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Executive Summary
In 1995 Congress repealed the National Maximum Speed Limit
(NMSL) of 55 mph (89 km/h), returning to the states the responsi-
bility for setting speed limits on major highways. Since then, 49 state
legislatures have taken the opportunity to raise speed limits on
Interstate highways--and, in some cases, on other major roads--
often to levels that had been in effect before the NMSL was estab-
lished in 1974. Some states are reexamining methods for determining
appropriate speed limits. Several are monitoring the effects of
changes in speed limits on driving speeds and safety outcomes.
In this study current practice in setting speed limits on all roads--
not just major highways--is reviewed, and guidance to state and local
governments on appropriate methods of setting speed limits and
related enforcement strategies is provided. The study is intended for
a broad audience of those involved in decisions about speed limits--
state and local legislators, traffic engineers, and law enforcement and
judicial officials, as well as the interested general public.
OVERVIEW OF CURRENT PRACTICE
Speed limits are one of the oldest strategies for controlling driving
speeds. Connecticut imposed the first maximum speed limit of 8
mph (13 km/h) in cities in 1901. Since that time, primary responsi-
bility for setting speed limits has remained with state and local gov-
1
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MANAGING SPEED
2
ernments. Nationally mandated speed limits such as the NMSL are
exceptions to the rule.
The current framework for speed regulation was developed in
the 1920s and 1930s. Each state has a basic statute that requires driv-
ers to operate vehicles at a speed that is reasonable and prudent for
existing conditions. Speed limits are legislated by road class (e.g.,
Interstate highway) and geographic area (e.g., urban district). They
generally apply to all roads of a particular class throughout a jurisdic-
tion. However, state and most local governments have the authority to
change the limits by establishing speed zones for highway sections
where statutory limits do not fit specific road or traffic conditions, and
to determine alternative maximum speed limits in these zones.
Legislated speed limits are established by state legislatures, city
councils, or Congress on the basis of judgments about appropriate
trade-offs among public safety, community concerns, and travel effi-
ciency. Legislated limits are established for favorable conditions--
good weather, free-flowing traffic, and good visibility. Drivers are
expected to reduce speeds as these conditions deteriorate. Speed lim-
its in speed zones are determined administratively. The most com-
mon approach sets the limit on the basis of an engineering study,
which takes into consideration such factors as operating speeds of
free-flowing vehicles, crash experience, roadside development and
roadway geometry (e.g., curvature, sight distance), and parking and
pedestrian levels to make a judgment about the speed at which the
limit should be set. In many speed zones, it is common practice to
establish the speed limit near the 85th percentile speed, that is, the
speed at or below which 85 percent of drivers travel in free-flow con-
ditions at representative locations on the highway or roadway section.
This approach assumes that most drivers are capable of judging the
speed at which they can safely travel.
REGULATION OF DRIVING SPEEDS
If most drivers are assumed to be capable of making reasonable judgments
about appropriate driving speeds, why are speed limits even necessary?
The primary reason for regulating individual choices is the signif-
icant risks drivers can impose on others. For example, a driver with a
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Executive Summary
higher tolerance for risk may decide to drive faster, accepting a higher
probability of a crash, injury, or even death in exchange for a shorter
trip time. This driver's decision may not adequately take into consid-
eration the risk his choices impose on the 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 imposition of risks on others that
are not adequately considered when the activity of a person or a firm
affects their welfare is a primary reason for government intervention
in many areas besides traffic safety, such as environmental protection
and product safety.
Another reason for regulating speed derives from the inability of
some drivers to correctly judge the capabilities of their vehicles (e.g.,
stopping, handling) and to anticipate roadway geometry and roadside
conditions sufficiently to determine appropriate driving speeds. This
reason may not be as relevant for experienced motorists driving under
familiar circumstances. However, inexperienced drivers or experi-
enced drivers operating in unfamiliar surroundings may underesti-
mate risk and make inappropriate speed choices. Even drivers
familiar with a particular road can make inappropriate decisions
because of fatigue or other factors.
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 risks imposed on others and the adequacy of information
about appropriate driving speeds vary 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.
Moreover, under normal conditions, drivers typically 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. In contrast, the risks
imposed on others by individual driver speed choices may be large on
urban arterials where roadside activities are numerous and traffic vol-
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MANAGING SPEED
4
umes are high for extended periods of the day, increasing the proba-
bility of conflict with other road users. These differences are impor-
tant factors for consideration in setting appropriate speed limits on
different types of roads.
