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How Should they be Determined?
By the late Professor Patricia F. Waller, Ph.D., (Emerita)
Dept. of Social and Behavioral Analysis, University of Michigan Transportation Research Institute
The following paper was given as an address to
the Sydney Chapter of the Australian College of Road Safety
September 6, 2001.
The primary purpose of our highway system is mobility, not safety. If it were the latter, we would set speed limits much lower. A 20 mph speed limit, with governors on all motor vehicles to prevent exceeding that speed, would certainly lower highway casualties. Of course, no one would consider such a measure, Clearly, we tacitly agree to accept a certain level of carnage in order to use the highways in ways we value. At the present time in the US, this tacit agreement says that it is acceptable to sacrifice between 40,000 and 42,000 lives annually.
seeking an acceptable balance between
mobility and safety, speed limits are a critical factor. To what
extent is speed a factor in the likelihood of a crash occurring in the
first place? An early and widely cited study (Solomon, 1964) found that as
speed deviated from the average speed of the traffic stream, crash
probability increased. He reported that speeds significantly below that of
the traffic stream were at least
as likely to cause crashes as those above the average speed. Solomon's
data were from the 1950s and were based on rural roads, most of which were
2-lane. Much has changed since then, and, not surprisingly, more recent
studies report somewhat different findings.
a review of speed and crash probability, Shinar (1999) cites extensive
evidence that speed variance increases crash probability. Yet
in 2000, 55 percent of fatal crashes in the US were single vehicle.
Almost one‑fourth of these involved a pedestrian or bicyclist, where
speed could have been a factor, but 42 percent of all fatal crashes
involved only one vehicle, with no pedestrian or cyclist (National Highway
Traffic Safety Administration, 2001). In these crashes, speed variance per
se is less likely to be a factor. Shinar
reports that, while there is some evidence that on a given road, crash
involvement rates for individual vehicles rise with their travel speed
there is no convincing evidence that across all road types, crash rates
increase with overall average speed. Of course, overall average speed is
highly correlated with road design, and higher speed limits are found on
roadways with safer design. While the role of speed deviation per se may
not be settled, there is unequivocal evidence that probability of injury
and fatality increases with increasing speed. Shinar concludes that:
"The overall cost of speed‑related crashes is much greater than
the relationship between speed and crash probability indicates. This is
because high‑speed crashes are associated with greater injury levels
than are low‑speed crashes." (p 271)
55 mph National Maximum Speed Limit
1973 the US experienced a fuel crisis that had repercussions throughout
the economy. Congress enacted legislation establishing a National Maximum
Speed Limit (NMSL) of 55 mph. Although enacted for purposes of fuel
economy, it resulted in a reduction in highway fatalities of over 9000 in the first year. When the immediate fuel
crisis had passed, and cars were no longed lined up at the gasoline
stations, an effort was made to retain the NMSL for safety reasons. There
was strong opposition. Congress requested an independent review of the
evidence, and in 1984 the Transportation Research Board issued their
report (Committee for the Study of the Benefits and Costs of the 55 mph
National Maximum Speed Limit, 1984), making the following points:
legislation was initially enacted for purposes of fuel economy. Gasoline
consumption declined in 1974, for a savings of about 255,000 barrels per
day (BPD) of petroleum, and it was estimated that in 1983, the lower speed
limits accounted for savings of about 167.000 BPD. Although additional oil
reserves have been discovered since 1974, there has been no absolute
increase in the total amount of fossil fuel on the planet Earth.
Consequently, the long‑term importance of conserving fuel has not
The review concluded that the unprecedented decrease in highway
fatalities, over 9000 lives, immediately following the enactment of the
NMSL resulted from many factors including reduced exposure and reduced
discretionary driving. However, taking into account other variables that
may have contributed to the safety benefits, the report concluded that the
NMSL continued to save lives. They estimated that, in the early years of
the 55 mph NMSL the lower speeds saved about 3000 to 5000 lives annually. In 1984, there continued to be between 2000 and 4000 fewer
fatalities, between 2500 and 4500 serious, severe, and critical injuries,
and between 34,000 and 61,000 fewer minor and moderate injuries as a
result of the lower speed limits (pp 166‑67).
a change in velocity of 50 mph, the probability of the driver being killed
is twice what it is at 40 mph. Incremental increases in speed add
disproportionately to the probability of serious or fatal injury,
Vehicle Injury Costs.
In the US, injury, both intentional and unintentional, is the
leading cause of death from age 1 to age 45. Because it so
disproportionately strikes the young, it is also the leading cause of lost
years of life prior to age 75, more than either cancer or heart disease.
Motor vehicle injury is the largest single component of these losses
(Committee on Injury Prevention and Control, 1999). A driver license from
one state enables one to drive in other states, and emergency care and
medical services must be provided wherever an injury occurs Even if one is
not injured, the costs incurred through taxes and health and auto
insurance premiums reflect any increases in injury‑associated costs,
The cost of motor vehicle injuries crosses state lines - it is a national
problem, not just a state or local one.
