triangles Departure Triangles HFG SIGHT DISTANCE Version 2.0 5-6

Discussion The two types of sight triangles used in calculating ISD are described below. Approach Sight Triangles: According to AASHTO (1), “Each quadrant of an intersection should contain a triangular area free of obstructions that might block an approaching driver’s view of potentially conflicting vehicles. The length of the legs of this triangular area [shown as “a” and “b” in the figure on the opposing page], along both intersecting roadways, should be such that the drivers can see any potentially conflicting vehicles in sufficient time to slow or stop before colliding within the intersection.” The vertex of the triangle that is nearest to the approaching driver represents the decision point at which the driver must begin to stop if the driver determines that a potential conflict is possible. Departure Sight Triangles: According to AASHTO (1), departure sight triangles provide “sight distance sufficient for a stopped driver on a minor-road approach to depart from the intersection and enter or cross the major road.” In this case, the vertex of the sight triangle is positioned over the driver of the stationary departing vehicle and the length of the triangle represents how far ahead the driver must be able to check for oncoming traffic that would make the maneuver unsafe. According to AASHTO (1), the length of the triangle is based on an acceptable gap time (which is independent of oncoming vehicle speed) that provides the departing vehicle with sufficient time to safely accelerate, cross the intersection and thus complete the maneuver. The gap time varies based on the vehicle type (e.g., passenger vehicle, combination truck, etc.) and distance that the vehicle must cross during the maneuver (e.g., number of lanes). Design Issues Although desirable at higher volume intersections, approach sight triangles are not necessary at intersections controlled by two-way and all-way stop controls or traffic signals because the stopping requirement is determined by the controls and not by approaching vehicles. Departure sight triangles should be provided in each quadrant of the intersection approach controlled by stop or yield signs and for some signalized intersections (see Case D (1)). Also grade adjustments are recommended if the departing vehicle’s rear wheels are on an upgrade that exceeds 3% at the stop line (1). Cross References Key Components of Sight Distance, 5-2 Determining Stopping Sight Distance, 5-4 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. 5-7 HFG SIGHT DISTANCE Version 2.0

DETERMINING WHEN TO USE DECISION SIGHT DISTANCE Introduction According to AASHTO (1, page 3-6), decision sight distance (DSD) represents a longer sight distance than is usually necessary for situations in which (1) drivers must make complex or instantaneous decisions, (2) information is difficult to perceive, or (3) unexpected or unusual maneuvers are required. DSD provides drivers with additional safety margin for error and affords them sufficient length to maneuver their vehicles at the same or reduced speed, rather than to just stop. Design Guidelines The following time values (t) and equations (from AASHTO (1)) should be used to calculate decision time in the following situations: Avoidance Maneuver A Stop on Rural Road B Stop on Urban Road C Speed/Path/ Direction Change on Rural Road D Speed/Path/ Direction Change on Suburban Road E Speed/Path/ Direction Change on Urban Road Time (t) 3.0 s 9.1 s 10.2–11.2 s 12.1–12.9 s 14.0–14.5 s Metric US Customary Metric US Customary a V Vtd t 039.0278.0 a V Vtd t 075.147.1 d = 0.278Vt d = 1.47Vt Equation t = time (see above) V = design speed (km/h) a = driver decel. (m/s2) t = time (see above) V = design speed (mi/h) a = driver decel. (ft/s2) t = time (see above) V = design speed (km/h) t = time (see above) V = design speed (mi/h) Common Examples - Guide signs, traffic signals - Intersection where unusual or unexpected maneuvers are required - The paved area of an intersection for (1) first intersection in a sequence or (2) isolated rural intersections - Lane markings indicating a change in cross section, overhead lane arrows - A change in cross section (lane drop, two lanes to four lanes, four lanes to two lanes, passing lane, climbing lane, optional lane split, deceleration lane, channelized right turn lane) - Lane closures in work zones - Time value t represents the sum of the PRT and MT components. - Deceleration values for Maneuvers A and B can be taken from SSD guideline (page 5-4). Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The figure below illustrates favorable and unfavorable conditions for Avoidance Maneuver E. 20 seconds from markings or gore point 7.4 sec from signs or markings/gore point Add 5 to 7.4 sec (depending on traffic volume) for each lane change Favorable Case: Visually uncluttered scene; easy-to-understand signs overhead or to right; conspicuous markings with PRPMs; unfamiliar driver Unfavorable Case: Poor markings/signing, deceptive appearance of site, unexpected features (e.g., Freeway left exit);lane change required HFG SIGHT DISTANCE Version 2.0 5-8

