U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This report is an archived publication and may contain dated technical, contact, and link information |
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Publication Number: FHWA-RD-97-135
Date: January 1998 |
Older Driver Highway Design HandbookGLOSSARY
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AAAFTS. American Automobile Association Foundation for Traffic Safety. back to top AASHTO. American Association of State and Highway Transportation Officials. back to top Ambient conditions. The visual background or surrounding atmospheric and visibility conditions. back to top Angular motion sensitivity. The ability of an observer to detect changes in the apparent distance and direction of movement of an object as a function of the change in the angular size of the visual stimulus on the observer's retina. back to top Angular velocity threshold. The rate of change in angular size of a visual stimulus that is necessary for an observer to discern that an object's motion has increased or decreased. back to top Annual average daily traffic (AADT). The total volume passing a point or segment of a highway facility in both directions for one year divided by the number of days in the year. back to top ATSSA. American Traffic Safety Services Association. back to top Attraction signing. Information/supplemental signs featuring logos or verbal messages pointing out places to visit or food, gas, and rest stop locations. back to top Barnes Dance timing. Type of exclusive signal timing phase where pedestrians may also cross diagonally in addition to crossing either street. Also referred to as scramble timing. back to top Brake reaction time. The interval between the instant that the driver recognizes the presence of an object or hazard on the roadway ahead and the instant that the driver actually applies the brakes. back to top Buttonhook ramp. J-shaped ramp that connects to a parallel or diagonal street or frontage road, which is often well removed from the interchange structure and other ramps. back to top Changeable message sign (CMS). Sometimes called portable changeable or variable message sign. This traffic control device has the flexibility to display a variety of messages to fit the needs of the traffic and highway situation. back to top Channelization. The separation or regulation of conflicting traffic movement into definite paths of travel by the use of pavement markings, raised islands, or other suitable means, to facilitate the safe and orderly movement of both vehicles and pedestrians. back to top Chevron signs. A chevron symbol (sideways "V") in black, against standard yellow background, on a vertical rectangle. Used as an alternate or supplement to standard delineators and to large arrow signs. back to top CIE. Commission Internationale de l'Éclairage (International Commission on Street/Highway Lighting). back to top Cloverleaf interchange. A form of interchange that provides indirect right-turn movements in all four quadrants by means of loops. Generally used where the turning and weaving volumes are relatively low. This type of interchange eliminates all crossing conflicts found in a diamond interchange but requires more area. The cloverleaf type of interchange can have one or two points of entry and exit on each through roadway. back to top Complete interchange lighting (CIL). Includes lighting in the interchange area on both the acceleration and deceleration areas plus the ramps through the terminus. back to top Composite photometry. Light measurement applied to a high-mast lighting system that employs a counterbeam arrangement, to take advantage of the efficiency with which pavement luminance can be increased with light directed upstream, while enhancing positive contrast through additivity of vehicle headlighting with the light directed downstream. back to top Concrete safety shaped barrier (CSSB). Commonly used median barrier where there is heavy vehicle travel and narrow medians. back to top Contrast. See luminance contrast. back to top Contrast sensitivity. Ability to perceive a lightness or brightness difference between two areas. Frequently measured for a range of target patterns differing in value along some dimension such as pattern element size and portrayed graphically in a contrast sensitivity function in which the reciprocal of contrast threshold is plotted against pattern spatial frequency or against visual angle subtended at the eye by pattern elements (such as bars). back to top Critical gap. The gap (distance to nearest vehicle) in oncoming or cross traffic that a driver will accept to initiate a turning or crossing maneuver 50 percent of the time it is presented, typically measured in seconds. back to top Dark adaptation. Adjustment of the eye to low levels of illumination, which results in increased sensitivity to light. back to top Decision sight distance (DSD). The distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard or its threat potential, select an appropriate speed and path, and initiate and complete the required safety maneuver safely and efficiently. back to top Depth perception. The ability to distinguish the relative distance of objects in visual space, used to interpret their motion over multiple observations. back to top Diamond interchange. The simplest and perhaps most common type of interchange.This type of interchange contains a one-way diagonal-type ramp in one or more of the quadrants. The diamond interchange provides for all movements to and from the intersecting road. back to top Diverge steering zone. Used with interchange/ramp exit models, it is the distance