National Academies Press: OpenBook

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

Chapter: Chapter 24 - Glossary

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Suggested Citation:"Chapter 24 - Glossary." 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 24 - Glossary." 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 24 - Glossary." 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 24 - Glossary." 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 24 - Glossary." 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 24 - Glossary." 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 24 - Glossary." 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 24 - Glossary." 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 24 - Glossary." 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.

Acceptable Gap Distance—The size of the gaps in major-road traffic typically accepted by drivers turning from a minor road to provide sufficient time for the minor-road vehicle to acceler- ate from a stop and complete a turn without unduly interfering with major-road traffic operations. Accessible Pedestrian Signals (APS)—Equipment for use at signalized intersections that com- municates pedestrian signal timing information in non-visual formats. Features include push- button locator tone, tactile arrow, pushbutton information message, automatic volume adjustment, alert tone, actuation indicator, tactile map, Braille and raised print information, extended button press, passive pedestrian detection, and clearance interval tones. AMBER Alert—An urgent broadcast regarding child abductions. Apparent Radius—The curve radius as seen from the driver’s perspective, which, in some cases, can make the curve appear distorted—either flatter or sharper—depending on topography and other road elements. Appropriate Message Length—Sign message lengths that drivers have time to read and com- prehend as they pass the sign. Arcminute—One-sixtieth (1/60) of one degree (1°). Arrow Panel Visibility—A roadway sign condition dependent on a number of factors, includ- ing the capability of the lamps in the panel, the type of roadway, the physical location of the panel, and the panel’s relation to horizontal and vertical curves, ambient light, and weather. Arrow-per-lane (APL) Signs—Large or grouped signs providing every individual lane with its own arrow to improve driver navigation. Behavioral Framework for Speeding—Conceptual overview of the key factors relevant to speed selection, as well as their relationship to potential speeding countermeasures. Bilingual Information—Information that is presented in more than one language on change- able message signs (CMSs). Blank-out/Blanking—The period of time, or scheduled phase, when sign readouts are not being used. Bollard—A thick vertical post sometimes used to control pedestrian and vehicular traffic. Bollard- mounted lighting can be used to provide vertical illuminance on pedestrians at crosswalks for improved visibility. Broad Spectrum—Light that contains a wide range of wavelengths (colors) across the visible spectrum. A broad-spectrum luminaire contains sufficient color content that humans can readily discriminate the colors of objects illuminated by it. Bulbout—A curb extension going past the sidewalk or curb line into the street. Bulbouts reduce the street pavement width in order to improve pedestrian crossings by shortening crossing distances, reducing the time pedestrians are exposed to traffic, improving pedestrian and motorist visibility, and reducing traffic speeds. Candela—The International System of Units (SI) base unit of luminous intensity. 24-1 C H A P T E R 24 Glossary

HFG GLOSSARY Version 2.0 24-2 Caution Mode Configuration—Arrow panel mode C, which provides flashing non-directional information to increase safety near highway work zones by providing early warning informa- tion to drivers indicating that caution is required while approaching and traveling through the work zone. Changeable Message Sign (CMS)—CMSs are electronic, reconfigurable signs placed above or near the roadway and are used to inform motorists of specific conditions or situations. Also referred to as variable message signs (VMSs) or dynamic message signs (DMSs). Clear Zone—The roadside border area that is available for drivers to safely stop or gain control of an errant vehicle. This area may include a shoulder, recoverable or non-recoverable slopes, and run-out areas that are smooth and clear of obstructions. See FHWA (2011). Clearance Interval—The period of time necessary for safe transitions in right-of-way (ROW) assignment between crossing or conflicting flows of traffic, including pedestrian activity; a com- bination of the yellow clearance interval plus the red clearance interval or an all-red interval. Clearing Distance—The distance a vehicle travels beginning at the time the signal changes to yellow and ending at the time the signal changes to red. Closed-Loop Compensatory Component—Part of the steering control process in which drivers continually monitor and adjust for deviations in position on the road based on feedback from near-field visual cues. Cognitive Preparation—The various active mental activities that can influence response times and decisions of drivers and includes such things as driver expectancies, situational awareness, a general sense of caution, and where attention is being directed by the driver. Color Spectrum—See Spectrum. Complexity—A function or level describing how much information is being provided and how difficult it is to process. Complexity of Sign Information—The number of information units being presented as part of roadway sign messages. Comprehension—The combination of completing a task at hand, e.g., reading a sign, plus the process of making the resultant decision, e.g., right or left turn in response to the sign’s information. Cone—The portion of the roadway scene on the right-hand side of the roadway where a driver would typically look for road signs. Conspicuity—The ease in seeing and locating a visual target, including signage, vehicles, bicycles, or pedestrians. In the context of road signs, it represents how easy it is to distinguish a sign from the surrounding visual environment. Continuation Distance—The distance that a vehicle travels prior to the descent of the entry gates at a railroad crossing. Counterbeam Lighting—A lighting technique whereby the light falls on objects from a direc- tion opposite to the traffic. Counterbeam lighting is characterized by a luminous intensity dis- tribution that is asymmetrical and has the maximum luminous intensity aimed against the direction of normal traffic flow. Crest Horizontal Curve—A horizontal curve that also contains a vertical, concave down, com- ponent of curvature. Critical Gap—For design purposes, the critical gap represents the gap between successive oncoming vehicles that average drivers will accept 50% of the time (and reject 50% of the time). Cross Section—The width of the lane. Cross Slope—The transversal slope of the roadway (described as a percentage) with respect to the horizon. Crossbuck—A railroad warning sign with two slats of wood or metal fastened together on a pole in a letter X formation with the word “Railroad” on one slat and “Crossing” on the other, black letters on a white background. Crossbucks are sometimes supplemented by other warning

