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2
Effects of Speed
The minutes
Some folks
Save through speed
They never even
Live to need
Burma Shave (Rowsome 1965)
The major reason for managing traffic speeds is safety. In this chap-
ter, what is known about the relationships among speed, crash inci-
dence, and crash severity is reviewed. Individual driver decisions
about appropriate travel speeds, however, are guided by more than
safety considerations. Thus, the relationship of speed to travel time,
fuel use, and other vehicle operating costs is also examined. In addi-
tion, driver decisions about speed affect other costs, such as vehicle
emissions, which contribute to air pollution in metropolitan areas
and to atmospheric changes that may increase the risk of global cli-
mate change. These costs, which are briefly reviewed, are borne
36
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37
Effects of Speed
largely by society as a whole rather than by individual drivers, at least
in the United States. The chapter concludes with an assessment of
the effects of speed on safety, travel time, and other related costs, and
their implications for managing speed.
DETERMINATION OF APPROPRIATE DRIVING
SPEEDS--MAKING TRADE-OFFS
How do people decide how fast to drive? Many factors come into
play including the characteristics of the road; the amount of traffic on
the road; weather conditions and time of day; the speed limit and its
enforcement; the length and purpose of the trip; the vehicle's operat-
ing characteristics, such as handling and stopping as well as fuel con-
sumption and emissions; and driver-related factors, such as the
propensity to take risks and the pleasure associated with driving fast.
Taking these and other factors into consideration, drivers face an
important trade-off between travel time and safety. By driving faster,
travel time is reduced and the destination is reached sooner if the trip
is safely completed. However, as discussed later in this chapter, a
driver who chooses to drive very fast relative to other traffic or very
fast for existing road conditions may increase the probability of being
involved in a crash as well as the severity of the crash. A driver can
reduce crash probability and severity by driving more slowly,
although driving too slowly relative to other traffic may also increase
the probability of crash involvement.
The theory underlying the trade-off between travel time and
safety is discussed in more detail in Appendix A. Conceptually the
trade-off is straightforward, but practically one could question
whether drivers really trade off safety and travel time when making
their trips. Some drivers indicate that this trade-off is not foremost
in their mind while traveling; others claim that they are not conscious
of making this trade-off.
For some drivers in many situations, the choice of driving speed is
heavily influenced by speed limits and their enforcement so that the
trade-off is, in a sense, made for them. But even in situations where
there is little or no speed limit enforcement and many drivers exceed
the posted speed limit, few motorists will drive as fast as their vehi-
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MANAGING SPEED
38
cles are capable of going. Something other than the fear of speed
limit enforcement causes drivers to drive at less than the maximum
possible speed. Similarly, when weather conditions such as fog, rain,
or snow cause visibility to deteriorate and traction to be reduced,
drivers may slow down, often to speeds well below the posted limits.
For many drivers faced with these conditions, their choice of a lower
speed and increased travel time is almost certainly made with safety
in mind. There is reason to believe, therefore, that where speed choice
is not constrained by speed limits and their enforcement, the driver
does trade off travel time and safety.
Even when visibility and weather conditions are good, drivers may
still make trade-offs. Rather than making them continuously, how-
ever, they may rely on rules of thumb based on driving experience.
For example, motorists may well rely on experience with particular
roads or types of roads to select a driving speed that has proven to be
a reasonable trade-off for them in the past. Only when they
encounter new conditions or conditions they face infrequently would
they be conscious of explicitly making such a trade-off. In this chap-
ter what is known about the key factors affecting drivers' choice of
speeds is reviewed.
RELATION OF SPEED TO SAFETY
The relation of driving speed to safety is investigated first because of
the importance that most drivers place on completing their trips
safely. The link between speed and safety is complex. Thus an in-
depth review of the literature on this topic was commissioned to help
shed light on the relationship of speed to crash causation and injury
severity. The results of that review, which can be found in its entirety
as Appendix B, are summarized in the following sections.1
1 These sections also draw on a second review, discussed more extensively in Chapter 3
and presented in its entirety as Appendix C.