Discussion Because some driving situations are particularly challenging (e.g., merging in moderate traffic during a lane drop), drivers require additional time to plan and execute the necessary maneuvers, or additional “safety margin” to compensate for errors they may make in the process. In these situations, use of DSD is appropriate because it incorporates the additional time that drivers need to complete more complicated driver actions. In particular, empirical data indicate that DSD is sufficiently long to accommodate the 85th percentile values in most challenging driving situations, even for older drivers. The DSD time specifically provides more time so that drivers can do the following: 1. Detect an unexpected or difficult-to-perceive information source or condition in a roadway environment that may be visually cluttered (PRT) 2. Recognize the condition or its potential threat (PRT) 3. Select an appropriate speed and path (PRT) 4. Execute the appropriate maneuver safely and efficiently (MT) In keeping with the components discussed in other sight distance guidelines (page 5-2), the first three of these tasks compose the PRT component while the fourth task is the MT component. Although application of DSD is typically based on roadway features, certain situational factors can also adversely impact driver responsiveness. The frequent occurrence of the following factors at a site may indicate that the use of DSD is appropriate for that site: High driver workload due to concurrent tasks (e.g., traffic merging, reading signs) Truck traffic that intermittently blocks the view Off-roadway clutter that can distract drivers Poor weather that increases driver workload and makes cues (especially markings) less conspicuous High traffic volume levels Design Issues An important assumption when using DSD is that drivers are provided with and able to respond to signage that allows them to prepare in advance of the roadway feature. Studies indicate that when this advance information is not available or easy to miss, drivers may require additional time beyond the DSD. In these situations, driver responses are based on when they are able to see the actual roadway feature (e.g., turn arrow pavement marking, gore point), rather than on their perception of advance signage. In this situation, the 85th percentile maneuver completion time (including the PRT) is between 20 and 23 s from the point at which the feature becomes visible (2, 3). Factors that may lead to these situations include the following: Dense traffic Poor marking and signing Deceptive appearance of site Features that violate driver expectancies (e.g., freeway left exit, add-drop lane) Another design issue that warrants mention concerns lane changes. Additional sight distance may be necessary if drivers are expected to make multiple lane changes to complete a maneuver. In particular, each additional lane change adds an average of 5 s/lane in light traffic ( 725 vehicles/h) and 7.4 s/lane in medium-density traffic (726 to 1225 vehicles/h) to the maneuver. Many design references use the term “design speed” to characterize the expected driving speed on a roadway. However, as noted in “Influence of Speed on Sight Distance” (page 5-12 of this document), neither design speed nor posted speed is always the best determinant of actual driving speed. When available, actual operating speeds should be used instead of design speed to help determine needed sight distance. Cross References Key Components of Sight Distance, 5-2 Determining Stopping Sight Distance, 5-4 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington DC. 2. Lerner, N., Huey, R.W., McGee, H.W., and Sullivan, A. (1995). Older Driver Perception-Reaction Time for Intersection Sight Distance and Object Detection. Volume I, Final Report (FHWA-RD-93-168). Washington, DC: FHWA. 3. McGee, H.W., Moore, W., Knapp, B.G., and Sanders, J.H. (1978). Decision Sight Distance for Highway Design and Traffic Control Requirements (FHWA-RD-78-78). Washington, DC: U.S. Department of Transportation. 5-9 HFG SIGHT DISTANCE Version 2.0

