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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).back to top
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.back to top
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.
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.
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 assure 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.
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.
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 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 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
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