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6
Guidance on Setting and
Enforcing Speed Limits
In this chapter the committee offers its guidance on appropriate
methods of setting and enforcing speed limits. Speed limits represent
trade-offs between risk and travel time for a road class or specific
highway section that reflect the decision makers' attempt to achieve
an appropriate balance between the societal goals of safety and
mobility. The process of setting speed limits is often viewed as a
technical exercise, but the decision involves value judgments and
trade-offs that are frequently handled through the political process in
state legislatures and city councils. Thus, the guidance offered here is
directed toward a broad audience of those involved in decisions about
setting and enforcing speed limits: state and local legislators, traffic
engineers, law enforcement and judicial officials, and the interested
general public.
The guidance attempts to be as specific as possible, recognizing
that decision makers are looking for practical advice. However, it
stops short of recommending numeric speed limits. Road conditions
vary too widely to justify a "one-size-fits-all" approach. There is no
186
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187
Guidance on Setting and Enforcing Speed Limits
single "right" answer in setting appropriate speed limits because pol-
icy makers in different communities may legitimately disagree on the
priority given to the factors--safety, travel time, enforcement expen-
ditures, community concerns--that affect decisions about speed lim-
its. Moreover, the available studies and data fall short of providing
decision makers with an adequate basis on which to quantify with
much precision the effects of changes in speed limits on such critical
factors as driving speeds, safety, and travel time by road class.
Technical input on how these factors should be weighed in different
situations, however, can help guide the decision. Thus, the focus here
is to identify the critical decision factors, elaborate what is known
and what is not known about their importance by road class, and
comment on the decision-making process itself. The primary focus is
on the effects of speed limit policies on safety, rather than on travel
time, energy consumption, or environmental pollution.
The chapter is organized as a series of questions and answers. It
begins with a general discussion of the rationale, purpose, and current
methods of setting speed limits both for broad road classes (legislated
limits) and specific road sections (speed zones). The decision process
for determining speed limits is then discussed. Next, a roadway clas-
sification scheme is offered, which distinguishes seven different road
classes and a category for special zones (i.e., school and work zones);
advice is provided on the key factors for consideration in determin-
ing appropriate speed limits for each category. The role and limits of
current enforcement and adjudication methods in regulating driving
speeds are then discussed. Finally, the potential for technology to
improve methods of determining and enforcing speed limits is con-
sidered. Key points are highlighted in bold in the text that follows.
WHY REGULATE DRIVING SPEEDS?
The argument can be made that most motorists drive in a reasonable
and prudent manner, selecting their driving speeds so as to arrive at
their destinations safely. If this is so, why not leave it up to the indi-
vidual driver to determine an appropriate speed? There are three
principal reasons for regulating drivers' individual speed choices:
(a) externalities, that is, the risks and uncompensated costs imposed
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MANAGING SPEED
188
on others because of individual driver choices about appropriate driv-
ing speeds; (b) inadequate information that limits a motorist's ability
to determine an appropriate driving speed; and (c) driver misjudg-
ment of the effects of speed on crash probability and severity.
The first reason derives from differences in drivers' risk tolerances.
For example, in selecting an appropriate driving speed, an older driv-
er is apt to assign more weight to the risks of mortality and injury to
himself and others than to travel time; other drivers might assign
greater weight to travel time. Even the same driver may make differ-
ent trade-offs between travel time and safety depending on trip
urgency, trip length, and familiarity with the road. The problem
arises because individual drivers' decisions about speed may be made
without adequate consideration of the effect of their choices on the
safety of other road users. Even a driver traveling alone who is
involved in a single-vehicle crash may impose medical and property
damage costs on society that are not fully reimbursed by the driver.
The potential costs imposed on others is a primary reason for regu-
lating speed.
The second reason for regulating speed is the inability of drivers to
judge vehicle capabilities (e.g., stopping, handling) and to adequately
anticipate roadway geometry and roadside conditions to determine
appropriate driving speeds. Drivers are generally able to modify their
driving speeds appropriately as traffic volumes increase and weather
conditions deteriorate. Unless they are familiar with conditions, how-
ever, they may not be as aware of appropriate speeds on roads with
poor geometrics or high levels of roadside activity.
A final reason for regulating speed, which is related to the issues
of information adequacy and judgment, is the tendency of some driv-
ers to underestimate or misjudge the effects of speed on crash prob-
ability and severity. This problem is often manifested by young and
inexperienced drivers and may be a problem for other drivers.
The effects of externalities and the availability of information
enabling motorists to anticipate conditions and select appropriate
driving speeds differ significantly by road class. For example, the
risks imposed on others by individual driver speed choices are likely
to be relatively small on rural Interstate highways where free-flowing
traffic creates fewer opportunities for conflict with other road users.