THE SAFETY CONNECTION AND THE ROLE
OF SPEED LIMITS
Drivers' speed choices impose risks that affect both the probability
and severity of crashes. Speed is directly related to injury severity in
a crash. The probability of severe injury increases sharply with the
impact speed of a vehicle in a collision, reflecting the laws of physics.
The risk is even greater when a vehicle strikes a pedestrian, the most
vulnerable of road users. Although injury to vehicle occupants in a
crash can be mitigated by safety belt use and airbags, the strength of
the relationship between speed and crash severity alone is sufficient
reason for managing speed.
Speed is also linked to the probability of being in a crash, although
the evidence is not as compelling because crashes are complex events
that seldom can be attributed to a single factor. Many driver attri-
butes and behavioral factors besides speed affect the probability of
crashes--driving under the influence of alcohol or other drugs, age,
attitudes toward risk, and experience of the driver--but speed has
been shown to play an important role.
The concept of speed itself is complex. It can relate to the speed of
a single vehicle or to the distribution of speeds in a stream of traffic.
Crash involvement on Interstate highways and nonlimited-access
rural roads has been associated with the deviation of the speed of
crash-involved vehicles from the average speed of traffic. Crash
involvement has also been associated with the speed of travel, at least
on certain road types. For example, single-vehicle crash involvement
rates on nonlimited-access rural roads have been shown to rise with
travel speed.
The primary purpose of speed limits is to enhance safety by reduc-
ing the risks imposed by drivers' speed choices. Speed limits enhance
safety in at least two ways. By establishing an upper bound on speed,
they have a limiting function; the objective is to reduce both the
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Executive Summary
probability and the severity of crashes. Speed limits also have a coor-
dinating function. Here the intent is to reduce dispersion in 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. A
related function of speed limits is to provide the basis for enforce-
ment and sanctions for those who drive at speeds excessive for con-
ditions and endanger others.
In setting speed limits, decision makers attempt to establish a rea-
sonable balance between risk (safety) and travel time (mobility) for a
road class or specific highway section. Thus, the posted speed limit
should inform motorists of maximum driving speeds under favorable
conditions that decision makers consider reasonable and safe for a
road class or highway section.
EFFECTIVENESS OF SPEED LIMITS
The principal objective of speed limits is improved safety, but sim-
ply posting a speed limit does not guarantee the desired change in
driving speeds or a reduction in crashes or crash severity. Recent
changes in speed limits in the United States provide an opportunity
to study these effects. In 1987 Congress allowed states to raise speed
limits from 55 to 65 mph (89 to 105 km/h) on qualifying sections of
rural Interstate highways. In the immediately following years, most
states that raised limits observed increases on the order of 4 mph
(6 km/h) in average speeds and 85th percentile speeds, and increases
in speed dispersion of about 1 mph (2 km/h). These speed changes
were generally associated with statistically significant increases
in fatalities and fatal crashes on the affected highways--a plausible
finding because of the strong link between even modest increases in
speed at higher speeds and increased crash severity. Although they
provided compelling evidence of higher fatalities on Interstate
highways, most studies did not examine the issue of broader network
effects, such as potential effects on safety from any traffic diversion
or redeployment of enforcement personnel. A more limited num-
ber of studies that attempted to look at such system effects reported
mixed results. One study that examined effects on non-Interstate
rural highways found evidence of spillover effects in higher fatalities
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MANAGING SPEED
6
on these roads. Two other studies that examined system effects on
a county- and statewide basis reported evidence of offsetting reduc-
tions in fatalities that resulted in neutral and even positive
systemwide net safety outcomes. Additional research and analysis
are needed to determine the extent and size of such systemwide
effects.
Studies have been conducted following repeal of federal maximum
speed limits in 1995; many of them focused on Interstate highways.
Most found results similar to the speed limit changes in 1987: mod-
est increases in average speeds and 85th percentile speeds and, in
some cases, speed dispersion on highways on which speed limits were
raised. Although not consistent across all states, most studies indi-
cated an increase in fatalities on highways on which speed limits were
raised. Most studies did not explore any possible system effects, and
the results should be considered preliminary because they are gener-
ally based on 1 year of data or less.
Most of the recent U.S. literature has focused on the effects of rais-
ing speed limits on Interstate highways. In the future, however, cir-
cumstances could warrant reductions in speed limits on some
Interstates and other major highways. An earlier Transporta-
tion Research Board study (1984) of the effects of the national
55-mph (89-km/h) speed limit found that the lower limit reduced
both travel speeds and fatalities, although driver speed compliance
gradually eroded. The report provides a comprehensive review of
studies that examined the effects of lowering speed limits on major
highways.