upstream from the exit gore at which a driver begins to diverge from the freeway. back to top Divided attention. The ability of a driver to allocate attention among tasks or stimuli in the roadway environment, where more than one task or stimulus is perceived to be important to safe performance at a given time. back to top Divided highway. Roadway that is separated by a median. back to top Dynamic visual acuity. Acuteness or clarity of vision for an object that has angular movement relative to the observer. Acuity depends on sharpness of retinal focus, sensitivity of nervous elements, oculomotor coordination, interpretative faculty of the brain, and contextual variables. back to top Edgeline visibility. The detection/recognition of painted pavement surface delineation along roadway edges. back to top Exit gore area. The area located immediately between the left edge of a ramp pavement and the right edge of the mainline roadway pavement at a merge or diverge area. back to top FARS. Fatal Accident Reporting System. back to top FHWA. Federal Highway Administration. back to top Full diamond interchange. Interchange with a one-way diagonal-type ramp in each quadrant. back to top Gap acceptance. The decision by a driver that there is sufficient time and/or distance ahead of an approaching vehicle to allow safe performance of a desired crossing or merging maneuver. back to top Gap judgments. The judgment of a driver of the time and/or distance ahead of an approaching vehicle traveling in a lane that the driver wishes to turn across or merge into. back to top Gap search and acceptance (GSA) zone. Used with interchange/ramp entry models, it is the zone in which the driver searches, evaluates, and accepts or rejects the available lags or gaps in the traffic stream for execution of a merging maneuver. back to top Guard (guide) rail. Protective barrier along a roadway to prevent vehicles from leaving the roadway. back to top Half-diamond interchange. An interchange with a one-way diagonal-type ramp in two adjacent quadrants. This type of interchange is appropriate to situations in which traffic demand is predominantly in one direction. back to top High-mast lighting. Illumination of a large area by means of a cluster of luminaires which are designed to be mounted in fixed orientation at the top of a high mast (generally 25 m [80 ft] or higher). back to top High-spatial-frequency stimulus. A visual target characterized by fine detail. back to top Horizontal alignment. The linear (tangent) character or specific degree of curvature describing the geometry of a defined section of highway in plain view. back to top IIHS. Insurance Institute for Highway Safety. back to top Illuminance. The density of luminous flux (rate of emission of luminous energy flow of a light source in all directions) incident on a surface; the quotient of the flux divided by the area of the surface, when the surface is uniformly illuminated. back to top Illumination. The amount of light falling onto a surface. back to top Initial acceleration (IA) zone. Used with interchange/ramp entry models, it is the zone in which the driver accelerates to reduce the speed differential between the ramp vehicle and the freeway vehicles to an acceptable level for completing the merge process. back to top In-service brightness level (ISBL). The brightness level of a delineation treatment at an intermediate point in its anticipated service life; this value varies by type of delineator, type of wear (traffic level), and environmental conditions. back to top Interchange (grade separation). A system of interconnecting roadways that provides for the movement of traffic between two or more highways on different levels. back to top Intersecting angle (skew). The angle formed by the intersection of two roadways (other than a 90-degree angle). back to top Intersection (at grade). The general area where two or more highways join or cross without grade separation, including the roadway and roadside facilities for traffic movements within it. back to top Intersection sight distance (ISD). The unobstructed view of an entire (at-grade) intersection and sufficient lengths of the intersecting highway to permit control of the vehicle to avoid collisions during through and turning movements. back to top ISTEA. Intermodal Surface Transportation Efficiency Act. back to top ITE. Institute of Transportation Engineers. back to top Joint flexibility. An aspect of the physical condition of the driver that can be assessed to determine whether the driver has sufficient strength to turn the steering wheel, apply the brakes, and generally control the vehicle. back to top Legibility Index (LI). Used to describe the relative legibility of different letter styles, it is calculated from the distance at which a character, word, or message is legible divided by the size of the letters on the sign. back to top Limited sight distance. A restricted preview of the traveled way downstream due to a crest vertical curve or horizontal curvature of the roadway, or to blockage or obstruction by a natural or manmade roadway feature or by (an)other vehicle(s). back to top Luminaire. A complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, to position and protect the lamps, and to connect the lamps to the power supply. back to top Luminance. The luminous intensity or brightness of any surface in a given direction, per unit of projected area of the surface as viewed from that direction, independent of viewing distance. The SI unit is the candela per square meter. back to top Luminance contrast. The difference between the luminance of a target area and a surrounding background