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189
Guidance on Setting and Enforcing Speed Limits
In addition, under normal conditions drivers tend to have adequate
information to determine appropriate driving speeds, because these
highways are usually built to the highest design standards, access is
limited, and roadside activity is minimal.1 In contrast, the risks
imposed on others by individual driver speed choices may be large on
urban arterials where traffic volumes are high for extended periods of
the day, roadside activities are numerous, and potential for conflict
with entering vehicles and with vulnerable bicyclists and pedestrians
is great. These differences are important factors for consideration in
setting appropriate speed limits on different types of roads.
WHAT IS THE PURPOSE OF SPEED LIMITS?
The primary purpose of speed limits is to regulate driving speeds to
achieve an appropriate balance between travel time and risk for a
road class or specific highway section. Speed limits have also been
imposed for fuel conservation when national maximum speed limits
were established on major highways during World War II and again
in 1974 following the oil crisis of the preceding year.2
Safety--more specifically, avoidance of crashes and mitigation of
crash outcomes--is the most important reason for imposing speed
limits. Many factors besides speed affect traffic safety--driving
under the influence of alcohol or other drugs, safety belt use, roadway
geometry, and weather--but speed has been shown to play an impor-
tant role.
Speed is directly related to injury severity in a crash through the
change in velocity (Delta-V) that occurs in a crash. The probability
of severe injury increases sharply with the impact speed through its
relation with Delta-V. The energy release is proportional to the
1 Not all rural Interstate highways are constructed to the highest design standards,
however. Some predate the Interstate construction program and were upgraded to
minimum Interstate standards.
2 The conservation effects, however, were not large. For example, Bloomquist (1984)
estimated that the 1974 National Maximum Speed Limit (NMSL) reduced fuel con-
sumption by 0.2 to 1.0 percent. Originally enacted as a temporary fuel conservation
measure, Congress made the 55-mph (89-km/h) speed limit permanent because of the
apparent safety benefits (TRB 1984, 15).
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MANAGING SPEED
190
square of the impact speed; the higher the impact speed, the greater
the potential Delta-V. The risk of severe injury is even greater when
a vehicle strikes a pedestrian, the most vulnerable of road users; mor-
tality risk for the pedestrian rises rapidly as impact speeds increase,
with the rapid rise beginning at very low speeds. The strength of the
relationship between speed and crash severity alone is sufficient rea-
son for managing speed.
Speed is also linked to the probability of being in a crash,
although the evidence3 is not as compelling because crashes are
complex events that seldom can be attributed to a single factor. In
addition, the association between speed and crash probability varies
by road type. Crash involvement has been associated with the devi-
ation of a driver's speed from the average speed of traffic regardless
of whether the deviation is above or below the average traffic speed.
Evidence of increased crash probability from traveling above the
average speed is found on many different road types, including
Interstate highways, nonlimited-access rural roads, and urban arteri-
als. Evidence of increased crash probability from traveling below the
average speed is found primarily on Interstate highways near inter-
changes where traffic slows to merge or exit and on rural roads where
vehicles slow at intersections or when negotiating turns. Crash
involvement has also been associated with a driver's selection of
speed on certain road types. For example, on nonlimited-access rural
roads, single-vehicle crash involvement rates have been shown to rise
with travel speed.
Speed limits are intended to enhance safety in at least two ways.
They have a limiting function. By establishing an upper bound on
speed, their purpose is to reduce both the probability and the sever-
ity of crashes. They also have a coordinating function; here the intent
is to reduce dispersion in driving speeds (i.e., lessen differences in
speed among drivers using the same road at the same time) and thus
reduce the potential for vehicle conflicts.
3 The reader is directed to Chapter 2 for a more complete review of the studies that
link speed and crash probability. In the interest of brevity, the specific references to
these studies are not repeated here. This protocol is also followed in other places in the
text where the reader is directed to specific chapters for more detailed information.
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191
Guidance on Setting and Enforcing Speed Limits
Speed limits are also established to provide motorists with a com-
mon set of rules about appropriate driving speeds. The purpose is to
encourage uniform driving behavior and an orderly flow of traffic.
Setting speed limits requires making implicit trade-offs among
road user safety, travel time, practicality of enforcement, and other
factors that may affect motorists' decisions about appropriate driving
speeds. Research can help inform the decision maker who must deter-
mine an appropriate speed limit, but the decision ultimately reflects
value judgments about acceptable levels of risk, the value of time, and
acceptable levels of enforcement.
WHAT INFORMATION SHOULD SPEED LIMITS
CONVEY TO THE DRIVER?
A speed limit sign should convey two basic messages: (a) the maxi-
mum speed for a reasonable and prudent driver traveling in free-
flowing traffic with good visibility and under fair weather
conditions, and (b) the speed that will be enforced within some tol-
erance for minor measurement error.
