U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590

Skip to content


eSubscribe Envelope

FHWA Home / Safety / Older Users


AAAFTS. American Automobile Association Foundation for Traffic Safety.

AADT. Annual Average Daily Traffic.

AASHTO. American Association of State and Highway Transportation Officials.

Ambient conditions. The visual background or surrounding atmospheric and visibility conditions.

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.

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.

Annual average daily traffic (AADT). The total volume passing a point or segment of a highway facility in both directions for 1 year divided by the number of days in the year.

Apron. The mountable portion of the central island of a roundabout that is adjacent to the circulatory roadway. An apron is generally required on smaller roundabouts to accommodate the wheel tracking of large vehicles.

ASTM. American Society for Testing and Materials.

ATSSA. American Traffic Safety Services Association.

Attraction signing. Information/supplemental signs featuring logos or verbal messages pointing out places to visit or food, gas, and rest stop locations.

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.

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.

Brightness. A term that refers to human perception of luminance. Whereas luminance is a photometrically measured quantity, brightness describes how intense a light source or lighted surface appears to the human eye.

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.

Candela (cd). A measure of luminous intensity.

Central island. The raised area in the center of a roundabout around which traffic circulates.

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.

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.

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.

CIE. Commission Internationale de l'Éclairage (International Commission on Street/ Highway Lighting).

Circulatory roadway. The curved path used by vehicles to travel in a counterclockwise fashion around the central island of a roundabout.

Circulatory roadway width. The width between the outer edge of the circulatory roadway and the central island, not including the width of any apron.

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.

Coefficient of luminous intensity (RI). The ratio of the luminous intensity (I) of a retroreflectometer in the direction of observation to the illuminance E at the retroreflectometer on a plane perpendicular to the direction of the incident light, expressed in candelas per lux.

Coefficient of retroreflected luminance (RL). A measure of retroreflection most often used to describe the retroreflectivity of pavement markings. Coefficient of retroreflected luminance is defined as the coefficient of luminous intensity per unit area.

Coefficient of retroreflection (RA). A measure of retroreflection used more often to refer to the retroreflectivity of highway signs. Coefficient of retroreflection is defined as the ratio of the coefficient of luminous intensity (RI) of a plane retroreflecting surface to its area (A), expressed in candelas per lux per square meter.

Complete interchange lighting (CIL). Includes lighting in the interchange area on both the acceleration and deceleration areas plus the ramps through the terminus.

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.

Concrete safety-shaped barrier (CSSB). A tapered concrete barrier used as a highway divider in narrow medians to prevent vehicle crossovers into oncoming traffic. It is referred to as a Jersey Barrier in some jurisdictions, as its first application was on the New Jersey Turnpike. CSSB's can be either permanent barriers or temporary portable barriers used in work zone applications.

Conspicuity. A measure of the likelihood that a driver will notice a certain target at a given distance against a certain background.

Contrast. See luminance contrast.

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).

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.

Crossbuck. White X-shaped retroreflectorized highway-rail grade crossing sign with the words RAILROAD CROSSING in black lettering, located alongside the highway at the railroad tracks. At multiple track crossings a sign indicating the number of tracks will be on the post of the Crossbuck.

Cutoff. A luminaire light distribution is designed as cutoff when the candlepower per 1,000 lamp lumens does not numerically exceed 25 (2.5 percent) at an angle of 90 degrees above nadir (horizontal); and 100 (10 percent) at a vertical angle of 80 degrees above nadir. This applies to any lateral angle around the luminaire.

Dark adaptation. Adjustment of the eye to low levels of illumination, which results in increased sensitivity to light.

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.

Deflection. The change in trajectory of a vehicle imposed by geometric features of the roadway.

Depth perception. The ability to distinguish the relative distance of objects in visual space, used to interpret their motion over multiple observations.

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.

Diverge steering (DS) 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.

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.

Divided highway. Roadway that is separated by a median.

Downstream. The direction toward which traffic is flowing.

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.

Edgeline visibility. The detection/recognition of painted pavement surface delineation along roadway edges demarcating the outside edge of the travel lane.

