Is speeding really a safety problem? Yes. "It
is clear that speeding does represent a significant traffic safety
problem," the National Highway Traffic Safety Administration
(NHTSA) reports. Deaths on rural interstates have increased as
a result of most states raising speed limits to 65 mph on these
roads during the late 1980s. Because speeding reduces the time
drivers have to avoid crashes, it increases not only the likelihood
of crashing but also the severity of crashes that do occur. According
to NHTSA, speeding is one of the most prevalent reported factors
associated with crashes. The agency estimates that speeding is
involved in 12 percent of all crashes and one-third of all fatal
crashes. (These estimates are likely to be conservative because
of the difficulty of establishing precrash travel speeds after
crashes.) NHTSA estimates that 13,909 fatalities and 77,277 moderate
to critical injuries occurred in speed-related crashes in 1991
and that the cost of all speed-related crashes was more than $18
billion.
Roadway design factors, including how far ahead a driver can see,
are compromised if vehicles travel faster than circumstances warrant.
Other vehicles and pedestrians are put at risk by speeding drivers
whose distances they may not be able to judge accurately. Recent
studies have shown that drivers who run red lights are likely
to be speeding and that motorcyclists who crash with other vehicles
making left turns are also likely to be speeding.
What is the relationship between how fast a car is going
and what happens in a crash? The higher the travel speed,
the greater the risk of serious injury or death in a crash. Vehicles
and their occupants in motion have kinetic energy that is dissipated
in a crash. The greater the energy that must be dissipated, the
greater the chances of severe injury or death. Motor vehicle crashes
are complex events with multiple causes, and individuals have
a wide range of tolerance to injury, but the amount of energy
that must be dissipated - thus the probability and severity of
injury in a crash - is related to a vehicle's speed at impact.
The laws of physics tell us that the energy of impact delivered
to vehicle occupants in collisions increases nonlinearly with
impact speed. In other words, crash severity increases disproportionately
with vehicle speed. A frontal impact at 35 mph, for example, is
one-third more violent than one at 30 mph. Using data from the
National Accident Sampling System, which is based on field investigations
of a national sample of police-reported crashes, NHTSA researchers
compared the relative severity of injuries in crashes with the
estimated crash impact speed. The percentage of occupants with
serious injuries consistently and dramatically increases with
increasing impact speed. For example, the rate of severe injury
for people involved in crashes at impact speeds of 21-30 mph is
11.1 - a rate that increases to 27.9 at impact speeds of 31-40
mph and to 54.3 at speeds of 50 mph or more. (The rate is calculated
as the number of occupants at a certain impact speed with severe
injuries, divided by the total number of occupants in crashes
at that impact level times 100.)
Who speeds the most? Young drivers speed more
often than older drivers. In a study of drivers on limited access
highways, high-speed drivers were more often male and more often
judged to be younger than 30. Studies in California have found
that the rate of speeding violations per mile traveled is at least
three times as high for drivers 16-19 years old as it is for drivers
age 30 and older. Although speeding is a problem among all driver
age groups, the crashes and violations of young drivers are much
more likely to be related to speed than is the case for drivers
of other ages - and the motor vehicle crash death rate per 100,000
people is especially high among 16-24 year-olds. A NHTSA analysis
found that the relative proportion of speed-related fatal crashes
decreases with increasing driver age. About 37 percent of all
drivers age 14-19 involved in fatal crashes were in speed-related
crashes, but the percentage among drivers 70 and older decreased
to 7 percent. At all ages, male drivers are more likely than female
drivers to be involved in speed-related fatal crashes.
Isn't speed variation - not speeding - the real problem?
No. Although research conducted in the 1950s on two-lane rural
roads did indicate that vehicles traveling much faster or much
slower than average were more likely to be involved in crashes,
this issue is not relevant on today's high-speed highways with
controlled access. The authors of this early study acknowledged
that their findings could not be extended to controlled access
freeways, but some proponents of higher travel speeds have attempted
to do so. Many differences in travel speeds are unavoidable because
of the slower speeds of turning or merging vehicles. Many crashes,
and nearly half of those resulting in occupant deaths, are single-vehicle
impacts in which differences among vehicle speeds play no role
or only a very minor one. Finally, the risk of death and severe
injury is a direct exponential function of speed, not speed differences.
What is the role of speed in crashes? Speed influences
crashes in four basic ways:
Why 55 mph? Isn't this just a holdover from the gas crisis?
Speed limit laws, which date back to 1901, have traditionally
been the responsibility of the states. Then Congress responded
to the oil shortage of 1973 by directing the U.S. Department of
Transportation to withhold highway funding from states that did
not adopt a maximum speed limit of 55 mph. The National Research
Council attributed 4,000 fewer fatalities to the decreased speeds
in 1974, compared with 1973, and estimated that returning the
speed limit on rural portions of the interstate highway system
to pre-1974 levels would result in 500 more fatalities annually,
a 20-25 percent increase on these highways. As concerns about
fuel availability and costs faded, however, speeds began to gradually
climb on U.S. highways. By the mid-1980s, a substantial majority
of vehicles on rural interstates were exceeding 55 mph. In response
to claims that the 55 mph limit had made the United States a nation
of law breakers and assertions that deaths and injuries would
not increase because people were already traveling at the speeds
at which they felt comfortable, Congress allowed states to increase
speed limits on rural interstates to 65 mph in 1987.
