Accidents on Seattle streets were less likely to cause injuries after the city lowered speed limits, a new study published today by the Insurance Institute for Highway Safety shows. In downtown Seattle, lowering default speed limits reduced the likelihood that a crash would involve an injury by a fifth on arterial roads, IIHS researchers found. Outside of the city center, where the new limits were less consistently implemented and publicized, there were smaller injury reductions, but these were not statistically significant.
Newton* understood this in his F=MA equation. That is, Newton’s second law says that force is proportional to what is required for an object of constant mass to change its velocity. This equals the object’s mass multiplied by its acceleration. The increase in weight (Mass) and speed (Acceleration) results in a substantial increase in the energy (Force) involved in such an accident or test of crashworthiness.
“When we talk about the Safe System approach, we always stress that nobody should have to die because of a mistake,” said IIHS President David Harkey. “These results illustrate the value of rethinking speed limits. Crashes still happened after Seattle’s changes, but they weren’t as dangerous.”
An earlier IIHS study showed that lowering speed limits in Boston resulted in less speeding. However, it did not investigate the impact on crash or injury rates. The study of Seattle’s program is one of the first to examine the effect of lower limits on injury rates in a large, populous U.S. city.
“Everybody thinks of highways when we talk about speed limits, but reducing speeds on city thoroughfares and residential streets is just as important,” Harkey noted. “Nationwide, nearly a third of crash deaths occurred on urban arterials in 2020.”
Higher speeds make crashes more likely by reducing the time a driver has to react and increasing the distance required to stop the vehicle. Higher speeds also increase the energy involved in a crash, raising the odds of an injury (See Newton above*).
As of November 2016, Seattle lowered the default speed limits from 30 mph to 25 mph on its arterial roads and from 25 mph to 20 mph on smaller, mostly residential streets, unless otherwise posted. To make drivers aware of the change, the city conducted a public outreach campaign and installed gateway signs indicating the new citywide limits on arterial roads into the city, highway off-ramps and ferry terminals.
At the same time, new 25 mph speed limit signs were installed on arterial roads within the downtown area. Most arterials outside the city center had higher posted speed limits, and these mostly remained in effect until 2018, when Seattle began installing new signs on more arterials outside downtown. By the end of 2019, speed limit signs had been changed in eight of the city’s 32 urban centers and villages, including downtown, and on some arterial corridors in other areas.
IIHS presumed that the largest impact would be a reduction in crash injuries – the main focus of the Safe System approach. Researchers looked at the proportion of crashes involving fatal, serious or evident injuries, comparing the before-and-after injury rates with three control cities in Washington where there were no known changes to speed limits over the study period.
In all four cities, a large majority of crashes occurred on arterials. The proportion of crashes with injuries dropped on these major roads in the three years after the speed limit reductions in the downtown area of Seattle, while the proportion of crashes involving injuries increased in the control cities. This was also true for all arterial and smaller roads in the downtown areas combined.
Controlling for weather, lighting conditions and other factors, Seattle’s speed limit reduction was associated with a statistically significant 17% drop in the odds of an injury crash downtown and a non-significant 7% drop outside the city center. On arterial roads only, there was a statistically significant 20% reduction in the odds of an injury crash downtown and a non-significant 11% decrease outside the city center.
After the period covered by this study, in 2020, Seattle lowered speed limits on most remaining arterial roads to 25 mph. At the same time, it removed about 750 old signs and installed ~2600 new ones in greater density.
“These results suggest that communities can reap substantial benefits by lowering speed limits,” said IIHS Senior Research Transportation Engineer Wen Hu, the lead author of the study. “To reduce injuries even further, communities should combine lower speed limits with engineering solutions, public education about the importance of reduced speeds, and high-visibility enforcement.”
On non-arterials, there was no significant change in the odds of an injury crash, either downtown or in other parts of the city. However, that might partly be due to the small sample size and the nature of the roads themselves. Many residential streets in Seattle are narrow lanes with on-street parking on both sides, so drivers may not be comfortable driving much faster than 20 mph regardless of the default limit.
It’s also likely that the program was more effective where speed limit signs were posted than in areas where drivers were expected to remember that the default limits had changed throughout the city. That explains why the largest effect was observed downtown, where new speed limit signs were put in place at the same time the limits were lowered.
“When it comes to reminders, a clearly visible speed limit sign is pretty hard to beat,” said Hu. “We expect Seattle to see even bigger benefits as the city completes the transition to new signage.”
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About Ken Zino
Ken Zino, editor and publisher of AutoInformed, is a versatile auto industry participant with global experience spanning decades in print and broadcast journalism, as well as social media. He has automobile testing, marketing, public relations and communications experience. He is past president of The International Motor Press Assn, the Detroit Press Club, founding member and first President of the Automotive Press Assn. He is a member of APA, IMPA and the Midwest Automotive Press Assn.
He also brings an historical perspective while citing their contemporary relevance of the work of legendary auto writers such as Ken Purdy, Jim Dunne or Jerry Flint, or writers such as Red Smith, Mark Twain, Thomas Jefferson – all to bring perspective to a chaotic automotive universe.