area, divided by the background luminance alone (e.g., lane marking minus lane pavement surface, divided by pavement surface.) back to top Measures of effectiveness (MOEs). Descriptions of driver or traffic behavior which quantify the level of safety or the quality of service provided by a facility or treatment to drivers, passengers, or pedestrians; examples include vehicle speed, trajectory, delay, and similar measures, especially accidents, plus indices of performance such as reaction time. In research studies, the MOEs are the dependent measures (e.g., the effects/behaviors resulting from introduction of a treatment or countermeasure). Median barriers. A longitudinal system of physical barriers used to prevent an errant vehicle from crossing the portion of a divided highway separating traffic moving in opposite directions. back to top Merge steering control (MSC) zone. Used with interchange/ramp entry models, it is the zone in which the driver enters the freeway and positions the vehicle in the nearest lane on the mainline. back to top Minimum required visibility distance (MRVD). The distance necessary to permit detection and comprehension, plus driver decisionmaking, response selection, and completion of a vehicle maneuver, if necessary. back to top MUTCD. Manual on Uniform Traffic Control Devices for Streets and Highways. back to top NCHRP. National Cooperative Highway Research Panel. back to top Negative offset. A term used to describe the alignment of opposing left-turn lanes at an intersection; this geometry exists when the left boundary of one left-turn lane, when extended across the intersection, falls to the right of the right boundary of the opposite left-turn lane. back to top NHTSA. National Highway Traffic Safety Administration. back to top No turn on red (NTOR). This message on signs is used to indicate that a right turn on red (or left-turn on red for one-way streets) is not permitted at an intersection. NTSB. National Transportation Safety Board. back to top Ocular media. The internal structure of the eye, including the aqueous, through which light entering through the cornea must be transmitted before reaching the photosensitive retina. back to top Ocular transmittance. The amount of light reaching the retina relative to the amount incident upon the cornea. back to top Osteoarthritis. A degenerative form of arthritis. back to top Parclo loop ramp. A (partial cloverleaf) interchange with loops in advance of the minor road with direction of travel on the freeway; and in the same interchange area, an interchange with loops beyond the minor road. back to top Partial interchange lighting (PIL). Lighting on an interchange that consists of a few luminaires located in the general areas where entrance and exit ramps connect with the through traffic lanes of a freeway (between the entry gore and the end of the acceleration ramp or exit gore and the beginning of the deceleration ramp). back to top Peak intensity. The maximum strength of a traffic signal maintained through a defined viewing angle; measured in candelas. back to top Pedestrian control device. A special type of device (including pedestrian signal indications and sign panels) intended for the exclusive purpose of controlling pedestrian traffic in crosswalks. back to top Pedestrian crosswalk. An extension of a sidewalk across an intersection or across a roadway at a midblock location to accommodate pedestrian movement. back to top Perception-reaction time (PRT). The interval between a driver's detection of a target stimulus or event and the initiation of a vehicle control movement in response to the stimulus or event. back to top Positive offset. A term used to describe the alignment of opposing left-turn lanes at an intersection; this geometry exists when the left boundary of one left-turn lane, when extended across the intersection, falls to the left of the right boundary of the opposite left-turn lane. back to top Post-mounted delineators (PMDs). Retroreflective devices located serially at the side of a roadway to indicate alignment. Each delineator consists of a flat reflecting surface, typically a vertical rectangle, mounted on a supporting post. back to top Raised pavement markers (RPMs). Used as positioning guides and/or as supplements or substitutes for other types of markings, these markers conform to the color of the marking for which they serve as a positioning guide, can be mono- or bi-directional, and are fastened into the pavement with the reflector surface visible above the road surface. back to top Reaction time (RT). The time from the onset of a stimulus to the beginning of a driver's (or pedestrian's) response to the stimulus, by a simple movement of a limb or other body part. back to top Rheumatoid arthritis. A usually chronic disease of unknown cause characterized by pain, stiffness, inflammation, swelling, and sometimes destruction of joints. Drivers with this condition sometimes require compensatory equipment for their vehicle. In acute conditions, individuals should not drive because of weakness and extreme tenderness in the joints of the wrists and hands. back to top Right turn on red (RTOR). Unless otherwise specified by traffic signal control signage, this practice permits a driver to proceed with a right turn on a red signal after stopping at signalized intersections. It provides increased capacity and operational efficiency at a low cost. back to top Route Marker Reassurance Assembly. Consists of a cardinal direction marker (i.e., east, west, north, and south) and a route marker. back to top Saccadic movement. A change in visual fixation from one point to another by means