Only a few states- Connecticut, Hawaii, Maryland, New Jersey,
New York, Pennsylvania, and Rhode Island - retain 55 mph maximum
speed limits on all highways. Delaware and the District of Columbia
do not have any rural interstates, and Massachusetts has fewer
than 100 miles posted at 65 mph. Fifty-five keeps down the proportion
of vehicles traveling at very high speeds. More vehicles travel
at 70 mph and faster in states with 65 mph speed limits than in
states that retained 55 mph speed limits on rural interstates,
which are designed for 70 mph travel only under the most favorable
conditions.
What is the effect of raising speed limits to 65 mph?
Higher travel speeds and more deaths. In states where speed limits
were raised to 65 mph in 1987, the higher limits are causing about
15-20 percent more deaths on rural interstates each year than
otherwise would be expected. Institute studies show that, in states
that raised rural interstate speed limits, about 400 lives are
lost each year because of the higher limits.
How are speed limits established? Speed limits
are typically set based on a roadway's design - for example, whether
it is a narrow two-lane byway or a modern controlled access freeway-
and whether the surrounding area is urban, suburban, or rural.
It has been argued that measuring the speed distribution of a
roadway and then setting the speed limit so that 85 percent of
motorists would be in compliance reduces the need for enforcement
and, at the same time, reduces crash risk by narrowing variation
among vehicle speeds. However, numerous studies of travel speeds
on rural interstate highways have shown that 85th percentile speeds
increased when speed limits were raised to 65 mph and then continued
increasing. Thus, the 85th percentile is not a stationary point.
It is rather a moving target that increases when speed limits
are raised. If speed limits are raised to meet a current 85th
percentile speed, a new 85th percentile that is higher will soon
result.
Others claim that, because interstate highways meet rigid design
standards for sight distance and roadway geometry, they should
be posted at their design speeds. The problem is that a design
speed is not necessarily a safe travel speed. The American Association
of State Highway and Transportation Officials defines design speed
as "the maximum safe speed that can be maintained...when
conditions are so favorable that the design features govern."
In other words, it is the maximum speed at which drivers can maintain
a safe level of vehicle control on a particular section of highway
under the conditions for which the highway was designed. Speed
limits are set somewhat lower because conditions are not always
favorable. Many motorists also assume there is a kind of built-in
tolerance factor in speed limit enforcement, so they exceed the
limit regardless of what it is.
Does research show that slightly faster speed limits would
reduce crashes? No. Advocates of higher speed limits
have claimed that research on speed variation indicates faster
speeds are not hazardous. They cite David Solomon's research from
the mid-1950s and similar studies that seem to show an increase
in crash likelihood among drivers traveling slower than the average
speed and a minimum of crashes at 5-10 mph above the average speed.
However, the travel speeds of crash-involved drivers in these
studies were obtained from driver reports and police estimates,
and it is well known that drivers exceeding speed limits are likely
to deliberately underestimate their speeds. Such underestimation
can account for much of the apparent underinvolvement of moderately
high-speed drivers in crashes. In addition, later research found
that simply removing the crashes involving intersections and turning
maneuvers eliminated the exaggerated overinvolvement of slower
drivers in crashes. More important, the Solomon research addressed
only speed variation, not speed limits. Raising speed limits raises
average speeds as well.
If going faster than 65 mph is prohibited, why do vehicle
speedometers go to 120 mph? There is no good reason.
Although a national law enforcement summit convened by the U.S.
Secretary of Transportation in 1990 called for reinstating a regulation
establishing 85 mph as the top speed on speedometers in all motor
vehicles, there is currently no regulation requiring it. NHTSA
noted in rulemaking revoked more than 15 years ago that "speedometers
that indicate speeds of 120 mph or more use more than half of
the dial to indicate illegal and dangerous speeds."
In the past, automakers often promoted their products by glamorizing
high speed. Although many automakers now compete in terms of safety
features, there still are exceptions. For example, a television
advertisement for the 1990 Nissan 300ZX Turbo - a car model with
a very high crash fatality rate - featured a 150 mph race between
a motorcycle, a race car, and a jet fighter plane. This ad was
withdrawn in response to protest from the safety community and
general public. Such ads "irresponsibly promote excessive
speed and exhibit a blatant disregard for public safety,"
the Institute observed at the time.
Should trucks have lower speed limits than cars?
Yes. Large trucks require much longer distances than cars to stop.
Lower speed limits for trucks make heavy vehicle stopping distances
closer to those of lighter vehicles. Slower truck speeds also
allow automobile drivers to pass trucks more easily. Crashes involving
large trucks not only can cause massive traffic tie-ups in congested
areas, but they put other road users at great risk - 98 percent
of the people killed in two-vehicle crashes involving a passenger
vehicle and a large truck are occupants of the passenger vehicles.