The 55 NMSL increased travel time, and it is this issue that generated the
greatest opposition. Interestingly, the report found that, "On the
basis of average trip lengths and average speeds, most personal vehicle
travel slowed down by the 55 mph speed limit involved time losses of less
than 3 minute? (p. 115). One committee member noted that, "Over a
year's time it may amount to about a day and a half for me, but if my day
were broken up into 2 and 3‑minute pieces, 1 don't think I could get
much done." Small units of time do not combine to create useable time
was further found that it cost "roughly
40 years of driving per life saved and serious injury averted."
However, "the average life expectancy of motor vehicle accident
victims in 1982 was about 41 year” (p 120). This comparison does not
take into consideration the enormous time costs incurred by family and
friends of the killed and seriously injured, or the time costs associated
with the many more injuries that are less serious, or crashes involving
property damage only. Furthermore, crashes are the primary cause of
traffic delays, creating enormous time costs to travellers. Thus,
when all time costs are considered, the total societal time costs are less
at 55 mph than they are at higher speeds.
There is also the issue of how time costs are distributed across the
population At the lower speed limit whatever time costs are incurred are
distributed fairly evenly across the entire travelling public. At the
higher speed limits, time costs are borne disproportionately by those who
are involved in crashes, especially those who are seriously or fatally
injured. Because for most trips, at the lower speed the additional time
involved is three minutes or less, the trade‑off between arriving at
a destination a few minutes later versus sacrificing a young life is not
one that should be difficult to choose.
issues either not covered or touched on only briefly in the report include
commercial trucking, composition of the vehicle fleet, consumer
expenditures, national security, balance of payments, environmental
impact, and our changing demographics. Each of these is discussed below.
In response to the enactment of the 55 mph NMSL the trucking
industry requested, and received, increases in allowable truck size and
weight. It was argued that additional cargo was needed to offset the lower
speeds, It was found, however, that the lower speeds led to savings in
fuel, maintenance, and safety, offsetting the time costs incurred. Most of
the trucking industry (excepting the independent operators) supported the
55 mph NMSL. It should also be noted that as speeds have increased, there
have been no corresponding decreases in truck size and weight limits.
One argument for increasing speed limits on some highways is that
they were designed for higher speeds. There is some truth to this, but the
design standards were for a vehicle fleet different from what we have
today. Passenger cars averaged around 4000 pounds and trucks were smaller
and lighter. In response to the fuel crisis, car size was reduced, and
trucks got larger and heavier. Although passenger vehicles are much safer
today than they were 30 or 40 years ago, the safety gains realized are
compromised by the higher speeds.
Even though, in the US, fuel prices are low compared to the rest of
the industrial world, in 1999 the average household spent over $1000 on
gasoline (Bureau of Labor Statistics, 2001). With today's higher prices,
it would be considerably more. Lower speeds translate into greater fuel
economy. They also reduce costs of vehicle maintenance and repair.
Reduced fuel consumption reduces our dependence on foreign oil
supplies. In 1973, in the US, our net imports of foreign oil comprised 35
percent of petroleum consumption. In 2000 this share had increased to over
50 percent and is expected to reach 64 percent by 2020 (US Department of
Energy, 2001a). In 1972, 43.5 percent of our net imported petroleum came
from OPEC countries. In 1999, it was 46 percent (US Department of Energy,
2001b). The costs of the Persian Gulf War were directly attributable to
our dependence on petroleum imports from that region, and we continue to
maintain a strong military presence there, as well as provide substantial
support to allies in that part of the world.
Our dependence on foreign oil is a major source of our negative
balance of payments. In 1974, imported petroleum exceeded exported
petroleum by almost $24 billion. In 2000, the excess came to almost $109
billion and accounted for 28 percent of our balance of payments deficit
(US Department of Energy, 2001 c; US Department of Commerce, 2001).
Reducing fuel consumption would reduce this deficit.
Motor vehicle fuel consumption is a major contributor to
environmental pollution, including global warming. In the US, of fuel
consumed in transportation of all kinds, passenger vehicles and small
trucks account for the largest single component. Transportation was also
the largest single source of carbon dioxide emissions in the US in 1999,
and motor gasoline was the largest contributor to these emissions.
Petroleum spills continue to be a source of water pollution, and traffic
is an increasing source of noise pollution in neighbourhoods (Bureau of
Transportation Statistics, 2001).
Perhaps the most significant issue in setting speed limits, as well
as the one most often overlooked, is the changing demographics of our
population. Global life expectancy has grown more in the last 50 years
than in the previous 5000. Prior to the Industrial Revolution, people aged
65 and over accounted for no more than 2 or 3 percent of the population in
the developed world. Today they account for about 14 percent, and are
estimated to reach 25 percent by 2030 (Peterson, 1999). In the US, Florida
presently has the highest proportion of older citizens, about 19%.
However, by 2025, 39 of our 50 states will have similar proportions
(Committee for Economic Development, 1999). Italy will achieve this status
in 2003; Japan and Germany in 2005 and 2006, respectively; and France and
Britain by 2016. Canada will be there in 2023 (Peterson, 1999).
changes in our population are not simply the result of greater longevity.