Entry width. The width of the entry to a roundabout, where it meets the inscribed circle, measured perpendicularly from the right edge of the entry to the intersection point of the left edge line and the inscribed circle.

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.

Flared approach. The widening of an approach at a roundabout, resulting in multiple lanes at entry to provide additional capacity at the yield line and storage.

FARS. Fatal Analysis Reporting System.

FHWA. Federal Highway Administration.

Footcandle. The English system's unit of illuminance. One footcandle is the illuminance on a surface that is everywhere one foot from a uniform point source of light of one candle and equal to one lumen per square foot. One footcandle equals 10.76 lux.

Footlambert. A unit of luminance equivalent to 1 lumen per square foot.

Full diamond interchange. Interchange with a one-way diagonal- type ramp in each quadrant.

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.

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.

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.

Guard (guide) rail. Protective barrier along a roadway to prevent vehicles from leaving the roadway.

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.

High-mast lighting. Illumination of a large area by means of a cluster of 3 to 12 luminaires which are designed to be mounted in fixed orientation at the top of a high mast (generally 18 to 46+ m [80 to 150+ ft] or higher).

High-spatial-frequency stimulus. A visual target characterized by fine detail.

Highway-rail grade crossing. The general area where a highway and a railroad's right- of-way cross at the same level, including the railroad tracks, highway, and traffic control devices for highway traffic traversing the area.

Horizontal alignment. The linear (tangent) character or specific degree of curvature describing the geometry of a defined section of highway in plain view.

IIHS. Insurance Institute for Highway Safety.

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.

Illumination. The amount of light falling onto a surface.

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.

Inscribed circle diameter. The basic parameter used to define the size of a roundabout, measured between the outer edges of the circulatory roadway. It is the diameter of the largest circle that can be inscribed within the outline of the intersection.

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.

Interchange (grade separation). A system of interconnecting roadways that provides for the movement of traffic between two or more highways on different levels.

Intersecting angle (skew). The angle formed by the intersection of two roadways (other than a 90-degree angle).

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.

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.

ISTEA. Intermodal Surface Transportation Efficiency Act.

TEH. Institute of Transportation Engineers.

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.

Lane control signals. Special overhead signals that permit or prohibit the use of specific lanes of a street or highway or that indicate the impending prohibition of their use.

Leading pedestrian interval (LPI). Also known as, "pedestrian head start," and "delayed vehicle green," an LPI allows pedestrians to begin crossing an intersection a few seconds before the vehicular green interval begins. This allows pedestrians to establish their presence in the crosswalk before the turning vehicles, thereby enhancing the pedestrian right of way.

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.

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).

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.

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.

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.)

Lux. The metric unit of illuminance. One lux is equal to the illuminance corresponding to a luminous flux density of one lumen per square meter.

Measures of effectiveness (MOE). 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 crashes, plus indices of performance such as reaction time. In research studies, the MOE's 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.

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.

Minimum required visibility distance (MRVD). The distance necessary to permit detection and comprehension, plus driver decision-making, response selection, and completion of a vehicle maneuver, if necessary.

Mountable. Geometric features (e.g., curbs) that can be driven upon by vehicles without damage, but not intended to be in the normal path of traffic.

MUTCD. Manual on Uniform Traffic Control Devices for Streets and Highways.

NCHRP. National Cooperative Highway Research Program.

Nearside priority. Priority given to drivers entering the circle of a roundabout.

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-facing boundary of the opposite left-turn lane.

NHTSA. National Highway Traffic Safety Administration.

Non-cutoff. The luminaire light distribution category when there is no candlepower limitation in the zone above maximum candlepower.

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.

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.

Ocular transmittance. The amount of light reaching the retina relative to the amount incident upon the cornea.

Offside priority. Priority given to traffic already in the circle of a roundabout.

Osteoarthritis. A degenerative form of arthritis.

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.

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).

Passive crossing control devices. Non-activated traffic control devices, including signs, pavement markings, and other devices located at or in advance of crossings to indicate the presence of a crossing and the possibility of a train.