DETERMINING PASSING SIGHT DISTANCE Introduction According to AASHTO (1), passing sight distance (PSD) is how far ahead a driver must be able to see in order to complete a passing maneuver without cutting off the passed vehicle before meeting an opposing vehicle that appears during the maneuver. The guideline provides the design values for passes made at different speeds provided in AASHTO (1). Design Guidelines Metric US Customary Assumed Speeds (km/h) Assumed Speeds (mi/h)Design Speed (km/h) Passed Veh. Passing Veh. Passing Sight Distance (m) Design Speed (mi/h) Passed Veh. Passing Veh. Passing Sight Distance (ft) 30 11 30 120 20 8 20 400 40 21 40 140 25 13 25 450 50 31 50 160 30 18 30 500 60 41 60 180 35 23 35 550 70 51 70 210 40 28 40 600 80 61 80 245 45 33 45 700 90 71 90 280 50 38 50 800 100 81 100 320 55 43 55 900 110 91 110 355 60 48 60 1000 120 101 120 395 65 53 65 1100 130 111 130 440 70 58 70 1200 75 63 75 1300 80 68 80 1400 Note: The passing vehicle is assumed to be traveling 19 km/h or 12 mi/h faster than the passed vehicle. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The figure below shows the lane change maneuver used by the white car to pass the black car. Perception Reaction Time (PRT) Left Tire Crosses Centerline Left Tire Crosses Centerline Passing Sight Distance Becomes Available Maneuver Time (MT) HFG SIGHT DISTANCE Version 2.0 5-10

Discussion The PSD encompasses both a PRT and an MT component. Mean PRTs to initiate a pass, measured from when PSD was available until when the right tire crossed the centerline, have been found to vary from 3.6 to 6.0 s, depending on the particular site on two-lane rural highways( 2). No information is available on subject variability, but 85th percentile PRTs will certainly exceed mean reaction times. Just as other PRTs are affected by age, gender, standard transmissions, and day versus night conditions, PSD PRT may be as well; however, no studies were found on this issue. The primary cue that a driver uses to determine whether it is safe to initiate a pass is the size of the image of the oncoming vehicle. Research suggests that drivers make reasonable estimates of the distance of an oncoming car but not of its speed. This inability to reasonably estimate speed may be a more pronounced problem for older drivers. MT is measured from the point at which either the left or right front tire (depending on study) of the subject vehicle crossed the centerline to the point at which the same front tire of the subject vehicle crossed the centerline back into the lane. One study found that on two-lane rural highways with approximately 96 km/h (60 mi/h) operating speeds and low traffic volumes (200 to 250 vehicles/h in the major direction and 85 to 175 vehicles/h in the minor direction), 65% to 75% of passes were attempted where there was no oncoming traffic, 25% to 35% of passes were attempted in the presence of oncoming traffic, and 0.8% of passes were aborted (3). In contrast, at high volumes (330 to 420 vehicles/h in the major direction and 70 to 170 vehicles/h in the minor direction), 51% to 76% of passes were made with no oncoming traffic, 26% to 50% of passes were in the presence of oncoming traffic, and 7.2 % of passes were aborted. The average time in the opposing lane was 12.2 s under low traffic conditions and 11.3 s with high traffic volumes (based on when the front left tire—not the right tire as in the PRT case—entered and left the opposing lane). Depending on site and direction, times varied from a low of 8.0 s to a high of 12.9 s and there was no clear association between length of available passing lane and time spent in the opposing lane. At a speed of 96 km/h (60 mi/h) the average times in the opposing lane are equivalent to distances of 325 m (1064 ft) for low traffic and 301 m (986 ft) for high traffic. Length of time spent in the passing lane is clearly related to the size of the time gap. In one study, drivers returning to their own lane with more than 10 s to spare averaged 12 s in the opposing lane. Drivers returning with 5 to 10 s to spare averaged 8.7 s and those with less than 5 s to spare, 6.8 s. Drivers who pass may approach a slower vehicle and pass immediately (a flying pass), or may adopt a short headway and wait for an opportunity (a delayed pass). In the second case, more time for acceleration is required. In either case, drivers may adopt a short headway just prior to the pass. A study on two-lane highways found that 40% of drivers following at short headways (0.5 s or less) were doing so in anticipation of passing (4). Design Issues In passing situations, drivers’ inaccurate estimates cannot be compensated for by increasing sight distance because the problem is that drivers misjudge the time they have to pass once they see the oncoming vehicle, and this problem remains the same regardless of how far down the road drivers can see. Instead, these types of crashes should be addressed through speed control measures or site factors that improve speed judgments. Factors that increase the time needed to execute a passing maneuver include (1) a passenger vehicle passing multiple vehicles, (2) a passenger vehicle passing a truck, (3) a truck passing another vehicle, and (4) the passing occurring on an upgrade. Many design references use the term “design speed” to characterize the expected driving speed on a roadway. However, as noted in “Influence of Speed on Sight Distance” (page 5-12 of this document), neither design speed nor posted speed is always the best determinant of actual driving speed. When available, actual operating speeds should be used instead of design speed to help determine needed sight distance. Cross References Key Components of Sight Distance, 5-2 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. 2. Hostetter, R.S., and Seguin, E.L. (1969). The effects of sight distance and controlled impedance on passing behavior. Highway Research Record, 299, 64-78. 3. Kaub, A.R. (1990). Passing operations on a recreational two-lane, two-way highway. Transportation Research Record, 1280, 156-162. 4. Rajalin, S., Hassel, S.-O., and Summala, H. (1997). Close following drivers on two-lane highways. Accident Analysis & Prevention, 29(6), 723-729. 5-11 HFG SIGHT DISTANCE Version 2.0