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39
Effects of Speed
Speed and the Probability of Crash Involvement
One of the more widely cited sources of statistics on speed and
crashes is the Fatal Analysis Reporting System (FARS) administered
by the National Highway Traffic Safety Administration (NHTSA),
the federal agency charged with regulating automotive safety. In 1996
NHTSA reported that speeding was a contributing factor in 30 per-
cent of all fatal crashes on U.S. highways in that year (NHTSA
1997a, 1). In addition to the 13,000 lives lost in these speeding-
related crashes, 41,000 people were reported critically injured at an
estimated economic cost to society of nearly $29 billion (NHTSA
1997a, 1).2 Thus speeding is singled out as "one of the most preva-
lent factors contributing to traffic crashes" (NHTSA 1997a, 1).
These figures must be interpreted with caution. The definition of
speeding is broad; for the purposes of coding crash-related informa-
tion, speeding is defined as "exceeding the posted speed limit or driv-
ing too fast for conditions" (NHTSA 1997a, 1). The determination
of whether speeding was involved in a fatal crash is based on the
judgment of the investigating police officer; fatal crashes receive a
thorough investigation.3 Even if speeding is listed as a contributing
factor in a crash, it may not have been the primary cause.
Furthermore, and perhaps most important, without knowledge of the
incidence of speeding in the driving population, the fatal crash data
2 Economic costs include productivity losses, property damage, medical costs, rehabil-
itation costs, travel delay, legal and court costs, emergency service costs, insurance
administration costs, premature funeral costs, and costs to employers. They do not
include any estimate of the value of lost quality of life associated with deaths and
injuries, that is, what society is willing to pay to prevent them.
3 To ensure reporting consistency, FARS analysts, who are state employees contracted
and trained by NHTSA, retrieve information about the crash from the police crash
report and other sources and put it in a standardized coding format. For each crash,
information is recorded at four levels--by crash, vehicle, driver, and person. Speed
appears in two places--(a) on the crash-level coding sheet where the speed limit is
recorded, and (b) on the driver-level coding sheet where speed-related violations are
recorded. Typical violations, noted in the 1996 FARS Coding Manual, include driving
at a speed greater than reasonable or prudent or in excess of the posted maximum, tow-
ing a house trailer at more than 45 mph (72 km/h), or driving too slowly so as to
impede traffic.
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MANAGING SPEED
40
cannot be properly interpreted. For example, a recent study suggests
that driver compliance with posted speed limits is poor, particularly
for limits less than 45 mph (72 km/h) on nonlimited-access highways
(Parker 1997, 43). The proportion of those driving above the posted
speed limit--hence "speeding" by NHTSA's definition--typically
exceeds the share of speeding drivers (approximately 20 percent
according to FARS) involved in fatal crashes.4 The literature review
attempts to examine the evidence that speeding is linked to the prob-
ability of being involved in a crash.
Theoretical Issues
At least three theoretical approaches link speed with crash involve-
ment: (a) the information processing approach, (b) the traffic conflict
approach, and (c) the risk-homeostasis motivational approach.
The first approach views the driver as an information processor
with a limited capacity to process information. As driving speed
increases, the rate at which the driver must process information about
the highway and its environment increases directly, even though the
total amount of information the driver has to process may stay con-
stant. At higher speeds there is less time for the driver to process
information, decide, and act between the time the information is pre-
sented to the driver (e.g., a child is running into the road) and the
time when action must be taken to avoid a crash.5 A crash is likely to
occur when the information processing demands exceed the atten-
tional or information processing capabilities of the driver (Shinar
1978).6 Unexpected events dramatically increase information pro-
4 Note that the 20 percent figure refers to the share of drivers involved in speeding-
related fatal crashes as a percentage of drivers involved in all fatal crashes, whereas the
30 percent figure cited earlier refers to the share of speeding-related fatal crashes as a
percentage of all fatal crashes.
5 More specifically, as speed increases, the distance covered during the driver's percep-
tion-reaction time and the minimum distance required for braking both increase. For
a vehicle on a level roadway, minimum braking distance increases with the square of
the speed (see glossary definition of braking distance).
6 Although drivers can increase their level of attention and concentration with increasing speed,
a heightened level of attention cannot be maintained for long periods because it is fatiguing.
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41
Effects of Speed
cessing requirements and hence the probability of a crash. This
approach leads to the conclusion that "speed kills"; as more drivers
increase their speed, the probability of information overload increases
along with the potential for crashes.