Traditionally, speed limits have been set to inform motorists of
appropriate driving speeds under favorable conditions. Drivers are
expected to reduce speed if conditions deteriorate (e.g., poor visibil-
ity, adverse weather, congestion, warning signs, or presence of bicy-
clists and pedestrians). In the future, variable speed limits may make
possible posted speed limits that vary with conditions.4 However,
until the technology becomes more widely available and less costly,
speed limits should inform the driver of the maximum appropriate
driving speed under favorable conditions. Minimum speed limits
have also been established on some high-speed roads to reduce dis-
persion in speeds. In this case, the speed limit informs the driver of
the minimum appropriate speed under favorable conditions.
4 Another alternative is to use prima facie limits more widely. Prima facie limits enable
drivers charged with a violation for exceeding the speed limit to contend that their
speed was safe for conditions existing at the time. Prima facie limits provide greater
flexibility to drivers to determine an appropriate speed for conditions and place a
greater burden of proof on the enforcement community that a violation has occurred.
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MANAGING SPEED
192
Speed limits should also inform drivers who exceed the maximum
(or fall below the minimum) limit that, with some tolerance for minor
measurement error, they can expect a citation. In other words, speed
limits should mean what they say.
HOW SHOULD SPEED LIMITS BE SET?
The approach currently in wide use to set speed limits is sound, that
is, speed limits are legislated by broad road class (e.g., Interstate high-
way) and geographic area (e.g., urban district). Where statutory lim-
its do not fit specific road or traffic conditions, speed zones may be
established administratively, and speed limits for that highway sec-
tion may be reduced from or raised above the statutory limit.5 The
system appropriately balances the desirability of uniform speed limits
(legislated limits for broad road classes) with the need for exceptions
(speed zones) to reflect local differences.
Establishing Legislated Speed Limits by Road Class
Decision Process for Determining Legislated Speed Limits
Legislated speed limits by road class are determined by state legisla-
tures and city councils for state and local roads, respectively.
Legislators should seek the advice of traffic engineers, law enforce-
ment officials, judges, public health officials, and the general public
in determining appropriate speed limits, and provision should be
made to monitor and enforce whatever decision is reached.
Consultation, however, does not ensure that all parties will reach
consensus or that tensions will be resolved between different inter-
ests, such as between commuters and residents on appropriate speed
limits on residential streets. The decision process requires trade-offs
and judgments concerning the relative importance of safety and
travel time and the feasibility of enforcement. There is no single
"right" speed limit, but, in addition to satisfying safety, the final
5 In those states with absolute speed limits, speed zone limits cannot be raised above
the maximum absolute limit for that road class.
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Guidance on Setting and Enforcing Speed Limits
selection of a speed limit should meet the requirements of enforce-
ability and acceptance by the community at large.
Nor should the process stop there. Roadway conditions, vehicle
safety features, driving behavior, and attitudes change over time. The
change in driver attitudes toward and compliance with the 55-mph
(89-km/h) NMSL is an example of how driver support for a legis-
lated speed limit can erode over time. Legislated speed limits should
be reviewed periodically and revised when necessary on the basis of
monitoring data on actual driving speeds and safety outcomes.
Roadway Classification Scheme
The committee identified seven road classes plus a category of spe-
cial zones (e.g., school zones, work zones) as the basis on which to
differentiate speed limits by road type. The categorization scheme
covers most major road classes and is expressed in terminology
appropriate for the general reader. The committee's classification
scheme differs from the more technical highway functional classifi-
cation system developed by the American Association of State
Highway and Transportation Officials (AASHTO), a classification
well known to the engineering community (Table 6-1). Data on
mileage, travel volume, and safety, which are useful in making deci-
sions about appropriate speed limits by road class, are only available
by the AASHTO functional classification scheme. Thus they are
provided here, although there is not always a one-to-one corre-
spondence between the AASHTO and the committee road classi-
fication systems.
Guidance on Setting Legislated Speed Limits by Specific Road Class
In determining appropriate speed limits by road class, decision mak-
ers should be guided by both the likely risks imposed on others by
individual driver speed choices and by the adequacy of cues provided
by the roadway to help drivers anticipate conditions and make
appropriate speed choices. They should also take enforcement prac-
ticality into consideration. Table 6-2 summarizes differences
between road classes on the basis of these general considerations.