INFLUENCE OF SPEED ON SIGHT DISTANCE Introduction Although posted speed has been found to have the strongest association with operating speed, some visual aspects or driving-task demands associated with the roadway environment can “unconsciously” influence drivers’ speed choice. Consequently, if operating speeds on a roadway significantly exceed design speed, sight distances on that roadway may be inadequate. In particular, drivers would have less time to react to an event or object at higher speed because they travel a greater distance during the initial PRT component of a response. Similarly, at higher speeds either vehicles take longer to stop/slow or maneuvers may become unsafe or overly difficult to perform. Design Guidelines If the operating speed of a roadway is substantially higher than the design speeds, increasing the sight distance to compensate for higher traveling speeds may be appropriate. Examples of how design elements can cause operating speed to vary from design speed are shown in the table. Design Element Impact of Design on Speed Lane Width Increasing lane width from 3.3 to 3.8 m is associated with an increase of 2.85 km/h (1.78 mi/h) in speed on high design standard two-lane rural highways. Alignment Speed on curves can be reasonably accurately predicted using models based on radius, curve deflection angle, and curve length. Once the curve radius exceeds 800 m, curves have similar speeds to tangents. Speed on tangents is much more difficult to predict and depends on a wide array of road characteristics such as tangent length, radius of curve before and after the section, cross section, grade, general terrain, and sight distance. Posted speed is a better predictor of speed on urban arterial tangents than it is on highway tangents. Pavement Surface Some studies show pavement re-surfacing can be associated with a small (≈2 km/h) (1.25 mi/h) increase in speed. Roadside Elements Elements close to the edge of the lane (e.g., parked vehicles, foliage) contribute to a reduction in driver speed. Results of one study of road sections posted at 50 km/h (31 mi/h) showed that 85th percentile speeds were 12 km/h (7.5 mi/h) lower in road sections with side friction due to the presence of pedestrians, bicyclists, parked vehicles, etc. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The table below describes the relationship between operating speed and design element from past studies (1). OPERATING SPEED RELATIONSHIP WITH DESIGN ELEMENT Element Direct Inconclusive None Sight Distance Stopping sight distance1 Decision sight distance; passing sight distance; intersection sight distance Horizontal Alignment Radius Superelevation Vertical Alignment Grades; climbing lanes Vertical curves Cross Section Lane width2; curb and gutter3; lateral clearance Cross slope Shoulder width Other Radii/tangent length combos3; number of lanes4, median type; access density 1with limits; 2weak; 3per one study; 4freeways; HFG SIGHT DISTANCE Version 2.0 5-12