In contrast to the extensive analysis of speed and safety changes on
Interstate highways, few studies have examined the effects of chang-
ing speed limits on lower-speed, nonlimited-access highways. Those
that were identified found little effect on driving speeds or crash rates
when speed limits were raised to near the 85th percentile speed or
lowered to near the 35th percentile speed in selected speed zones
on rural roads and on urban and suburban arterials. The results,
however, cannot be generalized to speed zones on all nonlimited-
access highways. Further, the lack of observed changes in driving
speeds may be explained to the extent that changes in posted speed
limits simply legalized existing driver behavior, that is, changed
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Executive Summary
compliance levels rather than speed behavior. Nevertheless, the find-
ings suggest the difficulty of altering behavior merely by changing
the sign.
ROLE OF ENFORCEMENT AND SANCTIONS
Managing speeds through speed limits requires a system of speed
laws and a process for establishing reasonable speed limits as well as
enforcement, sanctions, and public education, ideally all working
together. Enforcement is an integral part of such a system. Even if
reasonable speed limits are established through legislation or engi-
neering studies and most drivers comply within a small tolerance,
enforcement is still necessary to ensure the conformity of a minority
of drivers who will obey traffic regulations only if they perceive a
credible threat of detection and punishment for noncompliance.
The main difficulty with the traditional approach to speed
enforcement--radar enforcement using a mobile or stationary police
vehicle--is its short-lived temporal and spatial effect on deterring
speeding. Maintaining the deterrence effect requires a level of
enforcement intensity and expense that has proven difficult to sustain
because of competing enforcement priorities and limited resources
available for speed enforcement.
Targeted enforcement combined with focused publicity campaigns
can boost the effectiveness of traditional enforcement methods.
Automated enforcement, particularly photo radar, has been shown to
be efficient and effective where it has been used for speed control,
particularly on high-volume arterials. Photo radar could also be cou-
pled with variable speed limit systems on urban Interstate highways
where high traffic volumes can make traditional enforcement meth-
ods hazardous. Alternatives to enforcement to achieve desired driv-
ing speeds on local roads include physical measures known as "traffic
calming" (e.g., speed humps, roundabouts, and raised intersections).
Redesigning roads to achieve greater congruity between driver per-
ceptions of appropriate travel speeds and the cues provided by the
road itself (e.g., narrowing lanes) may also influence motorists'
speeds. A proper mix of these approaches can enable police to lever-
age their resources and deploy them efficiently.
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MANAGING SPEED
8
Traffic court judges are also important participants in effective
speed enforcement. They may overturn speeding violations if they
think the speed limits are unreasonable or reduce fines if they believe
the sanctions are too harsh. If judges are lenient in their treatment
of speeding offenses and routinely dismiss speeding citations, the
incentive for the police to enforce the speed limits may be reduced.
Thus it is important that traffic court judges--as well as the police
and motorists--perceive that speed limits are reasonable and
enforceable.
GUIDANCE
On the basis of its review of the purpose and methods of setting and
enforcing speed limits, the committee offers the following guidance to
responsible decision makers. Its primary focus is on the effects of speed
limit policies on safety rather than on travel time, energy consump-
tion, or environmental pollution. The committee attempted to be as
specific as possible, but the relevant studies and the data on which the
guidance is based fall short of providing sufficient support for quanti-
fying with much precision the effects of changes in speed limits on
driving speeds and safety.
General Framework for Establishing Speed Limits
The current general approach--legislated speed limits and administra-
tively established speed zones--is sound. It balances the desirability of
uniform speed limits (legislated limits for broad road classes) with the
need for exceptions (speed zones) to reflect local differences. The practice
of establishing speed limits to reflect a reasonable balance between travel
speeds and risks under favorable operating conditions is also sensible.
Making Decisions About Appropriate Speed Limits
Decisions about legislated speed limits reflect trade-offs and judg-
ments about the relative importance of safety, travel time, and feasi-
bility of enforcement. Legislators should consult with traffic
engineers, law enforcement officials, judges, public health officials,
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Executive Summary
and the general public in their deliberations. Consultation, however,
cannot ensure that all parties will reach consensus, particularly where
multiple interests are involved, such as residents and commuters on
residential streets. In addition to safety, final selection of a speed limit
should meet the requirements of enforceability and acceptance by the
community at large. Provision should also be made to monitor driv-
ing speeds and crash experience, and the decision should be reviewed
periodically because road conditions, vehicle safety features, driving
attitudes, and behavior change over time.