devices such as flashing lights, a bell, a “Yield” sign, a “Stop” sign, and/or a descending gate to prevent traffic from crossing the tracks. Decibel (dB) Level—A measurement that expresses the power or intensity magnitude of sound rel- ative to a specified or implied reference level. A decibel is one-tenth of a bel, a seldom-used unit. Decision Sight Distance (DSD)—DSD represents a longer sight distance than is usually necessary and is used 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. Design Consistency—Conformance of a highway’s geometric and operational features with driver expectancy. Dilemma Zone—The portion of the roadway formed between (1) the clearing distance to the intersection (the distance the vehicle travels between the time the signal changes to yellow to the time the signal changes to red) and (2) the stopping distance (the distance traveled by the vehicle between the times the signal changes to yellow to the time when the vehicle actually stops) when the stopping distance is greater than the clearing distance. The size of the dilemma zone is relative to the situation; it is not a fixed area. Driver Expectations—The driver’s readiness to respond to situations, events, and information in predictable and successful ways. Driver Fatigue—A general psycho-physiological state that diminishes an individual’s ability to perform the driving task by reducing alertness and vigilance. Drop-off—Deterioration of roadways caused when the edges of the pavement become destabi- lized and eroded, resulting in a difference in height between the pavement surface and the roadside surface. Dynamic Characteristics—Message properties that specify character movement such as time to display each message phase, to display blanking between phases of a multiphase message, and to flash one or more lines of a message. Dynamic Dilemma Zone—A road segment on approach to an intersection which varies in length based on fluctuations in vehicle speeds and number. Dynamic Late Merge Systems—These systems, developed for use in work zone lane closure sit- uations, utilize a series of changeable message signs and static work zone signs to provide merge information to the driver. The information is based upon the current traffic volume through the work zone and supports early merging when the traffic flow is light and late merg- ing (closer to the gore point) when the traffic volume is heavier. Dynamic Message Sign (DMS)—DMSs are electronic, reconfigurable signs placed above or near the roadway and are used to inform motorists of specific conditions or situations. Also referred to as changeable message signs (CMSs) or variable message signs (VMSs). Effective Length of the Passing Lane—The physical length of the passing lane plus the distance downstream to the point where traffic conditions return to a level similar to that immediately upstream of the passing lane. Effects of Roadway Factors on Speed—The impact of geometric, environmental, and traffic factors on driving speed under free-flow conditions in tangent roadway sections. Empirical Bayes—A method in which empirical data are used to estimate conditional probabil- ity distributions. Exit Gate Clearance Time—The amount of time provided to delay the descent of the exit gate arm(s) after entrance gate arm(s) begin to descend at a railroad crossing. Factors Affecting Acceptable Gap—These factors are the driver, environment, and other situ- ational factors—such as traffic volume, wait times, familiarity with the roadway or oncoming vehicle size—that cause most drivers or specific groups of drivers (e.g., older drivers) to accept smaller or larger gaps than they would otherwise accept under normal conditions. Fatal Accident Reporting System (FARS)—National Center for Statistics and Analysis (NCSA) data system. Four-Quadrant Gate—A set of four descending gates to stop traffic at railroad crossings which con- sists of one gate before and one gate after the railroad tracks for each of the two lanes of traffic. 24-3 HFG GLOSSARY Version 2.0

