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3
Managing Speeds:
Speed Limits
Angels
Who guard you
When you drive
Usually
Retire at 65
Burma Shave (Rowsome 1965)
The preceding chapter provided evidence of a close link between
speed and safety. Speed is directly related to injury severity in a crash,
reflecting the laws of physics. The link between speed and the prob-
ability of being in a crash is weaker, reflecting the fact that motor
vehicle crashes are complex events that can seldom be attributed to a
single factor.
The evidence presented, however, is sufficiently strong to reaffirm
the need for managing speed. In this chapter, one of the primary
77
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MANAGING SPEED
78
methods of managing drivers' choice of speed--the imposition of
speed limits--is discussed. Speed limits are part of a speed manage-
ment system, which involves laws and a process for setting reasonable
speed limits as well as enforcement, sanctions, and publicity. The
chapter begins with an explanation of why regulatory intervention is
justified. After a brief history of speed regulation, the major methods
of establishing speed limits are introduced and their strengths and
weaknesses summarized. The application of speed limits to different
road classes and roadway environments is considered next. A review
of what is known about the effect of speed limits on driver behavior
and safety follows, drawing heavily on studies of recent changes in
speed limits both in the United States and abroad. The chapter ends
with a discussion of the implications of these findings for speed limit
policies.
REGULATING SPEED--A THEORETICAL
JUSTIFICATION
Drivers continually make choices about appropriate driving speeds,
making their own assessment concerning the amount of risk they are
willing to bear. Because drivers have a strong incentive to complete
their trips safely, one could ask why they should not be left to choose
their own travel speeds. There are three principal reasons for regulat-
ing drivers' speed choices: (a) externalities,1 that is, the imposition of
risks and uncompensated costs on others because of inappropriate
speed choices made by individual drivers; (b) inadequate information
that limits a motorist's ability to determine an appropriate driving
speed; and (c) driver misjudgment of the effects of speed on crash
probability and severity.
The strongest case for regulatory intervention can be made on the
grounds of externalities. Drivers may not take into account the risks
imposed on others by their choice of an appropriate driving speed.
1 Externalities are defined as the "effect that occurs when the activity of one entity
(a person or a firm) directly affects the welfare of another in a way that is not trans-
mitted by market prices" (Rosen 1995, 91).
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Managing Speeds: Speed Limits
For example, drivers who choose to drive very fast relative to other
traffic or very fast for existing road conditions in exchange for a
shorter trip time may accept the higher risk of death or injury for
themselves, but their choice almost certainly increases the risk of
death and injury for other road users. Even a single-occupant, single-
vehicle crash imposes medical and property damage costs that are not
entirely paid for by the driver. Other externalities in the form of
higher fuel consumption or higher emissions resulting from higher
driving speeds are not directly paid for from current fuel or vehicle
operating taxes. Such externalities are the major theoretical justifica-
tion for the imposition of speed limits. (Speed limits, of course, are
not the only possible regulatory response.) The externalities--partic-
ularly the risks to others--may be relatively small on lightly traveled
Interstate highways but quite large on streets adjacent to schools or
in highly congested areas. The differences in the effects of the exter-
nalities are important to consider in setting appropriate speed limits
on different types of roads.
Regulatory intervention is also justified if drivers are systemati-
cally making "wrong" choices because of a lack of information or an
inability to understand the information presented to them. (A wrong
choice is defined as a choice that is different from the choice drivers
would make if they had and understood all the relevant information.)
For example, some drivers may not correctly judge the capabilities of
their vehicles (e.g., stopping, handling) or anticipate roadway geom-
etry and roadside conditions sufficiently to determine appropriate
driving speeds. These circumstances may not be as relevant for expe-
rienced drivers driving under familiar circumstances, although these
drivers can make inappropriate decisions because of fatigue or other
factors. Inexperienced drivers, or experienced drivers operating in
unfamiliar surroundings, are more likely to underestimate risk and
make inappropriate speed choices. For example, even experienced
drivers may not make informed choices when faced with entirely new
driving circumstances, such as the southerner confronting snow or the
easterner confronting a winding mountain road with no shoulders.