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Table 6-1 Road Class Categories and Characteristics (FHWA 1997)
Fatal Crashes Fatalities a Injuries c
Annual Travel in
Road Class Mileage
Road Class
Injury
(Percent of (Percent
100 MVM Fatality
(Percent of
(AASHTO (Percent
(Committee
Rate b
Rate b
Total) of Total)
(Percent of Total) Total)
Definition) of Total)
Definition)
2,324.5
Rural Interstate Rural Interstate 32,818 2,388 2,850 1.23 91,200 39
(9.4)
(0.8) (6.4) (6.8) (2.4)
5,093.5
Urban Interstate Urban Interstate 22,240 3,701 4,137 0.81 444,600 87
(20.5)
Other freeway and (0.6) (9.9) (9.9) (12.0)
expressway d
3,788.1
Rural multilane/high- Other principal 235,490 8,450 9,801 2.59 392,700 104
(15.3)
speed two-lane arterial, rural (6.0) (22.6) (23.4) (10.5)
Minor arterial,
rurale
3,777.2
Urban/suburban Other principal 52,973 4,977 5,434 1.44 731,900 194
arterial, urban f (15.2)
multilane (1.3) (13.3) (13.0) (19.6)
2,410.4
Rural lower-speed Major collector, 705,311 6,477 7,234 3.0 395,000 164
(9.7)
two-lane rural (18.0) (17.4) (17.3) (10.6)
Minor collector,
rural g
h 4,269.9
Not defined Minor arterial, 176,940 4,557 5,092 1.19 778,000 182
(17.2)
urban (4.5) (12.2) (12.1) (20.8)
Collector, urban
Not defined h 1,077.7
Local roads, rural 2,119,154 3,956 4,280 3.97 253,800 236
(4.3)
(54.1) (10.6) (10.2) (6.8)
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Urban residential street Local street, urban 574,524 2,080.8 2,845 3,079 1.48 646,600 311
(14.7) (8.4) (7.6) (7.3) (17.3)
Rural unpaved road N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A.
Note: Safety statistics are for 1996, the most recent year available. Year-to-year-variations by road class are small. AASHTO = American
Association of State Highway and Transportation Officials; MVM = millions of vehicle miles; FHWA = Federal Highway Administration;
N.A. = not applicable; 1 mi = 1.609 km.
a Includes vehicle occupant and nonoccupant fatalities.
b Fatality and injury rates are expressed as fatalities (or injuries) per 100 MVM.
c Includes vehicle occupant and nonoccupant injuries of all types, not just incapacitating. Estimates or 1995 data are used for 10 states plus the
District of Columbia because of incomplete reporting at the time of publication.
d The category "other freeway and expressway" includes roads with less than full control of access.
e AASHTO defines this road category as serving major statewide travel. The roads are designed for relatively high-speed travel and minimum
interference to through movement (AASHTO 1994, 11).
f This category also includes a substantial share (nearly 40 percent of the total mileage) of two-lane roads. The major purpose of these roads is
traffic circulation, not access.
g AASHTO defines this road category as serving intracounty travel where travel distances are shorter and speeds are more moderate than on
arterial routes (AASHTO 1994, 11). This category includes a small amount of four-lane divided highway mileage.
h The committee did not cover every possible road type. For example, the categories of "urban/suburban multilane roads" and "urban residential
streets" were thought to adequately capture the major urban road types. Hence, a separate category was not defined for urban minor arterial and
collector roads.
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Table 6-2 Major Factors for Consideration in Setting Speed Limits by Road Class
Available Information for
Road Class Effects of Externalities Motorist Speed Choice Enforcement Practicality
Rural Low--free-flowing traffic; Good--roads usually constructed Selective, targeted enforcement has
Interstates limited access; pedestrians to highest design standards; short-term positive effects, but
and bicyclists not generally drivers can generally anticipate coverage of complete mileage is
pres-ent; the best safety conditions difficult
record of all rural road
classes
Urban Somewhat greater than on rural Not as good as rural Interstates Mileage is easier to cover, but
Interstates Interstates because higher because of driver difficulties apprehending speeders in high-
traffic volumes and closer anticipating traffic bottlenecks traffic volumes on multilane seg-
interchange spacing increase or incidents, particularly during ments can be difficult
potential for vehicle conflict, congested periods
but still relatively low
Rural Greater than on rural Fair to good, depending on road- Coverage of very extensive mileage
multilane/ Interstate highways because way geometry and extent of is difficult
high-speed many highways are not access control--speed zones
two-lane divided and access is not can be used with lower speed
always limited, resulting in limits where driver anticipation
at-grade intersections with of appropriate speeds is likely
potential for vehicle con- to be poor
flicts
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MANAGING SPEED
204
chance of head-on collisions. Roadside hazards, such as trees and
utility poles, are often present on these roads. Fatal crashes and fatal-
ities are more than three times higher--and the fatality rate is
approximately twice as high--on this road class as on rural
Interstates (Table 6-1).11 Design speed should be an important con-
sideration in setting appropriate speed limits on these highways. The
cues available to motorists to select an appropriate driving speed vary
depending on roadway geometry and the amount of access control.
