Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 391
Appendix E
Glossary
The terminology required for a comprehensive discussion of the rela-
tionship between vehicle speed and safety has specific technical con-
notations that may differ from the meanings of these words in the
vernacular. This glossary describes several terms associated with vehi-
cle speeds on streets and highways, and with highway and traffic
engineering. Speed parameters customarily expressed in miles per
hour (mph) are cited in these units in this glossary (1 mph = 1.609
km/h).
10-mph Pace
The 10-mph pace is the 10-mph range encompassing the greatest
percentage of all the measured speeds in a spot speed study. It is
described by the speed value at the lower end of the range and the
percentage of all vehicles that are within the range; as such, it is an
alternative indicator of speed dispersion. Most engineers believe that
391
OCR for page 392
MANAGING SPEED
392
safety is enhanced when the 10-mph pace includes a large percentage
(more than 70 percent) of all the free-flowing vehicles at a location.
(Note: 10 mph = 16 km/h.)
85th Percentile Speed
The 85th percentile
speed is the speed at or
below which 85 percent
of the free-flowing vehi-
cles travel. Traffic engi-
neers have assumed that
this high percentage of
drivers will select a safe
speed on the basis of the Figure E-1 Eighty-fifth percentile speed.
conditions at the site.
The 85th percentile speed has traditionally been considered in an
engineering study to establish a speed limit. The 85th percentile
speed for a normal distribution is shown in Figure E-1. In most
cases, the difference between the 85th percentile speed and the aver-
age speed provides a good approximation of the speed sample's stan-
dard deviation.
Advisory Speed
At certain locations on the highway sys-
tem, such as horizontal curves, intersec-
tions, or steep downgrades, the safe speed
on the roadway may be less than the
posted speed limit. Rather than lowering
the regulatory speed limits at each of these
locations, traffic engineers often place
standard warning signs accompanied by a
square black-and-yellow advisory speed
Figure E-2 Advisory
plate as shown in Figure E-2. Although
speed plate.
this sign provides a warning to approach-
ing drivers, it is not legally enforceable.
OCR for page 393
393
Glossary
Arterial
Arterials provide the high-speed, high-volume network for travel
between major points in rural areas. They generally have minimum
design speeds of at least 37 mph (60 km/h). Most intersections are at
grade (i.e., at the same level), and access to abutting property is per-
mitted but controlled. Utilities are usually permitted within the
right-of-way. All rural arterials, including freeways, constitute about
9 percent of the rural highway length in the United States and carry
64 percent of the rural vehicle miles of travel.
The principal purpose of urban arterials is to provide mobility.
Design speeds may be as low as 31 to 37 mph (50 to 60 km/h), but
higher speeds are common, particularly for principal arterials. In
developed areas, principal arterials are often spaced at intervals of 0.6
to 1.2 mi (1 to 2 km). Principal arterials, including freeways, account
for 9 percent of the urban street length and carry 58 percent of all
urban travel.
Average Speed
The average (or mean) speed is the most common measure of central
tendency. Using data from a spot speed study, the average is calculated
by summing all the measured speeds and dividing by the sample size, n.
Basic Speed Law
The Uniform Vehicle Code (National Committee on Uniform
Traffic Laws and Ordinances 1992) and most state motor vehicle
laws include a basic speed law with wording similar to the following:
No person shall drive a vehicle at a speed greater than is reasonable
and prudent under the conditions and having regard for the weather,
visibility, traffic, and the surface and width of the roadway.
Braking Distance
Braking distance, assumed for design purposes to be on a wet pave-
ment surface, is the distance required to stop a vehicle from the
OCR for page 394
MANAGING SPEED
394
instant brake application begins. The minimum braking distance
for a vehicle on a level roadway increases with the square of the
speed:
V2
b
254f
where
b braking distance (m),
V initial speed (km/h), and
f coefficient of friction between tires and roadway.
The dashed line in Figure
E-3 shows braking distance as a
function of a vehicle's initial
speed. The solid line shows the
total stopping distance.
Figure E-3 Design values for braking
and stopping distance. (Note: 1 m =
3.28 ft and 1 km/h = 0.62 mph.)
Business District
For the purpose of establishing statutory speed limits, the Uniform
Vehicle Code (National Committee on Uniform Traffic Laws
and Ordinances 1992) defines a business district as the territory con-
tiguous to and including any highway when within any 180 m
along such highway there are buildings in use for business or
industrial purposes, including but not limited to hotels, banks,
or office buildings that occupy at least 90 m of frontage on one
side or 90 m collectively on both sides of the highway. (Note: 1 m =
3.28 ft.)