Another reason for regulatory intervention, which is related to the
issues of information adequacy and judgment, is the tendency of
some drivers to underestimate or misjudge the effects of speed on
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MANAGING SPEED
80
crash probability and severity. For example, drivers may have a good
sense of the relationship between driving speed and travel time, but
they may not have as good an appreciation of the effect of speed on
crash probability and crash severity. As noted in Chapter 1, there is
some evidence that drivers systematically overestimate their driving
skills and underestimate the risks of driving, particularly at higher
speeds. Other drivers may simply be indifferent to speed regulations
and will drive as fast as they can, ignoring the risks their speed
choices impose on others. The justification for imposing speed lim-
its, however, still leaves open the question of how the limits should
be set, the topic of the following section.
METHODS OF SETTING SPEED LIMITS
Brief History of Speed Regulation
The idea of regulating the speed of motor vehicle travel has a long
history. In fact, the first speed regulations predated the invention of
the automobile by some 200 years. The town of Newport, Rhode
Island, prohibited the galloping of horses on major thoroughfares in
an effort to prevent pedestrian deaths; Boston, Massachusetts, lim-
ited horsedrawn carriages to a "foot pace" on Sundays to protect
churchgoers (Ladd 1959). In 1901 Connecticut was the first state to
impose a maximum speed limit of 8 mph (13 km/h) in cities (Labatut
and Lane 1950).
A review of early speed legislation suggests that the primary pur-
pose of regulating speed was to improve public safety (Parker 1997,
1); another goal was uniformity in state speed regulations (UVC
1967, 436). The Uniform Vehicle Code (UVC), first published in
1926, provided the framework for speed control as it is known today.
The original code contained (a) a basic rule requiring motorists to
operate at speeds reasonable and prudent for conditions and (b) max-
imum general speed limits2 in business and residential districts and
other specific situations (e.g., grade crossings, in the vicinity of
2 These maximum limits were established as prima facie limits (UVC 1967, 437).
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Managing Speeds: Speed Limits
schools) (UVC 1967, 428, 437438). The 1934 version of the UVC
broadened the maximum speed limits to cover more general situa-
tions (e.g., urban districts) and introduced the concept of speed
zones, wherein state agencies would determine alternative maximum
speed limits at particular highway locations on the basis of engineer-
ing and traffic investigations (UVC 1967, 437, 446).3
With the rise of the traffic safety movement during the 1930s, the
National Safety Council organized a Committee on Speed
Regulation in 1936 to study the speed problem. Its report (NSC
1941) reiterated the framework laid out by the UVC--a basic rule,
maximum general speed limits, and authority for establishing speed
zones on the basis of a traffic engineering study. The committee rec-
ommended that state legislatures adopt uniform speed legislation
based on this framework (NSC 1941, 5, 15). Above all it recom-
mended a balanced approach toward speed control, directed at speed
"too fast for conditions" rather than at speed in excess of some arbi-
trary general limit (NSC 1941, 4). At the time the committee was
conducting its work, cars were capable of reaching speeds of 80 to
100 mph (129 to 161 km/h), but the general statewide speed limit in
many states was 35 to 45 mph (56 to 72 km/h) ( Joscelyn and Elston
1970, 32).
With the tremendous growth in motor vehicle travel and further
improvements in the highway system and the automobile during the
1930s and 1940s, the motoring public clamored for speeds higher
than the maximum posted limits, which were frequently ignored
because they were considered below those deemed reasonable by
motorists (The American City 1950). In response, traffic engineers
began to advocate an approach to setting speed limits (described sub-
sequently) that is based on operating speeds as well as other factors.
This method attempts to define a safe speed but also accommodates
drivers' desire for a reasonable speed.
3 The 1926 UVC had previously recommended that local authorities be empowered
to increase speed limits on through highways under their jurisdiction (UVC 1967,
450).
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82
Considerations in Establishing Speed Limits
General Speed Limits Versus Speed Zones
In any discussion of speed limits, it is important to distinguish
between general limits, which apply statewide or even nationwide,
and limits in speed zones, which apply to a particular section of road.