Discussion The design of a road affects drivers’ speeds through two major mechanisms. First, the design creates the driving task. Narrow lanes and sharp curves make the driving task more difficult and lead to reductions in speed. Second, drivers have expectations about the posted speeds—and comfortable speeds—based on various combinations of design elements. Users of this guide should be aware that operating speeds may be very different from posted speed when the road message and the posted speed are at variance. Thus design sight distances may be more appropriately determined based on operating, not posted, speed. The effects of different design features on speed are discussed below: Lane Width: Lane width influences speed because it influences the difficulty of the driving task. Narrower lanes require more frequent, smaller steering corrections, which correspond to more effort. Slowing down reduces the effort required. Alignment: Speed is strongly related to radius of curvature. Typically, models of predicted speed based on radius, deflection angle, and curve length account for more than 80% of the variance in speed. Similarly, one study of speeds in 176 curves on rural two-lane highways with posted speeds of 75 to 115 km/h found that V85 was most strongly related to radius and related, but less so, to grade and sight distance (R values .58 to .92) (2). Once the curve radius exceeded 800 m, curves had similar speeds to tangents. Speed on tangents is much more difficult to predict and is dependent on a wide array of road characteristics such as tangent length, radius of curve before and after the section, cross section, grade, general terrain, and sight distance. Accordingly, studies on urban arterials find posted speed limits typically account for only half of the variance in speed. Pavement Surface: One of the cues drivers use to estimate their own speed is noise level. When sound cues were removed through the use of earmuffs, drivers underestimated their actual speeds by 6 to 10 km/h (3). Also, some studies suggest re-surfacing a road can result in a speed increase of 2 km/h. Roadside Elements: Elements close to the edge of the lane—such as pedestrians, bicyclists, parked vehicles, and foliage—can strongly affect speed. One of the major cues used by drivers is the streaming of information in peripheral vision. Side friction increases the stimulus in peripheral vision, giving a sense of higher speed or greater hazard. In one study, drivers were asked to drive at 60 mi/h (96 km/h) with the speedometer covered. In an open- road situation, drivers averaged 57 mi/h (91 km/h). However, along a tree-lined route, drivers averaged 53 mi/h (85 km/h) (4). The trees, close by, provided peripheral stimulation, giving a sense of higher speed or greater hazard. The elements that create side friction—such as pedestrians, bicyclists, parked vehicles, and landscaping—also present various levels of hazard, likely influencing drivers to slow down to various degrees. In other words, pedestrian presence close to the road edge is more likely to affect speed than landscaping close to the road edge. Design Issues The relationship between several design elements and operating speed was investigated in a previous review of design elements (1). In some cases the relationship was found to be strong, such as for horizontal curves; however, for several other cases, such as lane width, the relationship was found to be weak. In all cases when a relationship between the design element and operation speed exists, there are ranges when the influence of the design element on speed is minimal. Cross References Key Components of Sight Distance, 5-2 Determining Stopping Sight Distance, 5-4 Determining Passing Sight Distance, 5-10 Key References 1. Fitzpatrick, K., Carlson, P.J., Brewer, M.A., Wooldridge, M.D., and Miaou, S.-P. (2003). NCHRP Report 504: Design Speed, Operating Speed, and Posted Speed Practices. Washington DC: Transportation Research Board. 2. Fitzpatrick, K., Carlson, P.J., Wooldridge, M.D., and Brewer, M.A. (2000). Design Factors that Affect Driver Speed on Suburban Arterials (FHWA/TX-001/1769-3). Washington, DC: FHWA. 3. Evans, L. (1970). Speed estimation from a moving automobile. Ergonomics 13, 219-230. 4. Shinar, D., McDowell, E., and Rockwell, T.H. (1977). Eye movements in curve negotiation. Human Factors, 19(1), 63-71. 5-13 HFG SIGHT DISTANCE Version 2.0