The second approach--the traffic conflict approach--assumes
that crash probability is related to the potential for conflict among
vehicles traveling in traffic. More specifically, the probability of an
individual driver being involved in a multiple-vehicle crash increases
as a function of the deviation of that individual driver's speed from
the speeds of other drivers. Drivers with speeds much faster or much
slower than the median traffic speed are likely to encounter more
conflicts (Hauer 1971).7 This relationship leads to the conclusion
that "speed deviation kills" and the prediction that on roads with
equivalent average traffic speeds, crash rates will be higher on roads
with wider ranges of speed. The theory, as formulated, relates only to
two-lane rural roads (Hauer 1971, 1).
A third approach--the risk-homeostasis motivational approach--
looks at speed and crash involvement from the perspective of driver
perception of risk. From this point of view, drivers are neither passive
information processors nor reactors to potential traffic conflicts.
Rather they adjust their speed according to the risks they perceive
(Taylor 1964) to maintain a subjectively acceptable level of risk
(Wilde et al. 1985).8 The issue is not the link between speed and
crash probability but between actual and perceived risk. Thus, driving
7 The number of conflicts between vehicle pairs is represented by the number of pass-
ing maneuvers. The number of passing maneuvers a driver must make increases with
his driving speed; the number of times a driver is passed by other vehicles increases as
he reduces speed. Hauer (1971) showed that the distributions of the two functions
(i.e., the number of times passing and the number of times being passed) have a min-
imum at the median traffic speed. The findings relate only to rural roads between
intersections (Hauer 1971, 1).
8 There is mixed empirical support for Wilde's risk-homeostasis theory. For example,
Mackay (1985) found that British drivers of newer and heavier cars drove faster than
drivers of older and lighter cars (with the exception of sport cars), and Rumar et al.
(1976) found that drivers with studded tires drove faster than those without such tires
on curves in icy but not in dry conditions. O'Day and Flora (1982), however, found
that restrained occupants actually had lower impact speeds in tow-away crashes than
unrestrained occupants, suggesting that drivers have different risk tolerances.
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MANAGING SPEED
42
at high speeds per se is not dangerous. Rather, the danger comes from
driving at a speed inappropriate for conditions, stemming from a
misperception of the situational demands or a misestimation of the
vehicle's handling capabilities or the driver's skills. This approach
would predict that, under most circumstances, drivers who increase
speed do not necessarily increase the risk of their crash involvement.
Review of Empirical Data
Several studies reviewed in this section (Table 2-1), many dating
back to the 1960s, have tested the theories about the relationship
between driving speed and crash involvement by analyzing actual
vehicle speeds and crash data on different classes of roads. Speed is
defined in several ways in these studies. It can relate to the speed of
a single vehicle or to the distribution of speeds in a traffic stream. In
the former case, the term speed deviation is used when referring to
the deviation of an individual driver's speed from the average speed
of traffic. In the latter case, when referring to the distribution of
speeds in a traffic stream, three measures of speed are typically con-
sidered: the average speed, the 85th percentile of the speed distribu-
tion, and the dispersion in travel speeds. Speed dispersion, in turn,
can be quantified by the variance, standard deviation, 10-mph pace,
or range (high minus low) of a sample of speed measurements.9 In
many studies, the standard deviation is approximated as the 85th per-
centile speed less the average speed.10
With several measures of speed, interpreting the results of these
studies is often difficult. Validity of the speed measures can also be a
problem. For example, it is nearly impossible to obtain a reliable mea-
sure of true precrash speeds for crash-involved vehicles because
crashes are not planned events. Thus, precrash speeds must be esti-
mated, but there is no way of validating their accuracy. In attempting
to isolate the effect of speed, many studies assume that everything
else remains equal. Of course, crash occurrence and injury severity are
9See definitions in glossary.
10The 85th percentile minus the average speed roughly corresponds to one standard
deviation (S), which is the positive square root of the variance (S2).