Because these highways carry a substantial amount of traffic (Table
6-1) and their function is to accommodate relatively high-speed
through travel (AASHTO 1994, 11), speed zones should be used
where speed limits should be appropriately lowered rather than
imposing lower speed limits throughout the system. Limited enforce-
ment coverage also supports this strategy.
Urban and Suburban Multilane Roads
This road class probably encompasses the greatest variation in road-
side conditions, thus making it difficult to specify a suitable sys-
temwide speed limit. Maximum speed limits should be set somewhat
lower than for rural multilane highways, and extensive use of speed
zones is recommended. The risk of vehicle conflicts is greater on
urban roads because of more roadside activity, access points, and cross
traffic. Bicyclists and pedestrians--the most vulnerable road users--
are apt to be more common on these roads. Drivers appear to make
some accommodation to these differences; fatal crashes, fatalities,
and the fatality rate are considerably lower on these roads than on
their rural counterparts, although injury levels and the injury rate are
nearly two times higher (Table 6-1). Roadway geometrics may not
differ greatly from those on rural multilane highways, but motorists
are apt to have greater difficulty determining appropriate driving
speeds in areas that are heavily developed. Speed adaptation can also
be a problem, particularly lowering speeds appropriately as drivers
travel from rural to suburban and urban areas. Enforcement is easier
from a coverage perspective; there are fewer miles of urban than rural
11 Injuries are more than four times higher and the injury rate is more than twice as
high (Table 6-1).
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Guidance on Setting and Enforcing Speed Limits
multilane roads (Table 6-1). Also, there is often room to pull over
speed limit violators. These roads may be candidates for photo radar
enforcement at high-crash or highly hazardous locations.
Rural Lower-Speed Two-Lane Roads12
The potential for vehicle conflicts is great on this road class because
of the absence of access control and limited opportunities for passing.
Fatal crashes, fatalities, and the fatality rate are among the highest for
all road classes; injuries and the injury rate are among the highest for
all rural road classes (Table 6-1). The roads are not designed to the
highest standards but rather to accommodate topography and
expected traffic (AASHTO 1994, 460). Thus, motorists may some-
times have difficulty determining appropriate driving speeds for con-
ditions. Design speed should be a key factor in establishing suitable
speed limits on these roads. Appropriate use of speed zones, warning
signs, and advisory speeds is recommended. Adequate coverage of
extensive road mileage poses a problem for enforcement. Thus, speed
limits must be reasonable for conditions and set at levels that are
largely self-enforcing. Fortunately, there is some evidence to suggest
that drivers do restrict their speeds on roads with lower design
speeds.13
Urban Residential Streets
The potential for inconsistent application of speed limits is high on
this road class. Neither vehicle operating speeds nor design speeds
are likely to provide useful input for determining appropriate speed
limits on residential streets; safety experience and enforcement prac-
ticality should be given higher priority. Neighborhood pressures may
result in setting speed limits very low, but often they are not enforced
and compliance is poor, even by neighborhood residents. Speed lim-
its based on vehicle operating speeds, however, may be inappropriate
because there is a high potential for vehicle conflicts, and drivers are
12The committee defines "lower-speed" as less than 55 mph (89 km/h).
13Agent et al. (1997) reported that the only roadway type in Kentucky where the 85th
percentile speed was less than 5 mph (8 km/h) higher than the posted speed limit was
two-lane rural roads without full-width shoulders (p. 11).
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MANAGING SPEED
206
not always aware of the danger they pose to bicyclists and pedestri-
ans with whom they share the road. The fatality rate on residential
streets is the highest for all urban road classes; the injury rate is the
highest for any road class (Table 6-1). Design speed also has limited
practical significance for determining speed limits on residential
streets; the frequency of intersections, presence of stop signs, and
amount of roadside activity (e.g., parking, driveways) have a greater
effect on actual vehicle speeds (AASHTO 1994, 429). The risks
imposed on other road users is sufficient reason for limiting driving
speeds on residential streets. However, speed limits should not be set
below enforceable levels.14 Even when there is a commitment to
enforcement, there are practical limits because of the extensive
mileage of residential streets. Alternative measures, such as traffic
calming and other highway design techniques, should be considered
to achieve desired driving speeds.
Rural Unpaved Roads
This is a road class for which posted speed limits are generally inap-
propriate. The basic law that drivers should adopt a reasonable and
prudent speed should govern. Risks of vehicle conflict are very low on
these roads; most are used by residents who are familiar with the
roads and their condition. Roadway geometry varies, and roadway
conditions can change rapidly depending on weather, season, and
amount of road maintenance, so that establishing an appropriate
speed limit is difficult even for favorable conditions. Finally, enforce-
ment is minimal on roads with such low traffic volumes.