The former are set by legislation--by state statute, municipal ordi-
nance, or Congress (ITE 1992, 347). Typically, general or legislated
limits apply to a category of highway (see glossary)--a freeway or an
arterial, for example--and reflect the design characteristics of the
particular road class. They also differ by area, distinguishing rural
from urban or local roads. By definition, general speed limits repre-
sent a compromise; they may be well suited for some roads but are
either "too high" or "too low" for others (Harwood 1995, 89).
Speed limits in speed zones, on the other hand, are established by
administrative action and are intended to be determined on the basis
of an engineering study (ITE 1992, 347). The limits are tailored for
a specific length of road where the general limit is deemed to be inap-
propriate. Guidance is abundant on how to conduct the requisite
engineering assessment of the traffic, road, and land use conditions
that should be considered in establishing an appropriate speed limit
in a speed zone (Harwood 1995, 89).
Uniformity
One might ask why speed zones are not established for every road
segment, thereby tailoring speeds to the particular characteristics of
the road and the location. Besides creating obvious resource problems
because of the requirement to both undertake the necessary engi-
neering studies and post signs on each highway section, a system of
frequently changing speed limits would create confusion for the driv-
er (Harwood 1995, 90). It could encourage a patchwork of different
speed limits that may or may not be consistent across road classes and
locations (Figure 3-1). The current system of statutory limits with
speed zones as exceptions has the merit of encouraging uniformity
and consistency of speed limits across a broad range of highways.
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Managing Speeds: Speed Limits
Figure 3-1 Parody of state response to repeal of 55-mph (89-km/h)
National Maximum Speed Limit (reprinted with permission of Joe
Heller, Green Bay Press-Gazette).
Objectives of Speed Limits
The primary purpose of speed limits is to enhance safety by reducing
the risks imposed by drivers' speed choices. Speed limits enhance
safety in two ways. They have a limiting function. By establishing an
upper bound on speeds, the objective is to reduce both the probabil-
ity and severity of crashes. Speed limits also have a coordinating
function--to reduce dispersion in driving speeds (Lave 1985); more
uniform speeds are associated with fewer vehicle conflicts. Another
function of speed limits, which is related to their coordinating func-
tion, is to achieve an orderly flow of traffic and improve traffic flow
efficiency. Once established, well-conceived speed limits help deter-
mine a reasonable standard for enforcement. Historically, speed lim-
its have also been established for energy conservation purposes
during times of national crisis.
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MANAGING SPEED
84
Those who set speed limits attempt to balance road user safety and
travel efficiency, among the many other factors that determine driv-
ers' speed choice. Determining the optimal trade-off between these
objectives depends, in part, on the function of the road. On limited-
access facilities built to move traffic efficiently, greater emphasis may
be placed on minimizing travel time without compromising safety.
On local roads, where the primary function is access to abutting
property, speed limits may be set to accommodate access rather than
the efficient movement of traffic (Harwood 1995, 90).
Informational Content and Reasonableness
Whatever trade-offs are made between safety and travel time in
establishing speed limits, posted limits ought to convey information
to drivers. According to current practice, the numerical value on
the sign advises the motorist of the maximum speed at which a driv-
er can lawfully proceed under favorable conditions (e.g., good
weather, daylight, and free-flowing traffic). Drivers are expected to
reduce their speeds as these conditions change. The maximum speed
limit should be related to the actual risk characteristics of the
road (e.g., curvature, lane width) if drivers are to perceive the speed
limit as credible and if adequate levels of voluntary compliance are to
be achieved (Fildes and Lee 1993, 22). State and local governments
do not have the resources--nor do they perceive it as a
good use of resources--to apprehend and penalize large numbers of
out-of-compliance drivers. Routine violation of speed limits by the
majority of drivers may breed contempt not only for speed limits
but also for other traffic regulations. As a general proposition, then,
speed limits should be set at levels that are largely self-enforcing
so that law enforcement officials can concentrate their efforts on
the worst offenders. This goal, however, may not be achievable on all
road classes (e.g., local streets) where lower driving speeds are desir-
able but speed compliance is poor. These roads may be candi-
dates for other speed management strategies, such as traffic calming.