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Table 2-1 Selected Studies of the Relationship Between Speed and Crash Probability
Authorship
and
Date of Road Class and
Study Speed Limit Levels Analysis Major Findings
Solomon Compared speeds of crash- Found U-shaped relationship between crash involvement and
Main rural roads, U.S.;
(1964) involved vehicles with speeds travel speeds. Lowest crash involvement rates at speeds
three-fourths were two-
of non-crash-involved vehi- slightly above average travel speeds. Highest crash involve-
lane rural roads with
cles ment rates at speeds well above and well below average traf-
speed limits of 55 to
fic speeds
70 mph (89 to 113
km/h) on 28 out of 35
sections
Cirillo Compared speeds of crash- Same finding as Solomon, but crash involvement rates were
Rural and urban Interstate
(1968) involved vehicles with speeds lower for all travel speeds suggesting importance of roadway
highways, U.S.; no
of non-crash-involved vehi- geometry to crash probability (i.e., higher design standards
speed limits given
cles; limited to daytime on Interstate highways than on rural two-lane roads)
travel and certain multiple-
vehicle crash types (i.e., rear-
end and angle collisions and
same-direction sideswipe
crashes)
RTI Compared speeds of crash- Found same U-shaped relationship between travel speed and
State and county high-
(1970); involved vehicles with speeds crash involvement, but the relationship was less extreme,
ways in Indiana with
West and of non-crash-involved vehi- particularly at low speeds, when crashes involving turning
speed limits greater
Dunn cles; separated out crashes vehicles were removed from the analysis
than or equal to 40
(1971) involving turning vehicles
mph (64 km/h)
(continued on next page)
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Table 2-1 (continued)
Authorship
and
Date of Road Class and
Study Speed Limit Levels Analysis Major Findings
Lave Six U.S. highway types-- Analyzed relationship between Speed dispersion significantly related to fatality rates for rural
(1985) rural and urban average traffic speed, speed Interstates and rural and urban arterials. After controlling
Interstates, arterials, and dispersion (measured as 85th for speed dispersion, average traffic speed not significantly
collectors; data from 50 percentile speed minus 50th related to fatality rates for any road type
states percentile speed), and two
nonspeed measures--traffic
citations per driver and
access to medical care--on
fatality rates
Garber Higher-speed roads [i.e., Analyzed relationship between Crash rates increased with increasing speed variance on all road
and with average traffic crash rates and average traf- classes. No correlation between crash rates and average traf-
Gadiraju speeds of 45 mph (72 fic speed, speed variance, fic speeds when data were disaggregated by road class
(1988) km/h) or above], design speed, and posted
including rural and speed limits
urban Interstates,
expressways and free-
ways, rural and urban
arterials, and rural col-
lectors in Virginia
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Harkey Rural and urban roads Compared speeds of crash- Found same U-shaped curve as Solomon and Cirillo; crashes
et al. with posted speed lim- involved vehicles with speeds limited to weekday, nonalcohol, nonintersection involve-
(1990) its of between 25 and of non-crash-involved vehi- ments
55 mph (40 and 89 cles
km/h) in North
Carolina and Colorado
Fildes et al. Two urban arterials with Compared free-flowing travel Found no evidence of Solomon's U-shaped relationship. Those
(1991) speed limits of 37 mph speeds and self-reported crash traveling at very fast speeds were more likely to report previ-
(60 km/h) and two rural histories of drivers who par- ous crash involvement than those traveling at slower speeds.
undivided roads with ticipated in a road safety sur- Self-reported crash involvements were lowest for those trav-
speed limits of 62 mph vey eling at speeds below average traffic speeds and highest at
(100 km/h), Australia speeds above the average with no advantage at the average
Baruya Urban roads with average Analyzed relationship between Both speed level and speed dispersion affected crashes.
and traffic speeds ranging personal injury crashes, speed Increased crashes were associated with increasing average
Finch from 21 mph (33 km/h) levels, and speed dispersion, traffic speeds. Decreased crashes were associated with reduc-
(1994) to 33 mph (53 km/h), defined as the coefficient of tions in speed dispersion at increasing speeds. The net
Great Britain variation of the speed distri- effect, however, was an increase in personal injury crashes
bution with increasing speeds
Kloeden et Speed zones with 37-mph Compared speeds of casualty Found statistically significant increase in probability of
al. (1997) (60-km/h) speed limits crash-involved vehicles with involvement in a casualty crash with increasing travel speed
in metropolitan speeds of control vehicles above, but not below, the speed limit. Probability of crash
Adelaide, Australia traveling in the same direc- involvement at speeds below the speed limit was not statisti-
tion, at the same location, cally different from traveling at the speed limit
time of day, day of week, and
time of year under free-flow-
ing traffic in daylight and
good weather
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MANAGING SPEED
46
influenced by other driver behaviors (e.g., drinking, not using safety
belts) and characteristics (e.g., age), vehicle characteristics (e.g., size
and weight), and road design (e.g., limited- or nonlimited-access
highways). To the extent that these contributory variables are not
taken into account, results of the studies must remain highly quali-
fied.