of a quick, abrupt movement of the eye. back to top Scissors off-ramp. A condition where one-way traffic streams cross by merging and diverging maneuvers onto exit ramps. Drivers tend to go straight ahead onto an off-ramp instead of turning left. back to top Selective attention. The ability, on an ongoing moment-to-moment basis while driving, to identify and allocate attention to the most relevant information, especially embedded when within a visually complex scene and in the presence of a number of distractors. back to top Senile miosis. An aging characteristic involving an excessive smallness or contraction of the pupil of the eye. back to top Sight distance. The length of highway visible to the driver. back to top Sight triangle. In plan view, the area defined by the point of intersection of two roadways, and by the driver's line of sight from the point of approach along one leg of the intersection, to the farthest unobstructed location on another leg of the intersection. back to top Situational awareness. The selective attention to and perception of environmental elements within a specified space and time envelope, the comprehension of their meaning, and the projection of their status in the near future. back to top Slip ramp. A diagonal ramp, more properly called a cross connection, which connects with a parallel frontage road. back to top Small target visibility (STV). A proposed criterion for roadway lighting. The concept assumes that increased target visibility results in both increased nighttime safety and improved nighttime driver performance, a surrogate for reduced accident risk. back to top Speed-change lane (SCL). Used in interchange/ramp exit models, it refers to the speed-change maneuver on deceleration lanes segmented components. back to top Steering control (SC) zone. Used with interchange/ramp entry models, it is the zone where positioning of the vehicle along a path from the controlling ramp curvature onto the speed-change lane is accomplished. back to top Stereopsis. Binocular visual perception of three-dimensional space based on retinal disparity. back to top Stopping sight distance (SSD). The sight distance required to permit drivers to see an obstacle soon enough to stop for it under a defined set of reasonable worst-case conditions, without the performance of any avoidance maneuver or change in travel path; the calculation of SSD depends upon speed, gradient, road surface and tire conditions, and assumptions about the perception-reaction time of the driver. back to top Temporary pavement marking treatment. This treatment primarily involves the application of paint or tape striping and has been shown to be important for effective vehicle guidance at highway work sites. back to top T-intersection. An intersection that involves three legs, where one leg is perpendicular to the other two legs. There are several types of this intersection, such as plain, with turning lanes, and channelized. back to top Traffic control device (TCD). The prime, and often the only, means of communicating with the driving public. These devices (e.g., signs, markings, signals, islands) must be used discriminately, uniformly, and effectively to assure correct driver interpretation and response. back to top Transient adaptation factor. A reduction in target contrast caused by the process of transient visual adaptation. back to top Transient visual adaptation (TVA). The process in which the (driver's) eye fixates upon roadway locations or surrounding environments at different luminance levels, continuously adapting to higher and lower levels; this process temporarily reduces contrast sensitivity. back to top TRB. Transportation Research Board. back to top Trumpet interchange. A three-leg interchange where a connecting highway terminates and where only a small amount of traffic moves between the terminating highway and one of the two legs of the freeway. The trumpet is laid out so that this minor traffic moves via a 200-degree loop. back to top Two-quadrant cloverleaf interchange. A type of partial cloverleaf where most traffic leaving one highway turns to the same leg of the intersecting highway. back to top Useful field of view (UFOV). That area surrounding the point of fixation within which one can perform more complex tasks. This might include discriminating among letters or geometric figures, identifying a target against a complicated background display, or combining a secondary task in the periphery with an ongoing task in the forward (central) field of view. back to top Variable message sign (VMS). See changeable message sign. Veiling glare. Stray light entering the eye that reduces the contrast of a target upon which the driver has fixated; this may result from the driver's direct view of light sources, such as opposing headlights or roadway luminaires, or from light reflected from surfaces near the target's location. back to top Vertical curve. The parabolic curve connecting the two approach grades on either side of a hill. back to top Visual acuity. The ability of an observer to resolve fine pattern detail. Acuity is usually specified in terms of decimal acuity, defined as the reciprocal of the smallest resolvable pattern detail in minutes of arc of visual angle. "Normal" or average acuity is considered to be 1.0 (a resolution of 1-min arc). back to top Visual clear (VC) zone. Used with interchange/ramp entry models, this refers to the zone that provides a buffer between the driver and the end of the acceleration lane, where the driver can either merge onto the freeway in a forced maneuver or abort the merge and begin to decelerate at a reasonable rate. back to top |