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Managing Speeds: Speed Limits
Primary Methods of Setting Speed Limits
The process of setting speed limits is often viewed as a technical
exercise. However, the decisions concerning appropriate limits
require value judgments and trade-offs that are appropriately handled
by the political process--by Congress in the case of setting national
speed limits and by state legislatures and city councils in determining
general limits for highways under their respective jurisdictions. In
this section, the major methods of determining speed limits are
described. The section begins with a description of the methods most
appropriate for setting statutory or legislated speed limits and con-
tinues with a discussion of the methods appropriate for setting speed
limits in speed zones (Table 3-1).
Statutory Limits
Statutory national speed limits were imposed twice in U.S. history,
both during times of national crisis. A federal speed limit of 35 mph
(56 km/h) was imposed during World War II. More recently,
Congress established the NMSL of 55 mph (89 km/h) during the
energy crisis of 1973 to reduce reliance on imported oil. In both
cases, the objective was to reduce energy costs rather than transpor-
tation costs. Safety benefits and travel time costs were a by-product
rather than an intrinsic part of the initial determination of an appro-
priate speed limit.4
Following repeal of the NMSL in 1995, most state legislatures
acted to raise speed limits on highways subject to the 55-mph (89-
km/h) speed limit. Many states reverted to the maximum general
speed limits in effect for these highways before the NMSL was
enacted. Other states established new speed limits on these high-
ways. Legislative decisions typically were accompanied by public
input and technical support provided by state departments of trans-
portation (DOTs). Before posting speed limit increases, many state
4 Benefit-cost considerations, particularly the trade-offs between loss of life, injury
costs, and time savings, were more directly considered and debated when Congress
decided to allow states to raise speed limits on rural Interstates in 1987.
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Table 3-1 Characteristics of the Primary Methods of Setting Speed Limits
Speed Limit Most Common Relation Appropriateness by Ease of Relation to
Method Application to Safety Road Class Implementation Enforcement
Statutory General limits Trade-offs among Statutory limits typi- Difficult to achieve Can be difficult to
limits safety, travel time, cally are estab- consensus on enforce if limits are set
and other objectives lished by road class national limits arbitrarily
are politically deter- and sometimes by except during
mined location (e.g., times of crisis--
rural) easier to establish
at state and local
level
Optimum General limits or Safety is balanced with Theoretically, should No known practical If implemented, could be
speed limits speed zones other objectives be adaptable for application--dif- difficult to enforce
(e.g., travel time) to any road class ficult to quantify because socially optimal
minimize social key variables speed limits are typically
highway transport lower than what individ-
costs ual drivers would select
Engineering Speed zones Not necessarily a safe May not be as appro- Well-established Helps establish a reason-
study speed for all road priate for urban methodology for able target of out-of-
method with classes; it depends, roads, particularly determining 85th compliance drivers for
speed limits for example, on the residential streets, percentile speed enforcement
set near the dispersion of speeds with greater mix of
85th per- between the slowest road users and func-
centile speed and fastest drivers tions than major
arterials and freeways
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Expert Speed zones Helps identify many Probably most useful Complex system to System has been used to
system factors, in addition and appropriate for develop, requir- target photo enforce-
based to vehicle operating roads in urban ing knowledge- ment (i.e., where recom-
approach speeds, that may areas where speed able experts and mended program limit is
affect safety limits based solely computer capa- substantially below driv-
on 85th percentile bility ers' preferred operating
speed may be inap- speeds)
propriate
Variable speed Freeways Not fully demon- Because of expense, Limited experience Systems are often com-
limits strated--some indi- most appropriate in United bined with photo radar
cation that more for highest-class States--new tech- enforcement
uniform speeds roads with large nologies are being
reduce crashes traffic volumes introduced