Foveal Vision—Central vision of the eye. The fovea, located in the pit of the retina, is the source of the eye’s high visual acuity capability. Free-Flow Speed—Free-flow speed is defined as conditions in which a driver has the ability to choose a speed of travel without undue influence from other traffic, conspicuous police pres- ence, or environmental factors. Fundus—The interior surface of the eye, opposite the lens, and includes the retina, optic disc, macula and fovea, and posterior pole. Gap—The time interval between two successive vehicles, measured from the rear of a lead vehicle to the front of the following vehicle, adapted from Traffic Engineering Handbook (Pline, 1999). Gate Delay—The length of time between the start of the flashing lights and the initiation of the descent of the entry gate arm at railroad crossings. Gate Interval Time—The length of time between the initiation of the descent of the entry gate and the initiation of the descent of the exit gate at a crossing with a four-quadrant gate device. Gate-rushing—Gate-rushing is when drivers do not stop at railroad crossings and drive under the gate arms as they are descending or drive around gate arms that are already in the lowered position. Glare—A visual phenomena which occurs when the intensity of a light source within the visual field is substantially greater than the visual adaptation level, causing physical discomfort or pain (discomfort glare) and/or reduced visibility (disability glare). Glare Screens—Visual barriers designed to shield drivers from glaring light from oncoming headlamps. Glare screens are often mounted on median barrier walls and come in a variety of forms, including vertical paddles, concrete barriers, wire or plastic mesh screens, etc. Shrub- bery and other landscaping elements can also be used as glare screens. Grade Severity Rating System—A simulation model that establishes a safe descent speed for the grade based upon a predetermined brake temperature limit. HAWK Signal—A new type of overhead beacon signal to assist pedestrians at unsignalized cross- walks on high-volume traffic streets. High Pressure Sodium (HPS) Lamp—A type of lamp for street lighting that operates using an electric arc through sodium vapor under high pressure. HPS lamps glow with a characteristi- cally yellow light. Highway Systems—The combination of three major components—the road (local roads, col- lectors, arterials and freeways), traffic control, and users with or without a vehicle. Horizontal Curves with Vertical Sag—A horizontal curve that also contains a vertical, concave up, component. Human Factors—A scientific discipline that tries to enhance the relationship between devices and systems and the people who are meant to use them through the application of extensive, well-documented, and fully appropriate behavioral data that describe and analyze the capa- bilities and limitations of human beings. Independent Alignments—Roadway design such that opposing lanes are developed independ- ently of each other. The opposing alignments may or may not run parallel to each other. Also, they may be separated horizontally by geographic, landscaping, or other features. Similarly, they may be separated vertically on hillsides or other steep grades. Induction Lamp—A type of lamp for street lighting that uses electromagnetic fields rather than electrodes to wirelessly transfer power to the interior of the lamp. Radio waves are typically used to energize either a bulb filled with sulfur or metal halides or a tube based on conven- tional fluorescent lamp phosphors. Information Units—A measure of the amount of information presented in terms of facts used to make a decision. Intersection Sight Distance (ISD)—The stopping sight distance required at intersections. Actual ISDs will differ, depending on the type of intersection and maneuver involved. HFG GLOSSARY Version 2.0 24-4