OCR for page 395
395
Glossary
Collector Roads and Streets
Collector roads and streets collect vehicles from local roads and abut-
ting properties and route them to arterials. Traffic volumes are rela-
tively low and design speeds may be as low as 31 mph (50 km/h).
Collectors have all intersections at grade and little access control.
They may also have pedestrians and parked vehicles. Collectors rep-
resent 23 percent of the rural highway length and carry 25 percent of
the rural vehicle miles of travel.
Collector streets in urban areas have design speeds of 31 mph (50
km/h) or greater. Their function is divided equally between mobility
and access. Collectors are more likely than minor arterials to accom-
modate parking, pedestrians, bicycles, and local buses. Collectors and
minor arterials account for 21 percent of urban street length and
carry 28 percent of all urban travel.
Compliance with Speed Regulations
There is no commonly accepted definition of compliance with speed
regulations. Motorists traveling less than the posted speed limit
might appear to be in compliance, but under certain weather, visibil-
ity, or traffic conditions, they may be violating the basic speed law. In
the more general case of free-flowing vehicles under favorable envi-
ronmental conditions, measures of compliance (actually, noncompli-
ance) include the percentage of vehicles exceeding the posted limit by
6 or 9 mph (10 or 15 km/h), or the percentage of vehicles exceeding
the roadway's design speed.
Costs of Motor Vehicle Crashes
In highway safety analyses, it is often necessary to assign costs to
traffic crashes. For example, the National Safety Council (NSC) rec-
ommends economic costs for crashes on the basis of productivity lost
and expenses incurred because of collisions. NSC also estimated
comprehensive costs for crashes, which included economic costs and
a measure of the value of lost quality of life associated with deaths
OCR for page 396
MANAGING SPEED
396
and injuries. The Federal Highway Administration (FHWA) has also
suggested collision costs based on two different injury scales: (a) the
KABC scale, with four injury levels ranging from Killed to Possible
Injury; and (b) the Abbreviated Injury Scale, with six injury levels
ranging from Killed to Minor. Table E-1 compares the costs recom-
mended by NSC (1996) and FHWA ( Judycki 1994).
Table E-1 National Safety Council and FHWA Traffic Crash Costs
Type of Accident Cost ($)
Abbreviated
Type of Injury Economic Comprehensive KABC Scale Injury Scale
Fatal 790,000 2,790,000 2,600,000 2,600,000
Critical 1,980,000
Severe 490,000
Incapacitating 41,200 138,000 189,000
Serious 150,000
Evident 13,900 35,700 36,000
Moderate 40,000
Possible 7,900 17,000 19,000
Minor 5,000
6,000 a 1,700 a
No injury-- 2,000
property damage
only
a NSC economic costs include minor injuries whereas comprehensive costs exclude
all injuries.
Crash Probability
In typical use, crash probability refers to the long-term likelihood
that a driver will be involved in a crash under a specified set of con-
ditions (e.g., on a given trip, during the coming year). Estimates of
national crash experience can be used to calculate average crash prob-
abilities. However, crash probability is known to vary with driver
characteristics, vehicle type, roadway features, and environmental
factors, so the crash probability for an individual motorist may be
substantially more or less than the average.
OCR for page 397
397
Glossary
Crash Severity
A fatal crash is a crash that results in one or more deaths within 30
days of the crash. A nonfatal injury crash is a crash in which at least
one person is injured, but no injury results in death. A property-dam-
age-only (PDO) crash is a collision that results in property damage,
but in which no person is injured.
Cross Section
The roadway cross section consists of those geometric features per-
pendicular to the direction of travel. Common cross-section elements
include the following:
· Number of lanes--determined by the projected traffic volume
for a facility.
· Lane width--must be sufficient to accommodate the design
vehicle, allow for imprecise steering maneuvers, and provide clear-
ance for traffic flow in adjacent lanes. It is dependent on the design
vehicle, design speed, volume, the presence or absence of shoulders,
horizontal alignment, and the presence of oncoming traffic.
· Cross slope--promotes drainage of surface water.
· Shoulders--used for emergency stopping and for lateral support
of base and surface courses.
· Medians--used to separate opposing directions of traffic on
multilane highways.