Special Zones
At least two situations--school zones and work zones--warrant spe-
cial handling in establishing speed limits. Risks to others are likely to
be great in both because of the presence of vulnerable road users--
children in school zones and workers in work zones. Drivers are
14Another option is to use prima facie speed limits more widely, which would provide
greater enforcement flexibility. This alternative would require legislative changes in the
two-thirds of the states that currently have only absolute speed limits.
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Guidance on Setting and Enforcing Speed Limits
unlikely to anticipate appropriate driving speeds for negotiating these
zones. Typically, they represent an exception to normal driving
speeds, and adequate speed adaptation is a problem as drivers
approach the zones. Moreover, work zones often have narrow lanes
and restricted alignments at detours and lane shifts that require speed
reduction. School zones are prime candidates for variable speed limit
systems because lower driving speeds are generally required only for
certain hours of the day. Changeable conditions in work zones as well
as limited hours of operation also make them amenable to variable
speed limits. Photo radar enforcement may be appropriate in those
circumstances where patrolling large numbers of school zones or
apprehending speeding violators in high traffic volumes in work
zones proves difficult.
Setting Speed Limits in Speed Zones
Speed zones are established for highway sections where legislated
limits for that road class do not fit specific road or traffic conditions.
Determination of appropriate speed limits in speed zones should be
made on the basis of an engineering study. Speed zones should be
reviewed periodically--with greater frequency where conditions are
changing rapidly (e.g., developing suburban areas)--to determine
whether changed conditions warrant an adjustment in the speed
limit or in the boundaries of the zone itself. California, for example,
has established a 5-year review cycle; police will use radar enforce-
ment in speed zones only if an engineering and traffic survey has
been conducted and reviewed within the past 5 years to set an appro-
priate speed limit. Traffic engineers or technicians under their super-
vision should conduct the engineering study. Consultation with law
enforcement officials is advised where this is not already accepted
practice so that the proposed speed limit is enforceable. Elected offi-
cials and citizen groups may also become involved when community
concerns have been expressed about driving speeds. In addition to
speed data, engineering studies can provide road-specific historical
data on crashes and information about hazards (e.g., pedestrian
crossings, intersecting streets with restricted sight distance) not read-
ily apparent to motorists. These data help the engineer in determin-
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MANAGING SPEED
208
ing an appropriate speed limit. However, unless the decision has the
support of enforcement officials and the general public, speed zoning
may not result in desired driving speeds, particularly if the speed limit
is set below the 85th percentile speed. On roads where enforcement
is infrequent and low speeds are desirable, such as residential streets,
alternatives like traffic calming should be investigated.
The most common factor considered in setting speed limits in
speed zones is the 85th percentile speed, although frequently the
limit is adjusted from this value on the basis of such factors as crash
experience, roadside development, roadway geometry, and parking
and pedestrian levels. Speeds are measured from spot-speed surveys
of free-flowing vehicles taken at representative locations in the pro-
posed speed zone. The speed limit typically is set near the speed at or
below which 85 percent of motorists are driving. The advantages of
setting the speed limit near the 85th percentile speed are that
(a) police are enabled to focus their enforcement efforts on the most
dangerous speed outliers, and (b) the 85th percentile speed is gener-
ally at the upper bound of a speed range within which crash involve-
ment rates are lowest, at least on certain road types according to some
studies that have examined the relationship between speed and crash
probability.
Setting the speed limit primarily on the basis of the 85th per-
centile speed is not always appropriate. The potential safety benefits
may not be realized on roads with a wide range of speeds. Basing the
speed limit on a measure of unconstrained vehicle operating speeds is
not appropriate on urban roads with a mix of road users, including
bicyclists and pedestrians, and with high traffic volumes and levels of
roadside activity. An expert-system approach, either formal or infor-
mal, could be developed to establish speed limits in speed zones.15
The expert-system approach deserves consideration because it pro-
vides a systematic and consistent method of examining and weighing
factors other than vehicle operating speeds in determining an appro-
priate speed limit.
15Details of Australia's expert system for setting speed limits are provided in
Chapter 3.
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Guidance on Setting and Enforcing Speed Limits
Differential Speed Limits
The committee remains neutral on the desirability of differential
speed limits for passenger cars and heavy trucks that have been
established in some states on some road classes. It did not find com-
pelling evidence16 to support more widespread application of differ-
ential speed limits. Neither did it find strong evidence that
differential speed limits should be eliminated where they are in use.
More research on and evaluation of the effects of differential speed
limits on driving speeds and safety outcomes are needed in the states
that have adopted them.
The committee found little evidence to suggest that motorists
decrease driving speeds at night when lower nighttime speed limits
are in effect. However, it did not find compelling evidence to suggest
that nighttime speed limits be eliminated in states that have adopted
them.
CAN DRIVERS BE INDUCED TO OBEY SPEED LIMITS
THROUGH ENFORCEMENT?