Lag—The time interval from the point of the observer to the arrival of the front of the next approaching vehicle (Lerner et al., 1995, pp. 58–59). Lane Drop Markings—Pavement markings that consist of short wide lines with short gaps used to delineate a lane that becomes a mandatory turn or exit lane. Legibility Distance—The minimum distance at which a sign must become legible to a typical driver. It is calculated as a function of the time it takes a driver to read the sign, interpret the sign, and execute maneuvers that comply with the sign’s message. Legibility Index—The distance at which a given unit of letter height is readable. Long-Range Guidance—Driving preview time for drivers of at least 5 s. Looming—One of several dynamic characteristics of message signs, this term refers to increas- ing the size of text or symbols over time in a message display. Low Pressure Sodium (LPS) Lamp—A type of lamp for street lighting that operates using an electric arc through sodium vapor. LPS lamps operate at lower pressure than HPS lamps, and they are nearly monochromatic yellow in color. Luminaire—A lighting fixture that consists of one or more electric lamps, lamp housings, reflec- tors, mast, wiring, and other necessary parts. Luminaire Cutoff Pattern—The distribution of intensities from a luminaire around the point at which the luminaire emits no light. A luminaire with full cutoff projects less than 10% of rated lumens beyond 80 degrees from nadir and no light at or above 90 degrees from nadir. The nadir is defined as the angle that points directly downward (0 degrees) from the luminaire. Luminance—Luminance is the luminous intensity per unit area of light measured as candela per square meter (cd/m2). Luminous Intensity—A measure of the perceived power emitted by a light source in a particu- lar direction per unit solid angle. Lux—The International System of Units (SI) unit of illuminance and luminous emittance. Maneuver Time (MT)—The amount of time required to safely complete a maneuver. MT is pri- marily affected by the physics of the situation, including vehicle performance capabilities, tire- pavement friction, road-surface conditions (e.g., ice), and downgrades, and to a lesser extent by driver-related factors (e.g., deceleration profile), although these factors are highly situation specific because the maneuvers encompass a broad range of actions (e.g., emergency stop, passing, left turn through traffic). Mental Models—The system user’s internal understanding and representation of an external reality. Mesopic Lighting—Light conditions under which visual sensitivity is shifted toward the blue/green portion of the visible spectrum compared to vision under photopic (daytime) lighting, thus making objects and clothing in this part of the spectrum more visible under these conditions. See photopic lighting. Metal Halide Lamp—A type of lamp for street lighting that operates using an electric arc through mercury vapor under high pressure. Metal halide salts added to the mercury glow at different wavelengths yielding a relatively white light. Most Meaningful Information (MMI)—Information sought by drivers for particular road loca- tion and point in time through scanning the road environment in front of, behind, and to the sides of the vehicle they are driving. Nighttime Driving—The situation in which motorists’ visibility while driving in darkness on rural roads is limited; roadway features, objects in the roadway, or pedestrians ahead are less visible depending upon headlamp intensity, ambient lighting, and presence or absence of oncoming headlamp glare. Open-Loop Anticipatory Control Process—Part of the steering control process in which drivers predict road curvature and required steering angle based on far-field visual cues. Optic Flow—The visual pattern caused by moving forward, in which points close to the point of expansion move outward slower than points more peripheral to it. This information is directly used by the driver’s visual system to perceive motion. 24-5 HFG GLOSSARY Version 2.0