· Marginal elements--curbs, gutters, sidewalks, roadside slopes,
and barriers.
Design Driver
A roadway's design must be compatible with drivers' capabilities and
limitations. The design driver embodies those specific human char-
acteristics that should be recognized in designing and operating the
road. It is inappropriate to design for the median driver because this
would potentially put half the drivers at risk. On the other hand, it is
OCR for page 398
MANAGING SPEED
398
probably not realistic to design for the 99th percentile value of every
human characteristic. Although the American Association of State
Highway and Transportation Officials (AASHTO) does not provide
an explicit description of the design driver, the following elements
certainly should be included:
· Familiarity: The designer should assume that motorists are driv-
ing on a roadway for the first time and that they have no familiarity
with its features.
· Driver age: Certain human performance characteristics deterio-
rate with age. Persons over the age of 65 constitute an increasing por-
tion of the driving population, and their special needs must be
considered in highway design.
· Vision: States specify a level of visual acuity (typically 20/30 cor-
rected) that drivers must satisfy to retain their license. Designers
must not only consider this requirement for their state, but also rec-
ognize that drivers from other jurisdictions with potentially inferior
visual acuity standards will be using their roads. Most states do not
test drivers for nighttime vision; nevertheless, the significant amount
of travel during the hours of darkness suggests that designers should
consider this factor.
· Eye height: The height of a driver's eye above the pavement
affects the length of road ahead that a driver can see; eye height is a
function of both the human and the vehicle. AASHTO's recom-
mended value (AASHTO 1994) of 1070 mm corresponds to the 7th
percentile driver in a passenger car.
· Impairment: Motorists may become impaired by fatigue, med-
ication, alcohol, and drugs. These imperfections, at least to the extent
that they are legal (e.g., a blood alcohol content below 0.08), should
be recognized by the designer. As a consequence, engineers must
design for the prudent, rather than the perfect, driver.
Design Speed
AASHTO defines a roadway's design speed as "the maximum safe
speed that can be maintained over a specified section of highway
when conditions are so favorable that the design features of the high-
OCR for page 399
399
Glossary
way govern" (AASHTO 1994). This is the maximum speed prudent
drivers would choose when environmental conditions are very good
and traffic volumes are light. Subject to the constraints of environ-
mental quality, economics, aesthetics, and social impacts, AASHTO
recommends higher design speeds to promote safety, mobility, and
efficiency. Certain highway design features, including curvature,
sight distance, and roadside elements, are highly sensitive to the
choice of design speed; others, including lane and shoulder widths,
do not change appreciably with design speed. In planning a roadway,
the engineer initially selects a design speed; that decision, in turn,
establishes upper or lower bounds on the facility's geometric design
parameters. This is the principal use of design speed. On a rural,
level, straight roadway with no access points and obstacle-free road-
sides, the concept of design speed is not meaningful.
Drivers exceeding the design speed by a small amount under
favorable conditions will not necessarily have a crash, principally
because AASHTO incorporates safety factors into its design recom-
mendations. For example, the stopping sight distance model assumes
a very conservative perception-reaction time and a wet roadway sur-
face; an alert driver can react quicker and a vehicle on a dry roadway
can decelerate to a stop in a much shorter distance than the design
value. Likewise, an attentive motorist can exceed a horizontal curve's
design speed without running off the roadway.
Higher design speeds enhance safety, principally by accommodat-
ing minor driver errors and providing greater opportunities for crash
avoidance. AASHTO strongly recommends consistency in design
speed along a roadway section to avoid misleading motorists.
Although it appears reasonable that the posted speed limit should not
exceed a highway's design speed, the existing roadway system includes
countless horizontal curves with safe speeds below the design speed or
posted speed limit; these situations are routinely handled with curve
warning signs and advisory speed plates (see Figure E-2).
Engineering Study
The Uniform Vehicle Code (National Committee on Uniform
Traffic Laws and Ordinances 1992) and state motor vehicle laws
OCR for page 400
MANAGING SPEED
400
authorize state and local highway agencies to determine whether the
statutory speed limit on a section of road is greater or less than is
reasonable under the conditions that exist at the location. This
determination must be based on an engineering study, which
requires data collection and analysis in the determination of an
appropriate limit. The data considered would typically include the
following factors:
· Area--rural, suburban, or urban;
· Results of a spot speed study, principally the 85th percentile and
10-mph (16-km/h) pace speeds;
· Crash experience, with particular attention to speed-related
crashes;
· Traffic volume and composition (i.e., types of vehicles);
· Existing traffic controls (regulatory and warning);
· Design features, including horizontal and vertical alignment,
sight distance, and lane width;
· Pavement surface condition;
· Parking;
· Presence and usage of driveways;
· Roadside hazards;
· Pedestrians and bicycles;
· Speed limits on adjacent roadway sections; and
· Existing level of speed enforcement.