Most experts agree that enforcement is critical to achieving compli-
ance with speed limits. Simply posting a speed limit sign will not
achieve desired driving speeds. Even if most motorists believe that
the speed limits are reasonable and they comply within a small toler-
ance, enforcement is still necessary to ensure the conformity of driv-
ers who will obey laws only if they perceive a credible threat of
detection and punishment for noncompliance.
The problem with traditional enforcement methods is their
short-lived effect in deterring speeding or other unwanted behavior.
Maintaining the deterrence effect requires a level of enforcement that
is difficult to sustain because of limited resources provided for speed
enforcement and competing enforcement priorities. Policy makers
can affect the level of enforcement through resource allocation, but
enforcement is expensive. Thus, the police should deploy enforce-
16 A review of many of the key studies concerning differential speed limits can be
found in Chapter 3.
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ment efforts strategically on those roads and at times when speed-
related incidents are most common or where road conditions are
most hazardous. (The infrequent nature of crashes, however, makes
targeting difficult.) There is some evidence17 that planned patrols
at varying time intervals and locations can extend the time- and
distance-halo effects of enforcement at particular locations, but only
after an initial period of continuous patrolling. In addition, the
patrols must be visible and sufficiently frequent to convince drivers of
a credible threat of detection for noncompliance. Police can boost the
longevity of the deterrence effect by combining enforcement initia-
tives with high-profile public information campaigns to increase
driver awareness that speed limits will be enforced. Publicity must be
followed up by actual enforcement if the approach is to successfully
deter speeding. Moreover, making permanent behavior changes
requires a long-term sustained effort.
Automated enforcement--for example, photo radar--can be used
to complement traditional enforcement methods, particularly where
roadway geometry or traffic volume makes traditional methods dif-
ficult or unsafe. Photo radar has been shown to be efficient and
effective where it has been used for speed control, particularly on
high-volume, major arterials where compliance with speed limits is
often poor. Photo radar enforcement could also be coupled with vari-
able speed limit systems for use on urban Interstates where high traf-
fic volumes make it difficult to apprehend speeding drivers. Photo
radar is controversial. Legal issues, such as privacy and owner (versus
driver) liability for speeding infractions, must be resolved. Successful
introduction of automated enforcement also requires funding, public
education, and careful deployment (i.e., on roads that are especially
hazardous and at high-crash locations where speeding is a contribut-
ing factor or where traditional enforcement methods are hazardous)
to ensure essential public support.
Other alternatives to traditional enforcement may be required in
some circumstances to achieve desired driving speeds. Traffic calm-
ing has successfully reduced speeding on many residential streets, but
17 Chapter 4 provides a more detailed discussion of studies on different enforcement
strategies.
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Guidance on Setting and Enforcing Speed Limits
it is difficult to determine net areawide effects on safety because of
the difficulty of accurately measuring traffic diversion and the small
size of "before" and "after" crash data that limits statistical analysis.18
Redesigning roads to achieve greater congruity between driver per-
ceptions of appropriate travel speeds and cues provided by the road
itself (narrowing lanes, etc.) has promise. The approach should result
in more consistent vehicle operating speeds, but additional study of
the relationship between operating speeds and roadway geometric
elements is required. In view of the size of the U.S. road network and
the cost and pace of road rehabilitation, road redesign is a long-term
strategy.
In the near term, speed limits should be set at levels that are
largely self-enforcing or at the lowest speed the police are able to
enforce.
HOW CAN THE JUDICIAL SYSTEM ASSIST IN
ACHIEVING DESIRED DRIVING SPEEDS?
Actions by law enforcement officials and the justice system can
undermine the effectiveness of speed limits in achieving desired driv-
ing speeds. The police can choose not to enforce speed limits where
they think the limits are unreasonable. Traffic court judges throw out
speeding violations or reduce fines in cases when they believe the
speed limits are inappropriate or the fines too harsh. Thus it is
important that the police and traffic court judges perceive that speed
limits are reasonable and enforceable.
When setting speed limits, care should be exercised that primary
consideration is given to safety, not revenue enhancement. Driver
perception of entrapment from speed limits set unreasonably low to
generate income erodes the credibility of traffic regulation.
Where the courts have broad discretion in assessing penalties for
speeding violations, inconsistent treatment of violations can lead to a
public perception that speed limit laws are arbitrary and capricious.
Development of sentencing guidelines and training for judges who
18 Chapter 5 provides a lengthier discussion of traffic calming and road redesign.
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212
handle speeding violations can help ensure consistent treatment of
violators. Licensing point demerit systems have already gone a long
way toward reducing inconsistencies in penalty assessments by pro-
viding a uniform system of graduated penalties for various traffic vio-
lations.
The deterrence effect of sanctions for speeding violations is often
limited by lengthy backlogs of cases in courts where traffic violations
are often perceived as minor infractions and not serious crimes.