Correlational Studies
This category of studies attempts to determine whether there is a link
between speed and crash probability. In the benchmark study con-
ducted by Solomon (1964), travel speeds of crash-involved vehicles
obtained from police reports were compared with the average speed
of free-flowing traffic on two- and four-lane, nonlimited-access rural
highways. Solomon found that crash-involved vehicles were overrep-
resented in the high- and low-speed areas of the traffic speed distri-
bution. His well-known U-shaped curve (Figure 2-1) showed that
crash involvement rates are lowest at speeds slightly above average
traffic speeds. The greater the deviation between a motorist's speed
and the average speed of traffic--both above and below the average
speed--the greater the chance of involvement in a crash. The corre-
lation between crash involvement rates and deviations from average
traffic speed gave rise to the often-cited hypothesis that it is speed
deviation, not speed per se, that increases the probability of driver
involvement in a crash. Hauer's subsequent theory of traffic conflict
(1971) provided a theoretical basis for Solomon's findings.11
Solomon's U-shaped relationship was replicated by Munden
(1967) using a different analytic method on main rural roads in the
United Kingdom, by Cirillo (1968) on U.S. Interstate highways
11 Some have interpreted these results to suggest that it is as unsafe to drive below as
above the average traffic speed. This ignores the fact that drivers involved in a crash at
higher speeds are at greater risk of injury than those driving at lower speeds, a rela-
tionship that Solomon confirms in his analysis of the relation between speed and crash
severity (see subsequent section).
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MANAGING SPEED
66
noninjured vehicle occupants, 17 percent of occupants sustaining inca-
pacitating injuries, and 34 percent of fatally injured occupants were
involved in speeding-related crashes (Bowie and Walz 1994, 34).
The relationship between speed and crash severity is perhaps most
dramatically demonstrated for vehicle crashes with pedestrians, the
most vulnerable road users. The study of vehicle-pedestrian crashes
in Helsinki (Pasanen and Salmivaara 1993) showed that the risk of
death for a pedestrian increased rapidly from very low speeds (15
mph or 24 km/h) to about 50 mph (80 km/h), where death was
almost certain (Pasanen and Salmivaara 1993, 308). A European
review of several studies of vehicle-pedestrian crashes confirmed
these results. It concluded that 5 percent of pedestrians are likely to
die if they are struck by a vehicle traveling at 20 mph (32 km/h) and
that risk levels rise sharply with speed--to a 45 percent probability of
fatality for the pedestrian at 30 mph (48 km/h) and an 85 percent
probability of fatality at 40 mph (64 km/h) (ETSC 1995, 11).
In summary, all of the studies that have investigated the link
between vehicle speed and injury severity have found a consistent
relationship. As driving speed increases, so does the impact speed of
a vehicle in a collision. Increased impact speed, in turn, results in a
sharp increase in injury severity because of the power relationship
between impact speed and the energy released in a crash.
RELATIONSHIP OF SPEED TO TRAVEL TIME
In addition to safety, travel time is a major factor affected by speed
that influences drivers' choice of an appropriate driving speed. The
importance and cost of travel time as a function of speed were amply
illustrated by the recent experience of the 55-mph (89-km/h)
National Maximum Speed Limit (NMSL). A review of the NMSL
(TRB 1984) estimated that in 1982 motorists were spending about 1
billion extra hours on highways posted at 55 mph because of slower
driving speeds compared with speeds on these highways in 1973, the
year before the NMSL was enacted (TRB 1984, 120). Most of this
additional travel time was expended by passengers in personal vehi-
cles (TRB 1984, 119). Frequently it involved small increments in
travel time for individual trips.
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67
Effects of Speed
Of course, any analysis of the time cost of travel has to take into
account the cost savings from reduced crashes and averted fatalities
and serious injuries from driving at lower speeds. When travel time
costs were compared with estimated lives saved and serious injuries
averted by the 55-mph (89-km/h) travel speed, the time cost worked
out to about 40 years of additional driving time per life saved and
serious injury avoided (TRB 1984, 120). The average remaining life
expectancy of motor vehicle crash victims in 1982 was about 41 years.