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MANAGING SPEED
128
speed dispersion (Davis 1998, 2, 1617). This experience was in
sharp contrast to another rural Interstate--I-10--where speed limits
had also been raised but where speeds remained relatively constant
and injury crashes and crash severity showed a slight decline (Davis
1998, 1). The major differences were attributed to rigorous enforce-
ment and the high percentage of heavy-truck traffic on I-10, which
tended to keep all vehicle speeds lower.62
Review of Studies of Changes in Speed Limits on
Nonlimited-Access Highways
Most U.S. studies have focused on changes in speed limits on lim-
ited-access highways. A recent study (Parker 1997), however, exam-
ined the effect of changes in speed limits--both increases and
decreases--in short speed zones [typically less than 2 mi (3 km)] on
rural and urban nonlimited-access highways. Changes in driving
speeds and crash experience at these sites were compared with closely
matched comparison sites where speed limits remained constant.63
The study found that changing posted speed limits had little effect
on driving speeds. Specifically, a review of before and after speed data
at the selected sites revealed that differences in average speeds, stan-
dard deviations of speeds, and 85th percentile speeds were generally
less than 2 mph (3 km/h) and were not related to the amount the
posted speed limit was changed (Parker 1997, 85).64 Part of the
explanation may lie in the fact that the speed limit changes--at least
increases in the speed limit--simply rationalized the speeds that
drivers were already driving. In fact, where speed limits were raised
62 The Doña Ana County Sherriff 's Office issued more than 1,000 citations for
speeding under a grant from the Traffic Safety Bureau during the time the speed data
were being recorded on I-10 (Davis 1996, 7).
63 The comparison sites could not be randomly drawn from the same population or
source, but every effort was made to match as closely as possible the geometric, vol-
ume, and speed characteristics of the sites where the speed limits had been changed
(Parker 1997, 9).
64 The researchers noted that the changes were statistically significant, primarily
because of large sample sizes, but "not sufficiently large to be of practical signifi-
cance" (Parker 1997, 87).
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Managing Speeds: Speed Limits
by 10 to 15 mph (16 to 24 km/h),65 there was a fourfold increase in
driver compliance levels (Parker 1997, 46). Conversely, where speed
limits were lowered, compliance levels declined sharply; drivers
appeared to ignore the new, lower speed limits at these sites (Parker
1997, 46). The author concluded that changing posted speed limits
alone--without additional enforcement, educational programs, or
other engineering measures--has only a minor effect on driver
behavior (Parker 1997, 87).
Not surprisingly, with such small speed changes, Parker found no
evidence of changes in total crashes or fatal and injury crashes when
posted speed limits were raised or lowered (Parker 1997, 86). The
study findings, however, cannot be generalized to all nonlimited-
access roads because the site selection process was not random.66 The
lack of observed changes in driver behavior raises the concern that, if
the planned speed limit changes simply legalized existing behavior,
the results could be significantly biased in favor of the finding that
the speed limit changes had little effect on driver behavior and thus
offer little insight into the independent effect of a change in speed
limits on the distribution of driving speeds.67
Nevertheless, Parker's results were confirmed in another recent
study of speed limit changes for a range of road types, mainly
nonlimited-access state highways (Agent et al. 1997). Data were col-
lected on speeds and crashes at more than 100 speed zones
in Kentucky where speed limits had been changed. In most cases,
the speed limit was lowered to near 35th percentile speeds; the
predominant change was from 55 to 45 mph (89 to 72 km/h).
The study found modest changes in 85th percentile speeds--less
than the change in the speed limit itself--whether the speed limit
was raised or lowered (Agent et al. 1997, 12). Where 85th percentile
speeds before the change were high relative to the new limit, modest
65 "Before" speed limit levels ranged from 20 to 50 mph (32 to 80 km/h) (Parker
1997, 9192).
66 It should be noted that safety issues and legal concerns are likely to preclude any
experimental design that involves random site selection for speed limit changes.
67 For a more detailed discussion of the Parker study, see the section on Posted Speed
Limits and Speeding Behavior in Appendix C.