Determination of appropriate speed limits in speed zones should
be made on the basis of an engineering study. Traffic engineers nor-
mally conduct the study; consultation with law enforcement officials
should be standard practice. Elected officials and citizen groups may
also become involved when community concerns have been raised
about driving speeds. Speed zones should be reviewed periodically--
with greater frequency where conditions are changing rapidly, such as
developing suburban areas--to determine whether a change in speed
limits or boundaries of the speed zone is warranted.
Methods of Setting Speed Limits
Legislated Speed Limits
The strong link between speed and crash severity supports the need for
setting maximum speed limits on high-speed roads (e.g., Interstate
highways, freeways, high-speed rural multi- or two-lane roads) to place
an upper bound on travel speeds to reduce crash probability and sever-
ity. The committee refrained from recommending a specific numeric
limit, however. Road conditions vary too widely to justify a "one-size-
fits-all" approach. Roads, even those in the same class, are not all built
to the same design standards, nor are traffic volumes uniform.
In determining appropriate speed limits for each road type, deci-
sion makers should be guided by both the likely risks imposed on
others by individual driver speed choices and the availability of infor-
mation to enable drivers to make appropriate speed choices. They
should take enforcement practicality into consideration. Decision
makers should also request technical information on the following
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MANAGING SPEED
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four factors to help guide their determination of appropriate legis-
lated speed limits for a specific road class:
· Design speed, that is, the design speed of a major portion of the
road, not of its most critical design features (e.g., a sharp curve);
· Vehicle operating speed, measured as a range of 85th percentile
speeds taken from spot-speed surveys of free-flowing vehicles at
representative locations along the highway;
· Safety experience, that is, crash frequencies and outcomes; and
· Enforcement experience, that is, existing speed tolerance (i.e.,
allowance for driving above the posted speed limit) and level of
enforcement.
The weight given to these factors, particularly those related to
speed, depends on the type of road. For example, on many rural
Interstate highways, vehicle operating speeds should be a major fac-
tor in setting speed limits. Design speeds provide little additional
information because restrictive design features are not generally pres-
ent on these highways; typically drivers can anticipate conditions and
determine appropriate driving speeds. In addition, risks to other road
users are small compared with other highways. Finally, maintaining
high levels of enforcement is difficult on long stretches of rural
Interstate. In contrast, design speeds should carry greater weight in
the determination of speed limits on nonlimited-access rural roads
where restrictive roadway geometry is likely to play an important role
in defining an appropriate driving speed. Poor safety records on these
roads support lower speed limits, but the limits must be reasonable
for conditions; enforcement is limited because of extensive rural road
mileage.
Safety and enforcement considerations should be given higher pri-
ority than design speeds or vehicle operating speeds on many urban
roads, particularly residential streets. Intersections and traffic signals
play a more critical role than design in limiting speed. Driver mis-
judgment about appropriate driving speeds poses high risks to vul-
nerable road users (e.g., pedestrians and bicyclists) on many urban
roads. Neighborhood pressures may result in setting very low speed
limits on residential streets, but often they are not enforced--or
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Executive Summary
enforcement tolerances are large--and compliance is poor even by
some neighborhood residents. Thus, where low speeds are desirable,
speed limits must be enforced, or alternatives such as traffic calming
should be considered for certain residential streets. More detailed
guidance for each of seven road classes plus one category for special
speed zones (e.g., school and work zones) is provided in Chapter 6.
Speed Limits in Speed Zones
Determination of appropriate speed limits in speed zones should be
made on the basis of an engineering study. The most common factor
considered in setting speed zone limits is the 85th percentile traffic
speed. Setting the speed limit at or near this level can be desirable on
some roads because it (a) enables the police to focus their enforce-
ment efforts on the most dangerous speed outliers and (b) is gener-
ally at the upper bound of a speed range where crash involvement
rates are lowest 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. Potential safety benefits may
not be realized on roads with a wide range of speeds (i.e., the spread
between the slowest and fastest drivers). Basing the speed limit on a
measure of unconstrained free-flowing travel speed is not appropri-
ate for urban roads with a mix of road users and high traffic volumes
and levels of roadside activity. Traffic engineers should consider an
expert-system approach, discussed in Chapter 3, which offers a sys-
tematic and consistent method of determining speed limits in these
speed zones.
Differential Speed Limits
No conclusive evidence could be found to support or reject the use of dif-
ferential speed limits for passenger cars and heavy trucks. More research
and evaluation of the effects of differential speed limits on driving speeds
and safety outcomes are needed in states that have adopted them.
Motorists do not appear to decrease speed at night when lower
nighttime speed limits are in effect. However, compelling evidence
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MANAGING SPEED
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could not be found to support the elimination of nighttime speed
limits in states that have adopted them.