Typically, the speed data--particularly the 85th percentile speed--
provide the first approximation of the speed zone limit. The limit
may be adjusted from this value on the basis of the other factors.
Externalities
"Externalities" refers to the risks imposed on others not taken into
account by an individual's decision. In the case of speed choice, the
term refers to the risks imposed on other road users (e.g., other driv-
ers and vehicle occupants, pedestrians, bicyclists) by an individual
driver's selection of a driving speed. For example, a driver's decision
to accept a higher risk of death or injury in exchange for a shorter trip
OCR for page 401
401
Glossary
time almost certainly increases the risk for other road users.
Externalities are one of the primary reasons for regulating speed.
Fatality Rates
There are four common methods of calculating fatality rates:
· Travel-based fatality rate--fatalities per 100 million vehicle-mi
of travel (100 mvm). In 1996, the United States had a travel-based
fatality rate of 1.7 fatalities per 100 mvm. This rate is commonly used
in the highway engineering community. (Note: 100 million vehicle-
mi = 161 million vehicle-km.)
· Registered vehicle fatality rate--fatalities per 100,000 registered
vehicles. In 1996, the United States had a registered vehicle death
rate of 20.8 fatalities per 100,000 registered vehicles.
· Population fatality rate--fatalities per 100,000 population. In
1996, the United States had a population death rate of 15.8 fatalities
per 100,000 people. This method of normalizing fatalities is com-
monly used by the health profession for infection and mortality rates.
· Driver fatality rate--fatalities per 100,000 licensed drivers. In
1996, the United States had a driver fatality rate of 23.3 fatalities per
100,000 licensed drivers.
Free Flow
A free-flowing vehicle is one whose driver has the ability to choose a
speed of travel without undue influence from other traffic, conspicu-
ous police presence, or environmental factors. In other words, the
driver of a free-flowing vehicle chooses a speed that he or she finds
comfortable on the basis of the appearance of the road.
In conducting a spot speed study, the field observer detects and
records the speed of free-flowing vehicles. Vehicles operating under
the following conditions are not free flowing and must be excluded
from the sample:
· Two vehicles in the same lane have a headway (time from the front
of one vehicle to the front of the following vehicle) of less than 4 s.
OCR for page 408
MANAGING SPEED
408
Sight Distance
Sight distance is the length of roadway ahead visible to the driver.
AASHTO design standards discuss four types of sight
distancedecision, intersection, passing, and stopping (AASHTO
1994).
Sight Distance, Decision
Decision sight distance Table E-2 Rural Decision Sight Distances
is the length of road- Decision Sight Distance (m)
way required for a driv- Speed (km/h) Stop Path Change
er to detect an 50 75 145
unexpected hazard in 60 95 175
70 125 200
the environment, rec-
80 155 230
ognize the hazard,
90 185 275
select an appropriate
100 225 315
speed and path, and
110 265 335
initiate and complete 120 305 375
the required maneuver Note: 1 m = 3.28 ft and 1 km/h = 0.62 mph.
safely and efficiently.
In contrast to stopping sight distance, this model assumes that the
driver will not simply slam on the brakes but rather will assess the sit-
uation, make an informed decision, and implement the action with-
out interfering with other traffic. Table E-2 indicates decision sight
distances on rural highways where the expected maneuvers are a con-
trolled stop and a speed or path change.
Sight Distance, Intersection
AASHTO identifies several intersection sight distance criteria that
must be considered by the designer (AASHTO 1994). At the risk of
oversimplification, intersections on high-speed rural highways must
provide sufficient sight distance for motorists under the following
conditions:
OCR for page 409
409
Glossary
· A driver approaching an intersection controlled by a Yield or
Stop sign or a traffic signal must have sufficient distance to see and
react to the traffic control.
· Drivers stopped at a Yield or Stop sign and preparing to cross or
turn onto a through highway must be able to see a sufficient distance
to make their maneuver with safety and without significantly inter-
fering with motorists on the through road.