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MANAGING SPEED
130
reductions in speed were recorded but were accompanied by a high
rate of noncompliance (Agent et al. 1997, 1213). The authors con-
cluded that motorists will drive at what they consider an appropriate
speed regardless of the speed limit (Agent et al. 1997, iii). Not sur-
prisingly, given the small changes in speed, no statistically significant
changes were observed in the total number of crashes or fatal or
injury crashes (Agent et al. 1997, 16).
The study exhibits many of the same limitations of the Parker
study, mainly nonrandom selection of sites, that limit generalization
of results. In addition, variability in data collection techniques for
speed measurement may have affected the reliability of results.
However, both studies suggest the need for reasonable speed limits
and the difficulty of changing driver behavior where drivers perceive
that an appropriate speed is other than the posted speed limit.
Review of International Experience on the
Effects of Changes in Speed Limits
In addition to the U.S. research on the relationship between changes
in speed limits and highway safety, a number of international studies
have examined this issue.68 International studies of the effects of
changes in speed limits on low-speed roads are numerous and were
summarized briefly earlier in the chapter. This section focuses on a
more limited number of recent studies of speed limit changes on high-
speed roads.
In contrast to the United States, where most studies have evaluated
the speed and safety effects of raising speed limits on limited-access
highways, international studies have primarily examined the effects of
reductions in speed limits. Studies of speed limit reductions in Sweden
(Nilsson 1990; Johansson 1996), the Netherlands (Borsje 1995),
Victoria, Australia (Sliogeris 1992), and Finland (Salusjärvi 1981) all
reported results that are a mirror image of those found in the United
States. Lower speed limits resulted in lower average speeds, although
68 This section also draws heavily on the review commissioned for this study, which
is presented in its entirety as Appendix C.
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Managing Speeds: Speed Limits
the changes were typically less than the absolute reduction in the speed
limit. Lower speed limits were also associated with reduced crash inci-
dence and, in some cases, with reduced crash severity. Many of the
studies, however, do not control for the potentially confounding effects
of other policies undertaken at the same time as the speed limit change
(e.g., public information campaigns, increased levels of enforcement)
or other factors that may have affected highway safety (e.g., changes in
amount of travel affecting exposure levels, safety belt legislation). Most
studies failed to consider systemwide effects of speed limit changes to
determine net safety effects. Any generalization of the results to the
United States, of course, must be mindful of differences in highway
networks, driving environment, and driving culture (there is more legal
high-speed driving in European countries).
SUMMARY
The potential adverse consequences of speeding, particularly the risks
imposed on others from an individual driver's speed choice, are suffi-
cient reason for regulating speed. Speed limits, one of the oldest
methods of managing speeds, are intended to enhance safety by
establishing an upper bound on speed to reduce both the probability
and the severity of crashes. They also have a coordinating function;
the intent is to reduce dispersion in driving speeds and thus reduce
the potential for vehicle conflicts.
Numerous methods are available for setting speed limits, ranging
from legislated limits on broad road classes, to limits in speed zones
determined on the basis of an engineering study, to limits established
by local ordinance on residential streets. Whatever method is used,
speed limits reflect implicit trade-offs among road user safety, travel
efficiency, and practicality of enforcement.
The trade-offs vary by roadway functional class and environment,
reflecting in part different levels of risk associated with driving on
different roadway types. Setting speed limits that give priority to
travel efficiency, for example, may be appropriate on rural freeways
where vehicles travel long distances under free-flowing traffic condi-
tions with little likelihood of conflict with other road users and where
the ability to enforce speed on extensive road mileage is limited. A
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MANAGING SPEED
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maximum speed limit is probably necessary, however, because of the
cause-and-effect relationship between high speeds and crash severity.
Speed limits that give priority to travel efficiency are less likely to be
appropriate in urban areas, where the roads must be shared with a broad
range of users, including vulnerable pedestrians and bicyclists, and
where roadside development increases opportunities for vehicle conflict
and raises the probability of an unexpected event. Because driver com-
pliance with urban speed limits has been poor, alternative methods for
managing and enforcing speeds may be necessary in these areas.
Most roads and roadway conditions fall between these extremes.
Appropriate use of speed zones can help establish speed limits suit-
able for conditions.