KEY REFERENCES FOR SIGHT DISTANCE INFORMATION Introduction Sight distance requirements, issues, and subtopics have been covered extensively in a range of standard reference sources for roadway design and highway. It is important for roadway designers and traffic engineers to recognize that most of the information presented in this chapter has been adopted from these other sources and for users of this HFG to know where to go to find alternative sources of sight distance information. Design Guidelines The list below summarizes source and chapter for sight distance information from key reference sources: A Policy on Geometric Design of Highways and Streets (2011) • Chapter 2, Design Controls and Criteria, discusses driver reaction time and related issues in Driver Performance and Human Factors subhead. • Chapter 3, Elements of Design, has a section on sight distance, with subsections on stopping sight distance, decision sight distance, passing sight distance for two-lane highways, and sight distance for multilane highways. • Chapters 5 (Local Roads and Streets), 6 (Collector Roads and Streets), 7 (Rural and Urban Arterials), and 9 (Intersections) all have a number of specific subsections on sight distance. Manual on Uniform Traffic Control Devices (MUTCD) (2009) • The MUTCD has several figures and tables relating minimum sight distance to speed. These include Table 3B-1 (for passing sight distance), Table 4D-2 (for traffic control signal sight distance), Table 6C-2 (for work zone longitudinal buffer space), and Table 6E-1 (for work zone flagger stations). • Section 2C.05, Placement of Warning Signs, describes a PRT model. Table 2C-4 (English units) shows advance warning sign placement as a function of speed based on PRT requirements. ITE Traffic Engineering Handbook (1999) • Chapter 2, Road Users, has sections on PRT and sight distance. • Chapter 11, Geometric Design of Highways, has a section on sight distance, with subsections on stopping sight distance, passing sight distance, decision sight distance, and intersection sight distance. ITE Traffic Control Devices Handbook (2001) • Chapter 2, Human Factors, has sections on driver PRT and maneuver time. • Chapter 11, Highway-Rail Grade Crossings, contains discussion of sight distance requirements for at-grade crossings. Guidelines and Recommendations to Accommodate Older Drivers and Pedestrians (2001) • Sections on Intersections (I) and Roadway Curvature and Passing Zones (III) contain discussions of sight distance. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data Highway Safety Manual (2010) • Chapter 2, Human Factors, has sections on PRT and factors that affect its duration. HFG SIGHT DISTANCE Version 2.0 5-14

Discussion The HFG focuses on key aspects of sight distance from the roadway users’ perspective and is not intended to provide a comprehensive or definitive presentation of sight distance. Additional data sources follow: A Policy on Geometric Design of Highways and Streets (1) provides guidance to roadway designers in the form of recommended values for a host of critical design dimensions. It is based on both established practices and standards, and reflects recent research. Most of the chapters contain sections or subsections that focus on user needs and characteristics; as noted above, Chapters 2, 3, 5, 6, 7, and 9 contain sight distance information. The Manual on Uniform Traffic Control Devices (2) is the national standard for traffic control devices installed on any street, highway, or bicycle trail open to public travel. MUTCD provides uniform standards for the design of all signs, signals, markings, and other devices that are used to regulate, warn, or guide traffic and that are placed on, over, or adjacent to streets, highways, pedestrian facilities, and bikeways. Though MUTCD does not address sight distance issues as comprehensively as A Policy on Geometric Design of Highways and Streets, it does provide a number of very accessible and useful figures and tables on sight distance. The Traffic Engineering Handbook (3) provides relevant key principles and techniques on “best” traffic engineering practices. The Traffic Control Devices Handbook (4) is intended to augment and supplement the MUTCD by providing additional information and background information on selected topics. Although sight distance is not addressed as a separate chapter, PRT and MT are addressed in Chapter 2, Human Factors, and sight distance requirements for at- grade crossings are covered in Chapter 11, Highway-Rail Grade Crossings. Guidelines and Recommendations to Accommodate Older Drivers and Pedestrians (5) focuses on older roadway users but includes relevant information from key sources relating to sight distance (see also the accompanying handbook for these guidelines, published as FHWA-RD-01-103). The Highway Safety Manual (6) has limited information about estimating sight distance; however, it includes some discussion of sight distance as a contributing factor in crashes. Design Issues None Cross References Key Components of Sight Distance, 5-2 Determining Stopping Sight Distance, 5-4 Determining Intersection Sight Distance, 5-6 Determining When to Use Decision Sight Distance, 5-8 Determining Passing Sight Distance, 5-10 Influence of Speed on Sight Distance, 5-12 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington DC. 2. FHWA (2009). Manual on Uniform Traffic Control Devices (MUTCD). Washington, DC. 3. Pline, J.L. (Ed.). (1999). Traffic Engineering Handbook, Fifth Edition. Washington, DC: ITE. 4. Pline, J.L. (Ed.). (2001). Traffic Control Devices Handbook. Washington, DC: ITE. 5. Staplin, L., Lococo, K., Bynington, S., and Harkey, D. (2001) Guidelines and Recommendations to Accommodate Older Drivers and Pedestrians (FHWA-RD-01-051). McLean, VA: FHWA, Research, Office of Safety R&D. 6. AASHTO (2010). Highway Safety Manual, 1st Edition. Washington, DC. 5-15 HFG SIGHT DISTANCE Version 2.0

