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C H A P T E R 24
Glossary
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.
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.
Candela--The International System of Units (SI) base unit of luminous intensity.
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).
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.
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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.
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, bi-
cycles, or pedestrians. In the context of road signs, it represents how easy it is to distinguish a
sign from the surrounding visual environment.
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.
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.
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 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.
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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.
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.
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.
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).
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.
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.
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. 5859).
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.
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.
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HFG GLOSSARY Version 1.0
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.
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.
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 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-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 vis-
ible 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 situa-
tions, 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.
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.
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 intersections
with yield control at entry, permitting a vehicle on the circulatory roadway to proceed, and
deflecting the approaching vehicle counter-clockwise around a central island (FHWA, 2009).
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.
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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, recognition, 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.
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
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entities. 28 CFR 35.151 New construction and alterations is available at http://www.ed.gov/policy/
rights/reg/ocr/edlite-28cfr35.html#S151.
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).
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).
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").
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