Passing Lane—A lane added in one or both directions of travel on a two-lane, two-way highway to improve passing opportunities. Passing Sight Distance (PSD)—The amount of distance 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. Pavement Classifications—The type and texture of paving material affects how light reflects from the road surface. The International Commission on Illumination (Commission Inter- nationale de l’Eclairage or CIE) has developed four road surface classifications. Class R1 sur- faces have primarily diffuse reflectances, Class R2 surfaces have mixed diffuse and specular reflectances, Class R3 surfaces have slightly specular reflectances, and Class R4 surfaces have mostly specular reflectances (see IESNA 2000). Pavement Drop-off—Drop-offs are caused when the edges of pavement are destabilized and eroded, resulting in a difference in height between the pavement surface and the roadside surface. Perception-Identification-Emotion-Volition (PIEV) Time—The total time from perception to completing a reaction. Perception-Reaction Time (PRT)—The time a driver takes to process information, 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. Per AASHTO (2004), bits of information on a scale from 0 to 6 bits is processed by the average driver at about 1 and 1.5 bits of information per second for unexpected and expected situations, respectively. Perceptual Requirements—The visual information about the roadway and surrounding envi- ronment that drivers need to judge road curvature, determine lane position and heading, etc. Phase (for message signs)—The text that is displayed at a single point in time on a message sign. Photopic Lighting—Light conditions under which visual appearance is stronger at the yellow portion of the visible spectrum compared to vision under mesopic (nighttime) lighting when visual sensitivity is shifted toward the blue/green part of the spectrum. See mesopic lighting. Point of Expansion—During forward motion, the point in the forward field that appears sta- tionary relative to the observer (the observers’ actual destination), and from which all other points are seen as moving away. Post-Mounted Delineators (PMDs)—A type of marking device used to guide traffic; a series of retroreflective devices mounted above the roadway surface and along the side of the roadway to indicate the alignment of the roadway. Preview Sight Distance (PVSD)—PVSD is a measure of driver sight distance based on the assump- tion that “the driver views or previews the roadway surface and other cues that lie ahead to obtain the information needed for vehicular control and guidance” (Gattis and Duncan, 1995). Psychomotor Requirements—The control actions (e.g., steering-wheel movements; foot move- ments to press brake, etc.) that drivers must make to maintain vehicle control or to facilitate other information acquisition activities. Raised Pavement Markers (RPM)—A variety of three-dimensional devices used in conjunction with pavement markings to mark lane boundaries. They often have a reflective surface to increase visibility and produce a noticeable vibration or physical sensation when in contact with vehicle tires. Red Light Running—Situations when drivers enter a signalized intersection when a red light is being presented. Retroreflective Raised Pavement Markers (RRPM)—Raised pavement markers affixed to the road surface that are designed to reflect light directly back to the light source. Retroreflectivity—The property allowing a surface to reflect a large portion of its light directly back to or near its source. Roadway Shoulder—See Shoulder. Roundabout Intersection—As defined by the MUTCD, roundabouts are circular inter- sections with yield control at entry, permitting a vehicle on the circulatory roadway to pro- HFG GLOSSARY Version 2.0 24-6

ceed, and deflecting the approaching vehicle counter-clockwise around a central island (FHWA, 2009). Roving Eye Treatments—Pedestrian or driver signals which include a pair of animated eyes as part of the lighted display, intended as a reminder to watch for vehicle movement (for pedes- trians) or to watch for pedestrian movements (for drivers). Safety Edge—A wedge-shaped asphalt material placed between the roadway and the shoulder, which can be used as a drop-off countermeasure. Serial Processing—A chain of events in which one step does not begin until the previous step is complete that is used to model some driver behavior. Shared-Use Lanes—Roadways or lanes used concurrently by vehicles, bicyclists, or pedestrians in either rural or urban areas. Sharrows—Shared-lane markings. Short-Range Guidance—Preview time for drivers of up to 3 s. Shoulder or Roadway Shoulder—A portion of the roadway contiguous with the traveled way for accommodation of stopped vehicles; for emergency use; and for lateral support of the sub- base, base, and surface courses. Also may be used by non-motorized traffic. Shoulder Drop-off—A difference in height between the pavement surface and the roadside sur- face caused when the edges of pavement become destabilized and eroded. Shoulder Rumble Strips (SRSs)—A raised or grooved pattern on the shoulder of a travel lane to provide a tactile or audio alert to the driver. Sight Distance (SD)—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. SD is based on (1) a perception-reaction time (PRT) required to initiate a maneu- ver (pre-maneuver phase) and (2) the time required to safely complete a maneuver (MT). Sight Distance at Left-Skewed Intersections—The available sight distance to the driver’s right side for a vehicle crossing a major road from a left-skewed minor road (where the acute angle is to the right of the vehicle). Sight Distance at Right-Skewed Intersections —The available sight distance to the driver’s left side for a vehicle crossing a major road from a right-skewed minor road (where the acute angle is to the left of the vehicle). Sign Comprehension—The driver’s or road user’s ability to interpret the meaning of a sign. The ability to comprehend and use signs is associated with three stages: legibility, recogni- tion, and interpretation. Sign comprehension can also consist of the sign reading task plus the process of making the resultant decision, e.g., right or left turn in response to the sign’s information. Sign Design—Design parameters of signs that impact the legibility of text placed on the sign, including retroreflectivity, legend color, font size, and font style. Sign Legend—The text and/or symbols composing the message of a sign. Sign Legibility—Specific design characteristics of signs that contribute to the drivers’ ability to perceive and understand the sign’s message. Sign Legibility Index—An index created by the USSC to calculate sign letter height. To deter- mine letter height divide the viewer reaction distance by the appropriate legibility index value (which varies depending on illumination, font style and case, as well as font color contrast to background). Sign Recognition—The ability of the driver to readily distinguish the sign, especially in the con- text of other signs and stimuli. Small Target Visibility Method—One method used to calculate or measure road lighting levels based on visibility of a small target. Visibility is calculated using target size and reflectivity, road reflectivity, veiling luminance, and other factors. Spectral Power Distribution (SPD)—The distribution of the power of each wavelength in the visual spectrum produced by a light source. The spectral power distribution of a luminaire 24-7 HFG GLOSSARY Version 2.0