· Drivers on the major roadway intending to turn left onto a cross
street must have adequate sight distance to make their maneuver with
safety.
AASHTO prescribes numerical values for these and other situations
at intersections; in all cases, the required sight distances increase with
the speeds of traffic approaching the intersection on the controlled
approaches and on the through highway. Many jurisdictions specify
intersection sight distances that are less stringent than those recom-
mended by AASHTO.
Sight Distance, Passing
Passing sight distance is the length of roadway that a motorist must
be able to see ahead in order to safely complete a passing maneuver
on a two-lane highway. The AASHTO model for passing sight dis-
tance design assumes that the passing maneuver, once initiated, will
be completed (AASHTO 1994). The passing sight distance model
uses a driver eye height of 1070 mm and a height for the opposing
vehicle of 1300 mm. The model also makes assumptions about the
relative speeds of the passing vehicle, the passed vehicle, and an
oncoming vehicle. AASHTO's assumptions for design purposes are
fairly conservative and result in long distances. By contrast, passing
sight distances for operational purposes assume that a partially com-
pleted passing maneuver may be aborted if an opposing vehicle
comes into view while the passing vehicle is in the left lane.
This assumption shortens the necessary sight distance considerably.
Values from the operational analysis are used by traffic engineers in
OCR for page 410
MANAGING SPEED
410
establishing the loca- Table E-3 Passing Sight Distances
tion and length of Minimum Sight Distance (m)
marked no-passing Speed (km/h) Design Operation
zones. Table E-3 com- 50 345 150
pares the passing sight 60 407 170
70 482 200
distances for design
80 541 240
and operational pur-
90 605 280
poses.
100 670 320
110 728 360
120 792
Note: 1 m = 3.28 ft and 1 km/h = 0.62 mph.
Sight Distance, Stopping
Stopping sight distance is the minimum distance for a vehicle travel-
ing at or near a highway's design speed on wet pavement to come to
a complete stop before reaching a stationary object (150 mm high) in
its path (AASHTO 1994). Adequate stopping sight distance, which
should be provided at every point along all roads, consists of two
components--the motorist's perception-reaction distance and the
vehicle's braking distance. Stopping sight distance may be calculated
using the following formula:
V2
d 0.278tV
254f
where
d minimum stopping sight distance (m);
t perception-reaction time, assumed to be 2.5 s;
V initial speed (km/h); and
f coefficient of friction between tires and roadway.
The solid line in Figure E-3 shows the relationship between stopping
sight distance and highway design speed. The difference between the
stopping and braking distances is the length of highway traveled dur-
ing the perception-reaction time.
OCR for page 411
411
Glossary
Speed Change Lanes
Speed change lanes include acceleration and deceleration lanes,
which are used in conjunction with interchange ramps to permit
entering vehicles to attain the speed of the through traffic and exit-
ing vehicles to decelerate outside of the through-traffic lanes.
Speed Dispersion
The speeds of individual vehicles on a street or highway vary, often in
the manner suggested by Figure E-1. Speed dispersion refers to this
spread in vehicle speeds. Speed dispersion can be quantified in various
ways including the standard deviation, variance, 10-mph pace, or range
(high minus low). There is general agreement that the safest conditions
occur when all vehicles at a site are traveling at about the same speed.
Speed Limit, Absolute
An absolute speed limit specifies a numerical value, the exceeding of
which is always in violation of the law, regardless of the conditions or
hazards involved. Many enforcement officers prefer absolute speed
limits because they reduce the incidence of challenged citations.
However, absolute speed limits lack flexibility, particularly in those
situations where traffic conditions vary widely. Approximately two-
thirds of the states have absolute speed limits. Prima facie speed lim-
its are the alternative to absolute limits.
Speed Limit, Differential
The motor vehicle codes in some states prescribe different speed lim-
its for different classes of vehicles. For example, the maximum speed
limit on a rural section of Interstate might be 75 mph (121 km/h) for
cars, pickup trucks, and vans, but 65 mph (105 km/h) for large trucks.
The primary rationale for this type of regulation is that large trucks
have much longer stopping distance than cars. In the absence of dif-
ferential speed limits, studies have found that large trucks travel 1 to
2 mph (2 to 3 km/h) slower than cars on level sections of rural
OCR for page 412
MANAGING SPEED
412
Interstate. This value may double when differential speed limits are
introduced, but the actual difference between car and truck speeds
rarely approaches the difference cited in the code.