Variable Speed Limits
Technology is available to support speed limits that change with con-
ditions, but more experimentation and evaluation 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 restricts their use to Interstate highways and freeways with
high traffic volumes or to selected segments of major roads where
weather (e.g., fog, visibility) is a frequent problem. Once their effec-
tiveness is more clearly established, broader application of variable
speed limit systems should be considered on urban Interstate high-
ways in the United States because they are well suited to addressing
temporal changes in traffic volumes, speed, and density on these
highways.
Enforcement and Other Speed Management Strategies
Policy makers can affect the level of enforcement through resource
allocation, but they must recognize that if drivers believe that a speed
limit is unreasonable, enforcement will be difficult and expensive. If
a low speed limit is posted on a road designed for higher speeds,
enforcement problems will be considerable. This occurred on many
Interstate highways under the NMSL. When speed limits were
raised by 10 mph (16 km/h) on sections of qualified rural Interstate
highways in 1987, average traffic speeds increased much less than the
change in the speed limit immediately following the change.
Apparently many drivers were already exceeding the old speed limits
because speeds had crept up since the NMSL went into effect.
Strategic deployment of traditional enforcement methods on roads
and at times when speed-related incidents are most common or
where road conditions are most hazardous can help focus resources
on potential problems. The relative infrequency of crashes, however,
can make it difficult to show systematic safety improvements from
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Executive Summary
targeted enforcement strategies. Planned patrols at varying times and
locations can extend deterrence effects temporally and spatially from
particular locations, but only after an initial period of continuous
enforcement. Patrols must be visible and sufficiently frequent to cre-
ate a credible deterrent. Police can improve compliance by combining
enforcement initiatives with high-profile public information cam-
paigns. Publicity must be followed up by enforcement actions, how-
ever, if the approach is to successfully deter speeding. Changing
fundamental attitudes about speeding requires a long-term sustained
effort.
Automated enforcement, particularly photo radar, can be used to
complement traditional enforcement methods, particularly where
roadway geometrics or traffic volume makes traditional methods dif-
ficult or hazardous. Photo radar is controversial. Its successful intro-
duction requires adoption of legal changes (e.g., resolution of
constitutional privacy issues, vehicle owner versus driver liability),
funding, and public education. It should be deployed selectively at
first--at locations that are hazardous and difficult to patrol with tra-
ditional methods and where speeding is a problem--to ensure essen-
tial public support. 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.
Speed limits alone will not be effective in all situations. Keeping
driving speeds at desired levels in urban areas poses a particular chal-
lenge. Traffic calming can be used judiciously on residential streets,
but community as well as resident support is important for its suc-
cess. Systemwide effects must also be considered so that the traffic
and safety problems will not simply migrate to other streets. Road
redesign has the potential to achieve greater consistency between
desired and actual operating speeds. Unfortunately, strict application
of current roadway design procedures does not ensure speed consis-
tency. Because of the size of the U.S. road network and the pace of
rehabilitation, road redesign is a long-term strategy, and more under-
standing concerning the overall safety benefits of alternative designs
is needed. The approach can yield satisfactory solutions, but addi-
tional study of the relationships between operating speeds and road-
way geometric elements is necessary.
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CONCLUDING COMMENT
Most states chose to raise speed limits on major highways following
repeal of the NMSL. The effects of their decisions on driving speeds
and safety outcomes, in particular, should be closely monitored.
Efforts to mitigate any adverse safety effects through enforcement
should be redoubled, and initiatives to promote safety belt use and
reduce driving while intoxicated--measures with large and proven
safety benefits--should be continued.
Technological advances may offer additional techniques for con-
trolling driving speeds on all types of roads. For example, technology
can help establish limits that are more sensitive to actual changes in
road conditions and thus provide drivers with better information.
Such technology can be installed in the vehicles and highways of the
future to monitor and control speed. Finally, it can help improve the
efficiency and effectiveness of enforcement. Further development,
experimentation, and evaluation are needed for many technologies to
realize their potential.
The issue of appropriate driving speeds, however, will persist as
long as there are individual drivers making choices about risk and
time efficiency. Ultimately, decisions about appropriate speed limits
depend on judgments about society's tolerance for risk, valuation of
time, and willingness to police itself. These judgments, in turn,
should be reviewed periodically in the light of changes in vehicles
and highway conditions and shifts in public perceptions of safety and
attitudes toward risk.
REFERENCE
ABBREVIATION
TRB Transportation Research Board
TRB. 1984. Special Report 204: 55: A Decade of Experience. National Research Council,
Washington, D.C., 262 pp.
Representative terms from entire chapter:
driving speeds