The effects of speed limits, particularly on safety, have been stud-
ied extensively. U.S. experience with raising speed limits on qualified
sections of rural Interstate highways in 1987 suggests that higher
speed limits resulted in higher average and 85th percentile speeds
and modest increases in speed dispersion. Higher speeds are linked
unequivocally with increased injury severity in a crash. Indeed, the
most methodologically sound studies found that higher speeds led to
increased fatalities and fatal crashes on rural Interstates in most
states. The studies were less clear about the absolute size of the safety
decrement, the extent and direction of any network effects, and the
role of enforcement in encouraging driver compliance with new
speed limits.
Preliminary data are available for speed and safety changes, pri-
marily on limited-access highways, in the first year following repeal
of the NMSL in 1995. Average and 85th percentile speeds rose less
than increases in the posted limit, reflecting, in part, poor driver
compliance with lower speed limits in effect before the change.
Speed dispersion increased in some states but not in others, in part
depending on what measure of speed dispersion was used.
Monitoring studies show some evidence of more high-speed driving
at levels that exceed the new speed limits, suggesting that at least
some drivers expect the same enforcement tolerance as at the lower
speed limits. Although the findings are not consistent across all
states, most studies indicated an increase in fatalities on highways
where speed limits were raised. Only one study examined possible
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Managing Speeds: Speed Limits
system effects, finding modest spillover effects. The results of these
studies must be considered preliminary because they are generally
based on 1 year of data or less.
The available studies of speed limit changes in speed zones on
nonlimited-access highways in the United States found little change
in speeds or crashes when speed limits were raised to near the 85th
percentile speed or lowered to a limit well below the 85th percentile.
Although methodological problems limit the generalization of study
results, the findings suggest the difficulty of changing driver behav-
ior merely by changing the sign.
Speed limits are likely to have more effect if the majority of driv-
ers perceive the limits as reasonable and related to the risks of driv-
ing, including risks to other road users. The increased probability of
a severe crash is sufficient reason to impose maximum speed limits
and direct enforcement at motorists who drive well in excess of the
speed limit. Speed limits alone, without enhanced enforcement or
innovative engineering measures, are insufficient to achieve compli-
ance with posted speed limits on many roads. In most cases, enforce-
ment is critical to ensure driver compliance. Appropriate
enforcement strategies--the subject of the next chapter--can help
persuade drivers that speed limits are in fact legal limits and not sim-
ply guidance on appropriate driving speeds.
REFERENCES
ABBREVIATIONS
ECMT European Conference of Ministers of Transport
ITE Institute of Transportation Engineers
NHTSA National Highway Traffic Safety Administration
NSC National Safety Council
RTI Research Triangle Institute
TRB Transportation Research Board
TRL Transport Research Laboratory
UVC Uniform Vehicle Code
Agent, K.R., J.G. Pigman, and J.M. Weber. 1997. Evaluation of Speed Limits in
Kentucky. KTC-97-6. Kentucky Transportation Center in cooperation with the
Kentucky Transportation Cabinet and the Federal Highway Administration. April,
90 pp.
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MANAGING SPEED
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Baerwald, J.E. 1953. Indiana Traffic Speeds 19421952. Joint Highway Research
Project. No. 73. Purdue University, Lafayette, Ind., March.
Baum, H.M., A.K. Lund, and J.K. Wells. 1989. The Mortality Consequences of
Raising the Speed Limit to 65 MPH on Rural Interstates. American Journal of
Public Health, Vol. 79, No. 10, Oct., pp. 13921395.
Baum, H.M., J.R. Esterlitz, P. Zador, and M. Penny. 1991a. Different Speed Limits
for Cars and Trucks: Do They Affect Vehicle Speeds? In Transportation Research
Record 1318, Transportation Research Board, National Research Council,
Washington, D.C., pp. 37.
Baum, H.M., J.K. Wells, and A.K. Lund. 1991b. The Fatality Consequences of the
65 MPH Speed Limits, 1989. Journal of Safety Research, Vol. 22, No. 4, Winter,
pp. 171177.
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Representative terms from entire chapter:
speed limit