Speed Limit, Posted
The posted speed limit is the value conveyed
to the motorist on a black-on-white regula-
tory sign such as the one shown in Figure E-
8. Standard engineering practice is to post
speed limits for freeways, arterials, and any
roadway or street where speed zoning has
altered the limit from the statutory value.
They are also used at any point where the
speed limit changes, including points beyond
Figure E-8 Speed
major rural intersections where traffic may
limit sign.
change from one road to another.
Speed Limit, Prima Facie
A prima facie speed limit is one above which drivers are presumed to
be driving unlawfully. Nevertheless, if charged with a violation, driv-
ers have the opportunity to demonstrate in court that their speed was
safe for conditions at the time and not in violation of the basic speed
limit, even though they may have exceeded the numerical limit.
Approximately one-third of the states have prima facie speed limits
or limits of each type (i.e., prima facie and absolute). Absolute speed
limits are the alternative to prima facie limits.
Speed Limit, Statutory
State motor vehicle laws specify numerical values for speed limits on
specific categories of streets and highways. For example, a code might
limit speeds to 25 mph (40 km/h) in residential areas, 30 mph (48
km/h) in business districts, and 55 mph (89 km/h) on all other roads.
Unless otherwise prohibited by law, these limits may be altered on the
basis of an engineering study.
OCR for page 413
413
Glossary
Speed Limit, Variable
The typical speed zoning process establishes a limit that is posted and
enforceable 24 h/d. In reality, streets and highways experience conditions
of traffic, weather, and incidents when lower limits would be appropri-
ate. In some cases, the conditions will be such that motorists could not
possibly travel at the posted speed limit. On the other hand, an urban
speed limit established in part because of daytime pedestrian traffic may
be unrealistically low for conditions at night. One method of addressing
these types of situations is through the use of variable speed limits.
An urban freeway variable speed limit system would operate in the
following manner. Detectors would monitor the actual volume, speed,
and density of traffic in sections of the freeway. This information
would be used to determine where congestion is causing traffic to
slow. In advance of these locations, electronic speed limit signs (simi-
lar to Figure E-8, but with changeable numbers) would be remotely
controlled to alter the posted speed limit. Motorists who comply with
these regulations would decrease their speed and not approach the end
of a stopped or slow-moving traffic queue at normal freeway speeds.
Speed Parameters
Field data from spot speed studies of free-flowing vehicles (see
Figure E-9) are processed to determine typical data parameters of
central tendency (average or median) and dispersion (standard devi-
ation, variance, 10-mph pace, and range).
Speed Standard Deviation
The standard deviation, which has the units of speed (km/h), is the
positive square root of the speed variance. Speed standard deviations
are often 3.7 to 4.3 mph (6 to 7 km/h) on urban streets and 5.6 to 6.8
mph (9 to 11 km/h) on freeways. The standard deviation's value is
strongly influenced by a few vehicles traveling at very high or very low
speeds; elimination of these vehicles will reduce the standard deviation.
The standard deviation is readily calculated from a sample of speed
measurements such as those shown in Figure E-9. It may be roughly
OCR for page 414
MANAGING SPEED
414
approximated by the speed range (largest observed speed minus the
smallest) divided by 6. The standard deviation may also be estimated
as the difference between the 85th percentile and average speeds.
Figure E-9 Sample speed data collection form.
Speed Variance
Speed variance for a spot speed study is calculated by summing the
squares of the differences between each measured speed and the aver-
age speed, and dividing the total by the sample size minus one (n 1).
The variance, which is the square of the standard deviation, thus has
OCR for page 415
415
Glossary
units of speed squared (km2/h2). Speed variance has little practical value
and is rarely cited as an output value from a spot speed study. The vari-
ance's principal application is in determining the standard deviation.
The technical literature includes studies in which analysts relied on
selected speed parameters, rather than having the original data such as
that shown in Figure E-9. Using speed study results that report only the
average and the 85th percentile speeds, these analysts have attempted to
quantify speed dispersion by calculating the numerical difference
between these two values. Although this difference usually provides a
good approximation of the speed sample's standard deviation, these
analysts have unfortunately and incorrectly labeled this result as "speed
variance." In reality, it is an estimate of standard deviation.