Thus, the number of years of extra driving time closely approximated
the number of years of life saved.44 Although the study committee
concluded that making a comparison between the value of a year of
life and the value of a year of driving time was not meaningful, it did
provide one framework for assessing the central trade-off between
travel time and safety involved in the decision to retain or relax the
55-mph speed limit (TRB 1984, 120).
Travel time costs are not equally distributed either by road type or
road user. For example, the 55-mph (89-km/h) NMSL exacted the
highest travel time costs for users of rural Interstate highways. At the
time of the introduction of the NMSL, these highways had the high-
est speeds, among the lowest crash rates, and carried the majority of
long-distance travel, particularly commercial travel. Lowering speeds
on these roads was estimated to cost motorists and truckers alike 100
years of additional driving per life saved--about four times as much as
on all other affected roads (TRB 1984, 123). The travel time costs to
motorists on other road classes were estimated to have much smaller
effects, in part a reflection of the role of congestion and roadway
geometry in limiting travel speeds on these nonlimited-access high-
ways.45 Given these results, it was not surprising that the relaxation of
the NMSL first occurred on rural Interstate highways.
44 A more recent analysis of the time-safety trade-off of raising speed limits on quali-
fied sections of rural Interstate highways in 1987 found that the 65-mph (105-km/h)
limit cost at least as much time as it saved when the years lost to deaths, injuries, and
travel delays were compared with the travel time saved (Miller 1989, 73).
45 The comparable figures were 31 years of driving per life saved on urban Interstate
highways and freeways, 28 years on rural arterials, and 14 years on rural collectors
(TRB 1984, 123).
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MANAGING SPEED
68
Travel time costs also tend to be unevenly distributed by road user.
Most of the additional travel time attributed to the NMSL, for
example, was borne by motorists engaged in personal travel.
However, the value of this travel depends on trip purpose and length.
For example, more highly valued work-related travel is relatively
insensitive to changes in speed limits and accounts for a sizeable
share of all local personal vehicle travel--most recently estimated at
32 percent in 1990 (FHWA 1992). However, commuting trips typi-
cally are short--the average trip length is about 11 mi (18 km)--and
average trip time is about 22 min (VNTSC 1994). Thus, slower
speeds generally result in adding small time increments. For many
work trips, congestion is likely to have more effect on driving speeds
and travel time than are reductions in speed limits.
Most personal travel (68 percent in 1990) is for shopping, family
and other personal business, and social and recreational purposes.
Because many of these trips are discretionary and do not have the
same economic purpose as work travel, the time value of these trips
is lower than for work travel, and, by extension, the incremental cost
of reduced driving speeds from lower speed limits is also lower.
Fortunately, most of these trips are short.
Particular groups of road users--commercial truckers and other
business travelers--may be more adversely affected by reduced driv-
ing speeds attributable to lower speed limits. These groups drive
more miles than the average motorist and often use high-speed
roads. The economic cost of increased travel time for these user
groups, particularly from lost productivity, can be substantial.46
RELATION OF SPEED TO FUEL USE AND OTHER
VEHICLE OPERATING COSTS
The primary motivation for the NMSL was to conserve energy by
reducing driving speeds. Today, because of low fuel prices, driver con-
cern for fuel economy plays a much smaller role in determining
appropriate driving speeds.
46 In the case of the NMSL, however, the lower speed limit did have some benefits for
truckers, such as lower fuel and maintenance costs.
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69
Effects of Speed
The most recent study of fuel efficiency (West et al. 1997 in Davis
1997, 3-50), based on a small sample of 1988 to 1995 model year auto-
mobiles and light-duty trucks, shows a clear relationship between fuel
economy and driving speed. Under steady-state, cruise-type driving con-
ditions, fuel economy peaks at about 55 mph (89 km/h) and then
declines at higher speeds, reflecting primarily the effect of aerodynamic
drag on fuel efficiency (Figure 2-5).47 At lower speeds, engine friction,
tires, and accessories (e.g., power steering) reduce fuel efficiency (TRB
1995, 63).