WHERE TO FIND SIGHT DISTANCE INFORMATION FOR SPECIFIC ROADWAY FEATURES Introduction The following table lists the information required to diagnose sight distance for specific roadway features. Although the roadway designer and the traffic engineer work with distances, sight distance needs actually originate from driver MT needs and speed choice. Therefore, to understand, diagnose, and address sight distance concerns, one must address the human factors issues of time and speed. Stopping sight distance is needed for all roadway features. Design Guidelines Feature or Problem Type of Sight Distance Requirement Information Required Location of Information Operating speed Determine Sight distance to hazard Determine All Roadway Features Stopping sight distance Required SSD AASHTO, Table 3-1 (1) Operating speed Determine Sight distance to hazard Determine Horizontal Curve Stopping sight distance Required SSD AASHTO, Table 3-2 (1) Curve recommended speed Determine Speed on approach Determine Sign location Determine Horizontal Curve Approach with Warning Sign Maneuver sight distance Sign placement guidelines MUTCD, Table 2C-4 (2) Operating speed Determine Vertical Curve Stopping sight distance Rate of vertical curvature, K AASHTO, Table 3-34 (1) Operating speed Determine Vertical Curve Passing sight distance Rate of vertical curvature, K AASHTO, Table 3-35 (1) Warning Sign Maneuver sight distance Warning sign placement guidelines MUTCD, Table 2C-4 (2) Guide Sign Maneuver sight distance Typical placement of route signs MUTCD, Figure 2D-6 (2) Operating speed Determine Avoidance maneuver C, D, or E Determine Signed Lane Drop Decision sight distance DSD AASHTO, Table 3-3 (1) Expert Judgment Based Primarily on Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SIGHT DISTANCE Version 2.0 5-16

Discussion The sight distance diagnostic procedure consists of a systematic on-site investigation technique to evaluate the highway environment to support sight distance needs. The highway location is surveyed, diagrammed, and divided into component sections based on specific driving demands (e.g., need to perform a specific maneuver). Then each section is analyzed in terms of its suitability to support the required task (e.g., information provided to the driver, allotted time to complete the required task or maneuver). This procedure enables the practitioner to compare the available sight distance with the sight distance required to perform the driving task safely. Procedures for measuring available sight distance are given in AASHTO (1) and the Manual of Transportation Engineering Studies (3). Available sight distance can be checked on plans for proposed designs or in the field for existing locations. Design Issues Many design references use the term “design speed” to characterize the expected driving speed on a roadway. However, as noted in “Influence of Speed on Sight Distance” (page 5-12 of this document), neither design speed nor posted speed is always the best determinant of actual driving speed. When available, actual operating speeds should be used instead of design speed to help determine needed sight distance. Cross References Tutorial 1: Real-World Driver Behavior Versus Design Models, 22-2 Tutorial 2: Diagnosing Sight Distance Problems and Other Design Deficiencies, 22-9 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. 2. FHWA (2009). Manual on Uniform Traffic Control Devices (MUTCD). Washington, DC. 3. Robertson, H.D., Hummer, J.E., and Nelson, D.C.(Eds.) (2000). Manual of Transportation Engineering Studies. Washington, DC: ITE. 5-17 HFG SIGHT DISTANCE Version 2.0