affects the perceived color of objects illuminated by it and may affect the ability to detect, iden- tify, or discriminate objects under mesopic lighting conditions. Spectrum—The full range of wavelengths (colors) of light produced by a light source. For exam- ple, a low pressure sodium luminaire contains nearly monochromatic yellow light, while an LED light source contains a variety of wavelengths with an abundance of blue relative to the other wavelengths in the spectrum. Speed Perception—A driver’s judgment of how fast he or she is traveling. Stopping Distance—The distance traveled by a vehicle beginning from the time a traffic signal changes to yellow and ending at the time when the vehicle actually stops. Stopping Sight Distance (SSD)—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 reach- ing that stopping requirement. SSD depends on (1) how long it takes for a driver to perceive and respond to the stopping requirement (PRT) and (2) how aggressively the driver deceler- ates (MT). This distance can be calculated as the sum of driver perception-reaction time + vehicle deceleration, under a range of visibility/traction conditions. Task Analysis—Identification of basic activities performed by drivers as they navigate different driving scenarios by successively decomposing driving segments into tasks and subtasks/infor- mation processing elements. Title II of the Americans with Disabilities Act (ADA) of 1990—Title II of the ADA is imple- mented in 28 CFR Part 35, which prohibits discrimination on the basis of disability by public entities. 28 CFR 35.151 New construction and alterations is available at http://www.ed.gov/policy/ rights/reg/ocr/edlite-28cfr35.html#S151. Train Delay—The amount of time between the gate descent and the train arrival at railroad crossings. Traffic Engineering—The definition from ITE’s Traffic Engineering Handbook is “that branch of engineering which applies technology, science, and human factors to the planning, design, operations and management of roads, streets, bikeways, highways, their networks, terminals, and abutting lands” (Pline, 1999). Truck Escape Ramp—A facility designed and constructed to provide a location for out-of- control trucks to decelerate to a stop, which is also available for use by other vehicles. Turnout—A widened, unobstructed shoulder area or lane that provides opportunities for slow-moving vehicles to pull out of the through lane and passing opportunities to follow- ing vehicles. Two-Quadrant Gate—A set of two descending gates to stop traffic at railroad crossings which consists of one gate before the railroad tracks for each of the two lanes of traffic. Veiling Luminance—Uniform luminance that washes over the retina causing a reduction of contrast. Veiling luminance is caused when the eye is exposed to a light source that is substan- tially more intense than the adaptation level. Viewer Reaction Distance—The distance a viewer will cover at a given rate of speed and reac- tion time, which can be calculated by speed of travel (ft/s) times perception-reaction time (s). Visual Conspicuity—Characteristics of a sign that enable a driver to differentiate the sign from its surrounding environment. Variable Message Sign (VMS)—VMSs are electronic, reconfigurable signs placed above or near the roadway and used to inform motorists of specific conditions or situations. Also referred to as changeable message signs (CMSs) or dynamic message signs (DMSs). Warning Time—At railroad crossings, the time between the initiation of the flashing light traf- fic control devices and the arrival of the train. Warrant Criteria—Criteria used to determine whether street lighting is warranted at an inter- section or other location. Various jurisdictions use nighttime/daytime crash ratios, average daily traffic, and other criteria to determine whether street lighting is likely to provide suffi- HFG GLOSSARY Version 2.0 24-8

cient safety improvements to justify the expense of installing and operating lighting at the location. Wayside Horn—An audible warning horn mounted at rail-highway crossings to alert drivers when a train is approaching. Work Zone Speed Limits—Reduced speed limits used in work zones to maintain safe traffic flow. Yellow Timing Interval—Duration of the yellow signal indication (also referred to as the “yellow change interval” or “yellow clearance interval”). Zip Merging—Vehicles merging one by one in an alternating pattern. 24-9 HFG GLOSSARY 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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