Fuel efficiency also varies as a function of vehicle class. Sport utility
vehicles, minivans, and pickup trucks--which represent a growing
share of the U.S. passenger vehicle fleet--have poorer fuel economy, on
Figure 2-5 Fuel economy as a function of speed, model year 19881995
automobiles and light-duty trucks (Davis 1997, 3-51). 1 mph = 1.609
km/h; 1 gal = 3.8 L.
47 Data on fuel economy as a function of speed for heavy trucks are older and more
sparse. The available information suggests that fuel economy for heavy-duty diesel
trucks declines sharply at speeds above about 50 mph (80 km/h), largely because of the
effect of aerodynamic drag (TRB 1995, 125).
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MANAGING SPEED
70
the average, than all but the heaviest automobiles for a wide range of
speeds (Davis 1997, 3-52). Similarly, their fuel economy peaks at lower
speeds, on the average, than does that of most passenger vehicles.
Other vehicle operating costs, such as tire wear, are also likely to
increase as a function of speed. Relative to fuel costs, however, these
other operating costs are small; speed-related changes in their costs
are not readily discernible by the average driver. Thus, they are not
likely to affect motorists' choice of appropriate driving speeds.
RELATION OF SPEED TO EMISSIONS
Speed is clearly linked with vehicle emissions that contribute to pol-
lution of the atmosphere, particularly to the degradation of metro-
politan air quality. According to current models, volatile organic
compounds (VOCs)--an ozone precursor--and carbon monoxide
(CO) are highest at very low speeds associated with heavily con-
gested stop-and-go traffic and rise again with high-speed, free-flow
highway driving (TRB 1995, 4952). At high speeds, increased
power demands on the engine cause CO and VOC emissions to
increase, but at exactly what speed this occurs and by how much
emissions are increased are unclear (TRB 1995, 122). Emissions of
oxides of nitrogen (NOx), another ozone precursor, are thought to
increase gradually at speeds well below free-flow highway driving,
but again there is considerable uncertainty about the speeds at which
this increase begins and the rate of increase (TRB 1995, 122).
Data on emissions of heavy trucks as a function of speed are far
more limited. The available data suggest that exhaust emissions of
VOC and NOx from heavy-duty diesel vehicles rise at high speeds
(TRB 1995, 122). Detailed data on diesel particulate emissions as a
function of speed are unavailable. This is particularly troubling
because particulate concentrations pose a significant health risk, and
heavy-duty diesel vehicles are the primary source of highway vehicle
particulate emissions (TRB 1995, 129).
In addition to being a source of pollutants that degrade metro-
politan air quality, transportation in general and motor vehicles in
particular are the largest source of carbon dioxide (CO2) emissions,
one of the principal greenhouse gases associated with global warm-
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71
Effects of Speed
ing.48 In 1994, motor vehicles accounted for about one-quarter of all
U.S. CO2 emissions (TRB 1997, 83). The United States, in turn, is
the largest emitter of CO2, accounting for one-quarter of global
emissions (TRB 1997, 84).
CO2 emissions--a by-product of any engine that burns fossil
fuels--are closely linked with fuel economy and thus speed. At high
speeds, where fuel economy is poor, vehicles emit more CO2.
Vehicle speeds are also associated with noise; noise levels rise at
higher vehicle speeds. Sonic pollution is of greatest concern to those
living near freeways and on residential streets with higher-speed
traffic.
Despite the link between driving speeds and adverse environmen-
tal effects, U.S. drivers do not directly pay for the costs that this pol-
lution imposes on society.49 Thus they are not apt to consider
environmental costs in their choice of an appropriate driving speed.
SUMMARY
In this chapter, the role of speed has been considered as it relates to
the major factors motorists take into account in determining appro-
priate driving speeds. The relation of speed to safety--a major con-
cern for most drivers--is complex. Driving speed is clearly linked
with crash severity. Injury severity in a crash rises sharply with the
speed of the vehicle in a collision, reflecting the laws of physics. At
equivalent impact speeds, injury severity for pedestrians, the most
vulnerable of road users, is dramatically greater than for vehicle occu-
pants. Furthermore, the incidence of speeding as a contributing fac-
tor in crashes is higher the more severe the crash. The strength of the
relationship between speed and crash severity alone is sufficient
grounds for managing speed.
