BUFFALO, N.Y. -- Texas recently raised the speed limit on a
portion of its interstate highway to 80 mph. Based on the mantra
"Speed Kills," fatalities on that West Texas roadway should
"Not so fast," say emergency medicine researchers in the Center
for Transportation Injury Research and Calspan University at
Buffalo Research Center (CUBRC).
"'Variance Kills,' would be a more accurate slogan."
The difference in speeds, or "variance," among vehicles
traveling on the same roadway, plus the difference between the
posted speed and what is known as "design speed" increases the risk
of accidents, not speeds alone, they reported recently at the
American College of Emergency Physicians' Research Forum in New
The research suggests that consideration should be given to
setting speed limits based on topography -- rather than having one
standard speed limit -- for stretches of interstate highways
outside of high-population areas.
Design speed refers to the maximum speed a vehicle can maintain
safely based on conditions and the road's topography: A curved road
would have a lower design speed than a straight road, and a flat
road would have a higher design speed than a hilly road.
"If the roadway's design and weather conditions allow cars to
travel safely at 80, but the posted speed is 55, some people will
observe the speed limit, while some will drive at 75 or 80," said
Dietrich Jehle, M.D., associate professor of emergency medicine in
the UB School of Medicine and Biomedical Sciences, and first author
on the study.
"In that situation there will be a lot of weaving in and out of
traffic, more crashes and more fatalities," noted Jehle, who also
is director of emergency services at Erie County Medical Center in
"But when the speed limit is 65 on a roadway with design speed
of 80, some people will drive at 62 and some will drive at 75 or
80," he continued. "When all cars are traveling at fairly similar
speeds, there is less variance and fewer accidents."
The study was based on the number of fatalities and vehicle
miles traveled on individual roadways extracted from New York State
Department of Transportation reports.
Absolute mortality decreased by 28.3 percent on the New York
State Thruway, the primary focus of the study, when the speed limit
was raised from 55 miles per hour to 65 miles per hour., the study
found. Adjusted for vehicle miles traveled, there was a 42.5
percent decrease in mortality.
Supporting the importance of speed variance in fatalities, the
percentage of traffic traveling more than 10 miles over the speed
limit dropped from 39 percent to 8 percent when the speed limit
increased from 55 mph to 65 mph on the Thruway, results showed.
Total accident rates and injury rates also declined slightly. Over
this same time period, traffic volume on New York State interstates
increased by 13 percent.
To control for improvements in auto design over the time period
studied, which may have made cars safer, the researchers also
studied fatalities on major roads where the speed limit did not
change. On those roads, fatalities increased 20.7 percent during
the three years after the speed limit was raised on
Drivers on New York interstates apparently are not vigilant
about design speed, however. On flat roads, mortality dropped by
30.2 percent after the speed limit increased, but on mountainous
roads mortality increased by 17.6 percent.
Jehle noted that in individual crashes, the faster the vehicles
are traveling the greater the fatality rate, whereas speed variance
increases overall fatality rates.
"The message here for those responsible for highway safety is
that factors other than absolute speed are important," said Jehle,
"Variance in speed and road topography both play a major role in
crash fatalities on interstate highways.
"Given these findings, it might be a good idea to set speed
limits based on topography on stretches of interstates outside of
high population areas, rather than having one standard speed
limit," said Jehle.
Sarah Connolly, M.D., an emergency medicine resident, and
Michael Godzala, UB medical student, also contributed to the
The research was supported by a grant from the Federal Highway
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