WHERE TO FIND SIGHT DISTANCE INFORMATION FOR INTERSECTIONS Introduction The following table lists the information required to diagnose sight distance at various intersection types. Although the roadway designer and the traffic engineer work with distances, sight distance needs actually originate from driver MT needs and speed choice. Therefore, to understand, diagnose, and address sight distance concerns, one must address the human factors issues of time and speed. Stopping sight distance is needed for all roadway features. Design Guidelines Feature or Problem Type of Sight Distance Requirement Information Required Location of Information Sight triangle Determine Operating speed Determine Uncontrolled Intersection Intersection sight distance Length of sight triangle legs AASHTO, Table 9-3 (1) Operating Speed Determine ISD-Case B1 AASHTO, Table 9-6 (1) ISD-Case B2 AASHTO, Table 9-8 (1) Two-Way Stop Intersection Intersection sight distance Case B1 Case B2 Case B3 ISD-Case B3 AASHTO, Table 9-8 (1) Operating Speed Determine ISD-Case C1 AASHTO, Tables 9-9 & 9-10 (1) Intersection with Yield Control on Minor Road Intersection sight distance Case C1 Case C2 ISD-Case C2 AASHTO, Table 9-12 (1) Time gap AASHTO, Table 9-13 (1) Operating speed Determine Left turns from Major Road Intersection sight distance—Case F ISD-Case F AASHTO, Table 9-14 (1) Four-Way Stop Intersection Intersection sight distance—Case E None required None required for ISD Signalized Intersection Intersection sight distance—Case D None required for basic None required for ISD signal operation Operating speed Determine Stopping sight distance Required SSD AASHTO, Table 3-1 (1) Sight triangle Determine Roundabout Intersection sight distance Length of conflicting leg Roundabout Guide, Exhibit 6-33 (2) Speed of vehicle Determine Speed of train Determine Distance from rail to stop line Determine Railroad-Highway Grade Crossing RHGC sight distance sight triangle Case A Case B Required RHGC sight distance AASHTO, Table 9-32 (1) Operating speed Determine Avoidance maneuver A or B Determine Approach to Stop Condition Decision sight distance DSD AASHTO, Table 3-3 (1) Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data HFG SIGHT DISTANCE Version 2.0 5-18

Discussion The sight distance diagnostic procedure consists of a systematic on-site investigation technique to evaluate the highway environment to support sight distance needs. The highway location is surveyed, diagrammed, and divided into component sections based on specific driving demands (e.g., need to perform a specific maneuver). Then each section is analyzed in terms of its suitability to support the required task (e.g., information provided to the driver, allotted time to complete the required task or maneuver). This procedure enables the practitioner to compare the available sight distance with the sight distance required to perform the driving task safely. Procedures for measuring available sight distance are given in AASHTO (1) and Robertson, Hummer, and Nelson, (4). Available sight distance can be checked on plans for proposed designs or in the field for existing locations. Tustin, Richards, McGee, and Patterson (3) and Robertson et al. (4) provide additional information that may be useful for determining sight distance. Design Issues Many design references use the term “design speed” to characterize the expected driving speed on a roadway. However, as noted in “Influence of Speed on Sight Distance” (page 5-12 of this document), neither design speed nor posted speed is always the best determinant of actual driving speed. When available, actual operating speeds should be used instead of design speed to help determine needed sight distance. Cross References Tutorial 1: Real-World Driver Behavior Versus Design Models, 22-2 Tutorial 2: Diagnosing Sight Distance Problems and Other Design Deficiencies, 22-9 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. 2. FHWA (2000). Roundabouts: An Informational Guide (Publication FHWA-RD-00-067). Washington, DC. 3. Tustin, B., Richards, H., McGee, H., and Patterson, R. (1986). Railroad-Highway Grade Crossing Handbook, Second Edition (FHWA-TS- 86-215). Fairfax, VA: Tustin Enterprises. 4. Robertson, H.D., Hummer, J.E., and Nelson, D.C. (2000). Manual of Transportation Engineering Studies. Washington, DC: ITE. 5-19 HFG SIGHT DISTANCE Version 2.0

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 600: Human Factors Guidelines for Road Systems: Second Edition provides data and insights of the extent to which road users’ needs, capabilities, and limitations are influenced by the effects of age, visual demands, cognition, and influence of expectancies.

NCHRP Report 600 provides guidance for roadway location elements and traffic engineering elements. The report also provides tutorials on special design topics, an index, and a glossary of technical terms.

The second edition of NCHRP 600 completes and updates the first edition, which was published previously in three collections.

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