Institute studies have shown that lower speed limits for trucks
on 65 mph highways lower the proportion traveling faster than
70 mph without increasing variation among vehicle speeds. In one
study, only 4 percent of large trucks exceeded 70 mph on Ohio's
rural interstates with 55 mph speed limits for large trucks and
65 for cars, but up to six times the proportion of trucks exceeded
70 mph on rural interstates in three other states with uniform
65 mph speed limits. Another study found that the percentage of
trucks going faster than 70 mph was twice as large in states with
uniform 65 mph limits (14 percent in Arizona, 9 percent in Iowa)
as it was in states with different car and truck speed limits
(4 percent in California, 3 percent in Illinois).
How do speed limits and safety records in other countries
stack up alongside the United States? Germany does not
even have speed limits on most autobahns, for example, and longstanding
claims point to the safety of these roads. Yet the death rate
per 100 million vehicle miles traveled on U.S. interstate highways
has generally been lower than the rate on autobahns. Fatality
rates have been declining in both countries for many years, but
the German rate had historically been much higher than the U.S.
rate. For example, there were 1.24 deaths per 100 million vehicle
miles traveled on U.S. interstate highways in 1975, compared with
2.75 deaths per 100 million miles on German autobahns. During
1984-86, U.S. and German rates were similar, but when speed limits
on rural interstates were raised to 65 mph beginning in 1987,
the U.S. interstate death rate became higher than the rate on
the autobahn. After German reunification, autobahn fatality rates
fluctuated but declined to 0.94 in 1992 while the U.S. rate reached
an all-time low of 0.85.
Eighty percent of autobahns do not have speed limits, but several
other factors have contributed to declining death rates on these
roads. Germany has achieved a very high belt use rate - about
95 percent compared with about 50 percent in the United States.
The minimum age for a drivers license in Germany is 18 years old,
which eliminates a risky subset of drivers from the road. Passing
on the right is illegal on autobahns, and drinking-driving laws
are tougher in Germany than in the United States. All of these
factors should have contributed to even lower death rates on German
autobahns, but the U.S. rate has continued to decline ahead of
the German rate.
Are speed-related crashes a problem on urban as well as
rural roads? Yes. Speed-related fatal crashes affect
all roadways, but the percentage varies by road type. According
to NHTSA, about 36 percent of all fatal crashes on rural roads
are related to speed, compared with about 30 percent on urban
roads. Rural roads account for 40 percent of all vehicle miles
traveled, but they account for 61 percent of all speed-related
fatal crashes.
Are pedestrians at risk from speeding vehicles on city
streets and suburban roads? Yes. The second largest category
of motor vehicle deaths, after occupants, is pedestrians, and
69 percent of pedestrian deaths occur in urban areas. The speed
of vehicles involved in pedestrian impacts is a major determinant
of the severity and outcome of injury. There is a much higher
ratio of deaths to injuries where speed limits are higher - nine
times as high where the speed limit is 55 mph as on roads where
it is 30 mph or lower. A federal study of pedestrian crashes found
a positive correlation between speed and injury severity. In addition,
vehicle speed influences the likelihood that a pedestrian will
be struck in the first place because a driver cannot stop quickly
enough. One study found that, even in residential zones, almost
20 percent of vehicles were traveling at more than 30 mph when
they struck pedestrians. Some cities are using new approaches
to slowing urban traffic to reduce pedestrian crashes, especially
in school and work zones.
Does the speed limit matter? Don't drivers speed anyway? Many
drivers tend to drive somewhat faster than posted speed limits,
no matter what the limits are. Although people often opt to travel
somewhat faster than the posted limit, they do not completely
ignore it but choose a speed they perceive as unlikely to result
in a ticket. The more important speed-related safety issue on
freeways involves the proportion of vehicles traveling at very
high speeds, not the proportion violating the speed limit. The
Institute's frequent monitoring of free-flowing travel speeds
on interstate highways where the 55 mph speed limit was retained
and speeds on roads where limits have been raised to 65 mph shows
that, in general, higher speed limits lead to greater proportions
of cars traveling faster than 70 mph. For example, in New Mexico,
the first state to raise its limits to 65 mph on rural interstates,
the proportion of motorists exceeding 70 mph grew from 5 percent
shortly after speed limits were raised to 36 percent in 1993.
On urban interstates that stayed at 55 mph, only 13 percent of
cars and 2 percent of tractor-trailers traveled faster than 70.
In Maryland, which retained 55 mph limits, the proportion traveling
faster than 70 mph remained virtually unchanged at 7 percent during
1988-93. By 1994, 12-15 percent of cars were exceeding 70. In
neighboring Virginia, which switched to 65 mph limits, the percentage
exceeding 70 mph went from 8 percent in 1988 to 29 percent by
1992 and 39 percent by 1994.
Governor's Office of Highway Safety
100 Peachtree Street, Suite 2000
Atlanta, GA 30303
(404) 656-6996 FAX (404)651-9107
or 1-800-342-9819
Or Contact the DeKalb Police Star Team
Revised 6/17/96
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