In the developed nations, fertility rates have fallen far below
replacement level (Commission on Global Aging, 2000). The younger
population is decreasing in proportion, and, in many countries, in
absolute numbers as well. The US is somewhat better off than most other
industrialised nations because of our high immigration.
increase in life expectancy, with a corresponding decrease in birth rate,
heralds a much larger elderly population driving and using our roadways.
Older people are by far the fastest growing segment of our driving
population. They are increasing their numbers and proportions in the
licensed population, and both their trips and their total mileage are
increasing. Unfortunately, age takes its toll and there is some loss of
driving proficiency with increasing age. While
some of this loss is undoubtedly attributable to medical conditions
that occur with increasing frequency with increasing age, some of the loss
accompanies "normal" aging. For most people, vision changes
occur by the mid‑40s and continue thereafter, and divided attention
becomes more difficult with increasing age.
there is an increase in crash risk per mile driven, beginning in the late
50s and early 60s, with more marked and accelerating increases in the 70s
and older (National Highway Traffic Safety Administration, 2000; Peck and
Romanowicz, 1993/94). This increase occurs even though everything we know
about older drivers tells us that, as a group, they try very hard to limit
their driving to the safest times and places. Older drivers are
underrepresented in crashes involving speed, alcohol, or reckless driving.
Because age is also correlated with increasing vulnerability to injury,
older people are more likely to be seriously or fatally injured in a crash
of specified dimension (Evans, 1991).
are the implications of these changing demographics for speed and speed
limits? First, we are seeing an increasing number of older drivers on our
roads. Second, there is no question that, as a group, their driving
proficiency decreases with increasing age. Third, their transportation
needs do not decrease simply because they can no longer drive. Data from
the Nationwide Personal Transportation Survey show that, for all ages,
work‑related trips account for fewer than one‑third of trips
taken. Up until the mid‑80s, older people take about as many, if not
more, non‑work‑related trips as those aged 16 to 64 (Rosenbloorn,
in press). Fourth, there will be fewer younger drivers available to meet
the transportation needs of these older people. There is clearly a need to
enable older drivers to continue to meet their own transportation needs
for as long as possible.
is currently considerable effort to do just that. Federal Highway
Administration is seriously addressing modifications in the highway system
to assist older drivers, and hence everyone else who is not functioning at
optimum level (Staplin, Lococo, and Byington, 1998). Other work continues
on facilitating the driving task, with a focus on older drivers (Schieber,
in press., Staplin, in press), including how intelligent transportation
systems may be of assistance (Caird, in press). In the private sector,
vehicle manufacturers are at last addressing the needs of older drivers
(Pike, in press).
is also considerable attention being paid
to licensing, original and renewal, of older drivers, but here the
problems are much more challenging. In some states, the laws specifically
forbid any special requirements for licensure based on increasing age.
Thirty states have no safety-related provisions whatsoever for renewal
licensure of older drivers. Some "special provisions" for older
persons are designed to make it easier for the older applicant, e.g., in
Tennessee, licences issued to people aged 65 and older do not expire, and
in North Carolina, applicants aged 60 and older are exempt from parallel
parking on the road test. The most common special provision is a shortened
renewal cycle or a requirement for in‑person renewal (Insurance
Institute for Highway Safety, 2001). There is some evidence that vision
testing of older drivers is effective in reducing crashes (Shipp, 1998),
and that more sophisticated vision testing may possibly be more effective
(Owsley, in press). However, on the whole, measures currently used in
driver licensing renewal have shown little or no relationship to prior or
subsequent driving performance as indicated in driver records. Although
there is considerable research being conducted in this area, and from time
to time, we hear that someone has discovered the “magic bullet," we
still do not know how to identify those older drivers who are more likely
to have future crashes.
do know, however, that as drivers age and become less proficient, they
respond as other drivers do when the task becomes more complex - they
reduce their speed. Older drivers are increasing their presence on our
roadways, and many of them cannot drive safely at the higher speeds. To
ban them from the interstate or other high speed roadways means
excluding them from the safest roads in existence. In 1995, our
Congress saw fit to remove the federal requirement that states retain the
55 mph speed limit on rural interstates. In those states that raised the
speed limit, highway fatalities increased on interstates, an average of 15
percent, and fatality rates increased 17 percent (Farmer, Retting and
speeds reduce time to respond to emergency situations, increase stopping
distance, and increase both crash risk and crash severity. On virtually
every dimension considered, including fuel economy, safety, consumer and
societal costs, environmental pollution, national security, and time
costs, the lower speed limits make more sense. Most importantly, they make
more sense in light of the unprecedented changes occurring in our
population and the increasing presence of older drivers on our highways.
It is to everyone's advantage to enable these drivers to continue to meet
their own transportation needs without posing undue hazard to themselves
or other road users. If we are fortunate, eventually we will all be the
beneficiaries of such a policy.
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