Automated enforcement has the potential to relieve court backlogs
through the use of administrative adjudication procedures for many
speeding infractions. Such procedures can reduce the number of
court hearings as well as the cost of processing speeding violations.
WHAT POTENTIAL DOES TECHNOLOGY OFFER
TO IMPROVE METHODS OF DETERMINING AND
ENFORCING SPEED LIMITS?
Technology can advance the state of the art, but it does not hold the
complete solution. Many applicable technologies already exist or are
currently being enhanced as computing capability has grown. The
difficulty in using them often lies less with the technologies them-
selves--although cost is an important consideration--than with
political and legal hurdles to be overcome in deploying them widely.
The most promising technologies for near-term adoption are dis-
cussed here.
The technology to support variable speed limits and improve traf-
fic flow efficiency is available, but more experimentation and evalu-
ation are needed to determine the effectiveness of these systems
from a safety and traffic efficiency perspective and to learn where
variable speed limits can be deployed most usefully. The current high
cost of variable speed limit systems limits opportunities for their
deployment to urban Interstates and freeways with large traffic vol-
umes or to selected segments of major roads where weather (e.g., fog,
visibility) is a frequent problem.
Automated enforcement, particularly photo radar, can provide an
effective complement to traditional enforcement methods, particu-
larly where police patrol vehicles cannot be deployed effectively or
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Guidance on Setting and Enforcing Speed Limits
safely. Successful introduction of automated enforcement may
require adoption of legal changes; strong public support is essential
for its success.
Intelligent Transportation System technologies that support more
efficient and safer travel are being developed and demonstrated on
U.S. vehicles and highways. New techniques for communicating
information to drivers about appropriate driving speeds are under
development, and advances in vehicle control technologies have the
potential to automate some speed-related driving functions. Some
technologies, such as "smart" cruise control, are close to commercial-
ization. Other vehicle-related technologies, such as frontal-collision
and lane-departure avoidance systems, are still in the research and
development phase. Key concerns include reliability, liability, driver
control, and acceptance. The technologies represent important inno-
vations that require watching and evaluation.
CONCLUDING COMMENT
The issue of appropriate driving speeds and safety will persist as long
as there are individual drivers making choices about driving speeds.
Most states have recently raised speed limits on many major high-
ways following repeal of the NMSL. Close monitoring of effects,
particularly changes in driving speeds and safety outcomes, is desir-
able; vigilant enforcement is needed; and redoubled efforts should be
taken to mitigate adverse safety outcomes by such continuing initia-
tives as increased safety belt use and reductions in driving while
intoxicated--measures with large and proven safety benefits.
The speed-safety problem may become more acute with increased
numbers of older drivers, who may not themselves speed but who
have reduced capacity to handle high speeds. Congestion--which is
unlikely to abate in the near term--contributes to the problem by
increasing driver frustration and encouraging unsafe driving behav-
iors, such as speeding to avoid red lights or high-speed weaving on
crowded Interstates and freeways.
Speed limits are one of the oldest and most widely used methods
of controlling driving speeds, but speeds limits alone are not effective
in all situations. Technology can help establish limits that are more
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MANAGING SPEED
214
sensitive to actual changes in conditions and thus provide drivers
with better information. It can help outfit the vehicles and highways
of the future with speed monitoring and control devices. Finally,
technology can help improve the efficiency and effectiveness of
enforcement. But the efficacy of speed limits will continue to depend
largely on driver perception of the reasonableness of the limits and
the willingness of the police and the courts to enforce the limits and
punish violations. Where they are not present and limiting speed is
desirable, alternative measures to managing driving speeds will have
to be sought.
REFERENCES
ABBREVIATIONS
AASHTO American Association of State Highway and Transportation
Officials
FHWA Federal Highway Administration
NHTSA National Highway Traffic Safety Administration
TRB Transportation Research Board
AASHTO. 1994. A Policy on Geometric Design of Highways and Streets. Washington,
D.C.
Agent, K.R., J.G. Pigman, and J.M. Weber. 1997. Evaluation of Speed Limits in
Kentucky. KTC-97-6. Kentucky Transportation Center, Kentucky Transportation
Cabinet, and Federal Highway Administration, April, 90 pp.
Bloomquist, G. 1984. The 55 mph Limit and Gasoline Consumption. Resources and
Energy, Vol. 6, March, pp. 2139.
FHWA. 1997. Highway Statistics 1996. U.S. Department of Transportation,
Washington, D.C.
NHTSA. 1998. Life in the Fast Lane. DOT-HS-808-670. U.S. Department of
Transportation, Feb.
TRB. 1984. Special Report 204: 55: A Decade of Experience. National Research Council,
Washington, D.C., 262 pp.
Representative terms from entire chapter:
speed limit