Speed Zone
Speed zoning is the process of establishing a reasonable and safe
speed limit for a section of roadway where the statutory speed limits
given in the motor vehicle laws [e.g., 30 mph (48 km/h) in business
districts] do not fit the road or traffic conditions at a specific loca-
tion. The limits may be altered on the basis of an engineering study.
To be enforceable, the new limits must be posted along the roadway
using a standard regulatory sign such as the one shown in Figure
E-8. In addition, speed limits that are increased or decreased as a
result of the speed zoning process must be recorded in documents
maintained by an appropriate agency (e.g., state supreme court
library). Speed zones should be periodically restudied.
The basic principles of speed zoning should also be applied to special
situations such as school crossings and roadway construction areas. In
addition, they may be used to establish minimum speed limits for freeways.
Spot Speed Study
Engineers conduct spot speed studies by measuring and recording the
speeds of a sample of free-flowing vehicles as they pass a point on a street
or highway. The measurements are usually made with a hand-held radar
or laser speed meter. The field data are typically recorded on a data form
similar to the one shown in Figure E-9. This study is an essential ele-
OCR for page 416
MANAGING SPEED
416
ment in the more comprehensive engineering study required for speed
zoning. Unless there is an interest in other conditions, a spot speed study
is normally conducted on a straight, level road during daylight, off-peak
hours. Speed data are collected separately by direction. Minimum sam-
ple sizes of at least 100 vehicles are necessary to properly represent the
speed characteristics of the traffic at the study site.
Traffic Calming
Traffic calming is a term used to identify various engineering tech-
niques to physically control vehicle speeds and/or volumes on local
streets. The techniques, which include speed humps, traffic diverters,
narrow roadways, and staggered alignment, are deployed in response
to complaints by adjacent property owners of speeding traffic or
excessive traffic volumes. Although these techniques have been found
effective on local streets, they must be planned and implemented
carefully to ensure that the original problems are not simply moved
to another local street.
Vehicle Alignment
A roadway's vertical alignment consists of grades, where the elevation
changes at a fixed rate per unit distance along the highway, and ver-
tical curves, where the highway grade increases or decreases. These
features are portrayed in Figure E-10. As indicated in Table E-4,
AASHTO recommends maximum grades for rural highways as a
function of highway classification and type of terrain (AASHTO
1994). Maximum grades on urban freeways are identical to those for
rural freeways, but grades steeper than those given in Table E-4 are
permitted on urban arterial,
collector, and local streets.
Minimum lengths of crest ver-
tical curves are a function of
the approach and departure Figure E-10 Vertical alignment
grades as well as the stopping features.
sight distance for the road-
way's design speed.
OCR for page 417
417
Glossary
Table E-4 Maximum Vertical Grades for Rural Roads
Maximum Grade (%)
Terrain Freeway Arterial Collector Local
Level 34 35 47 58
Rolling 45 46 510 611
Mountainous 56 58 612 1016
Vehicle Miles of Travel
The total amount of travel on a roadway segment or on an entire
roadway system is typically expressed in vehicle miles of travel
(VMT). The numerical value may be obtained by multiplying the
length of a section (in miles) by average traffic volume (vehicles per
day), summing these values for all sections of interest, and expanding
the results to an annual value. VMT is commonly used to character-
ize the amount of travel on different classes of roadway and as a nor-
malizing factor in calculating crash or fatality rates.
REFERENCES
ABBREVIATIONS
AASHTO American Association of State Highway and Transportation
Officials
NSC National Safety Council
AASHTO.1994. A Policy on Geometric Design of Highways and Streets. Washington,
D.C.
Chowdhury, M.A., D.L.Warren, H. Bissell, and S. Taori. 1998. Are the Criteria for
Setting Advisory Speeds on Curves Still Relevant? ITE Journal, Vol. 68, No. 2,
Feb., pp. 3245.
Judycki, D. 1994. Motor Vehicle Accident Costs. HHS-10. Federal Highway
Administration, U.S. Department of Transportation.
Messer, C., J. Mounce, and R. Brackett. 1981. Highway Geometric Design Consistency
Related to Driver Expectancy, Volume 2. Report FHWA-RD-81-036. Federal
Highway Administration, U.S. Department of Transportation.
National Committee on Uniform Traffic Laws and Ordinances. 1992. Uniform Vehicle
Code and Model Traffic Ordinance.
National Safety Council. 1996. Accident Facts.
OCR for page 418
Representative terms from entire chapter:
speed limit