Peak intensity. The maximum strength of a traffic signal maintained through a defined viewing angle; measured in candelas.

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.

Pedestrian crosswalk. An extension of a sidewalk across an intersection or across a roadway at a midblock location to accommodate pedestrian movement.

Pedestrian refuge. An at-grade opening within a median island that allows pedestrians to safely wait for an acceptable gap in traffic.

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.

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-facing boundary of the opposite left-turn lane.

Post-mounted delineators (PMD). Retroreflective devices located in a series 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.

Raised. Geometric features (e.g., curbs) with a sharp elevation change that are not intended to be driven upon by vehicles at any time.

Raised pavement markers (RPM). 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.

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.

Retroreflective. Capable of returning light to its source.

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.

Right turn on red (RTOR). Unless otherwise specified by traffic signal control signing, 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.

Roundabouts. Circular intersections with specific design and traffic control features that include yield control of entering traffic, channelized approaches, and appropriate geometric curvature to ensure that travel speeds on the circulating roadway are typically less than 50 km/h (30 mph).

Route marker reassurance assembly. Consists of a cardinal direction marker (i.e., east, west, north, and south) and a route marker.

Saccadic movement. A change in visual fixation from one point to another by means of a quick, abrupt movement of the eye.

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.

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.

Senile miosis. An aging characteristic involving an excessive smallness or contraction of the pupil of the eye.

Short range delineation. Delineation that is useful to the driver for tracking the roadway at night under poor visibility conditions.

Sight distance. The length of highway visible to the driver.

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.

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.

Slip ramp. A diagonal ramp, more properly called a cross connection, which connects with a parallel frontage road.

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 crash risk.

Speed-change lane (SCL). Used in interchange/ramp exit models, it refers to the speed- change maneuver on deceleration lanes segmented components.

Splitter island. A raised or painted area on an approach to a roundabout used to separate entering from exiting traffic, deflect and slow entering traffic, and provide storage space for pedestrians crossing the road in two stages. It is also referred to as a median island or separator island.

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.

Stereopsis. Binocular visual perception of three-dimensional space based on retinal disparity.

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.

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.

Threshold contrast. The minimum difference in luminance of a target and luminance of that target's background at which the target is visible. Also defined as the luminance contrast detectable during some specific fraction of the times it is presented to an observer, usually 50 percent.

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.

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 ensure correct driver interpretation and response.

Transient adaptation factor. A reduction in target contrast caused by the process of transient visual adaptation.

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.

TRB. Transportation Research Board.

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.

Two-quadrant cloverleaf interchange. A type of partial cloverleaf where most traffic leaving one highway turns to the same leg of the intersecting highway.

TWLTL. Two-way, left-turn lane.

Two-way stop-control. Stop signs are present on the approaches of the minor street and drivers on the minor street (or a driver turning left from the major street) must wait for a gap in the major-street traffic to complete a maneuver.

Upstream. The direction from which traffic is flowing.

Useful field of view. Also known as the "functional field of view", or "attentional window," this area refers to a subset of the total field of view where stimuli can not only be detected, but can be recognized and understood sufficiently to permit a timely driver response. As such, this term represents an aspect of visual information processing, rather than a measure of visual sensitivity.

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.

Vertical curve. The parabolic curve connecting the two approach grades on either side of a hill.

Visual accommodation. The process by which the eye changes focus from one distance to another

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).

Visual adaptation. The process by which the retina becomes accustomed to more or less light than it was exposed to during an immediately preceding period. It results in a change in the sensitivity of the eye to light.

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.

Yield line. A pavement marking used to mark the point of entry from an approach into the circulatory roadway of a roundabout, and is generally marked along the inscribed circle. Entering vehicles must yield to any circulating traffic coming from the left before crossing this line into the circulating roadway.

Zebra crossing. A crossing marked by transverse white stripes where vehicles are required to yield to pedestrians.

Page last modified on December 29, 2015
Safe Roads for a Safer Future - Investment in roadway safety saves lives
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000