48 Unlike most other vehicle emissions, CO2 is not toxic. Along with other greenhouse
gases, its effect in the upper atmosphere is to trap heat and warm the earth; hence the
term greenhouse effect.
49 Drivers do pay for the cost of pollution controls on vehicles, emission inspections,
and improved fuels.
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MANAGING SPEED
72
Speed is also related to the probability of being in a crash,
although the evidence is not as compelling. Theory, the results of
empirical studies, and clinical analyses of crash causation all link
speed with crash probability. However, crashes are complex events,
and isolating the effect of speed from all the other factors that con-
tribute to crash probability to establish causality unequivocally is not
practicable. Moreover, the concept of speed itself is complex. Crash
involvement has been associated with the dispersion in traffic
speeds--in particular, with the deviation of an individual driver's
speed from the average speed of traffic at both higher and lower
speeds than the average. Those who drive at high speeds, well above
the average speed of traffic, pose the greatest safety concern to them-
selves and others because of the clear link between speed and crash
severity. Crash involvement has also been associated with a driver's
speed of travel. For example, single-vehicle crash involvement rates
have been shown to rise with travel speed.
The relationships among speed, speed dispersion, and crash prob-
ability also appear to vary by road class. However, data are limited for
many road types, and thus the observations that can be drawn are
suggestive rather than conclusive. Speed dispersion poses an impor-
tant safety concern on high-speed, nonlimited-access highways, such
as rural, two-lane, undivided highways; wider speed dispersions are
associated with higher crash involvement rates. Crash probability is
also associated with speed dispersion on Interstate highways, partic-
ularly on urban Interstates near interchanges. The potential for vehi-
cle conflict is high on most urban streets, where pedestrians and
parked vehicles augment normal vehicle conflicts. On these roads,
however, lower driving speeds reduce injury severity if a collision
occurs. Vehicle-pedestrian crashes are an exception, because pedes-
trian injuries tend to be severe even at low impact speeds. Both speed
and speed dispersion appear to play a role in crash likelihood on
urban arterials; speed deviation above average traffic speeds and
higher speeds in general are closely linked with crash probability on
these roads. Little is known about the relationship between safety
and speed on residential streets.
Crash probability also varies by crash type. Speed dispersion is a
contributing factor in the occurrence of multiple-vehicle rear-end
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73
Effects of Speed
and angle collisions, particularly for those driving well below average
traffic speeds. Driving at high speeds is associated with a greater inci-
dence of single-vehicle crashes.
Travel time is another major factor affected by speed that influ-
ences motorists' selection of an appropriate driving speed. However,
travel time costs are not equally distributed either by road class or by
road user. The highest travel-time costs occur on high-speed roads,
particularly Interstate highways and freeways, where speed regula-
tion, if enforced, can increase driving time under free-flowing traffic
conditions. Commercial truckers and business travelers are heavy
users of these types of roads and typically drive more miles than the
average motorist. Consequently, the economic cost of increased travel
time and lost productivity from speed reduction measures can be sub-
stantial for these road users.
Currently, fuel and other vehicle operating costs play a relatively
minor role in motorists' selection of an appropriate driving speed.
The relationship between speed and fuel use is unambiguous--fuel
economy is inversely related to driving speeds above about 55 mph
(89 km/h) for passenger vehicles, on the average, and at somewhat
lower speeds for light and heavy trucks. After more than a decade of
low fuel costs, however, drivers have little incentive to consider fuel
costs in their choice of speed.
Driving speed is clearly linked with vehicle emissions that contribute
to metropolitan air pollution and emissions of CO2, a greenhouse gas
closely associated with global warming. High driving speeds are also
associated with noise pollution. U.S. drivers, however, have never
directly paid for these costs. Thus, at present, the choice of an appropri-
ate driving speed is not affected by consideration of environmental costs.
These findings have several implications for managing speed. First,
the unambiguous relationship between speed and crash severity alone
is sufficient justification for controlling driving speeds. Second, if they
are enforced, speed limits--the most common method of managing
speed--can help restrict travel speeds, particularly at the very high
speeds where the injury consequences of crashes are the greatest.
Third, deviation of driving speeds from the average speed of traffic is
associated with crash involvement. Thus, speed limit policies should
attempt to minimize speed dispersion.
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74
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Representative terms from entire chapter:
speed dispersion