Above all, decades after he first drove a car, Zino still revels in the sound of the exhaust as the throttle is blipped during a downshift and the driver’s rush that occurs when the entry, apex and exit points of a turn are smoothly and swiftly crossed. It’s the beginning of a perfect lap.
AutoInformed has an editorial philosophy that loves transportation machines of all kinds while promoting critical thinking about the future use of cars and trucks.
Zino builds AutoInformed from his background in automotive journalism starting at Hearst Publishing in New York City on Motor and MotorTech Magazines and car testing where he reviewed hundreds of vehicles in his decade-long stint as the Detroit Bureau Chief of Road & Track magazine. Zino has also worked in Europe, and Asia – now the largest automotive market in the world with China at its center.
Seattle Streets Safer Because of Lower Speeds
Accidents on Seattle streets were less likely to cause injuries after the city lowered speed limits, a new study published today by the Insurance Institute for Highway Safety shows. In downtown Seattle, lowering default speed limits reduced the likelihood that a crash would involve an injury by a fifth on arterial roads, IIHS researchers found. Outside of the city center, where the new limits were less consistently implemented and publicized, there were smaller injury reductions, but these were not statistically significant.
Newton* understood this in his F=MA equation. That is, Newton’s second law says that force is proportional to what is required for an object of constant mass to change its velocity. This equals the object’s mass multiplied by its acceleration. The increase in weight (Mass) and speed (Acceleration) results in a substantial increase in the energy (Force) involved in such an accident or test of crashworthiness.
“When we talk about the Safe System approach, we always stress that nobody should have to die because of a mistake,” said IIHS President David Harkey. “These results illustrate the value of rethinking speed limits. Crashes still happened after Seattle’s changes, but they weren’t as dangerous.”
An earlier IIHS study showed that lowering speed limits in Boston resulted in less speeding. However, it did not investigate the impact on crash or injury rates. The study of Seattle’s program is one of the first to examine the effect of lower limits on injury rates in a large, populous U.S. city.
“Everybody thinks of highways when we talk about speed limits, but reducing speeds on city thoroughfares and residential streets is just as important,” Harkey noted. “Nationwide, nearly a third of crash deaths occurred on urban arterials in 2020.”
Higher speeds make crashes more likely by reducing the time a driver has to react and increasing the distance required to stop the vehicle. Higher speeds also increase the energy involved in a crash, raising the odds of an injury (See Newton above*).
As of November 2016, Seattle lowered the default speed limits from 30 mph to 25 mph on its arterial roads and from 25 mph to 20 mph on smaller, mostly residential streets, unless otherwise posted. To make drivers aware of the change, the city conducted a public outreach campaign and installed gateway signs indicating the new citywide limits on arterial roads into the city, highway off-ramps and ferry terminals.
At the same time, new 25 mph speed limit signs were installed on arterial roads within the downtown area. Most arterials outside the city center had higher posted speed limits, and these mostly remained in effect until 2018, when Seattle began installing new signs on more arterials outside downtown. By the end of 2019, speed limit signs had been changed in eight of the city’s 32 urban centers and villages, including downtown, and on some arterial corridors in other areas.
IIHS presumed that the largest impact would be a reduction in crash injuries – the main focus of the Safe System approach. Researchers looked at the proportion of crashes involving fatal, serious or evident injuries, comparing the before-and-after injury rates with three control cities in Washington where there were no known changes to speed limits over the study period.
In all four cities, a large majority of crashes occurred on arterials. The proportion of crashes with injuries dropped on these major roads in the three years after the speed limit reductions in the downtown area of Seattle, while the proportion of crashes involving injuries increased in the control cities. This was also true for all arterial and smaller roads in the downtown areas combined.
Controlling for weather, lighting conditions and other factors, Seattle’s speed limit reduction was associated with a statistically significant 17% drop in the odds of an injury crash downtown and a non-significant 7% drop outside the city center. On arterial roads only, there was a statistically significant 20% reduction in the odds of an injury crash downtown and a non-significant 11% decrease outside the city center.
After the period covered by this study, in 2020, Seattle lowered speed limits on most remaining arterial roads to 25 mph. At the same time, it removed about 750 old signs and installed ~2600 new ones in greater density.
“These results suggest that communities can reap substantial benefits by lowering speed limits,” said IIHS Senior Research Transportation Engineer Wen Hu, the lead author of the study. “To reduce injuries even further, communities should combine lower speed limits with engineering solutions, public education about the importance of reduced speeds, and high-visibility enforcement.”
On non-arterials, there was no significant change in the odds of an injury crash, either downtown or in other parts of the city. However, that might partly be due to the small sample size and the nature of the roads themselves. Many residential streets in Seattle are narrow lanes with on-street parking on both sides, so drivers may not be comfortable driving much faster than 20 mph regardless of the default limit.
It’s also likely that the program was more effective where speed limit signs were posted than in areas where drivers were expected to remember that the default limits had changed throughout the city. That explains why the largest effect was observed downtown, where new speed limit signs were put in place at the same time the limits were lowered.
“When it comes to reminders, a clearly visible speed limit sign is pretty hard to beat,” said Hu. “We expect Seattle to see even bigger benefits as the city completes the transition to new signage.”
AutoInformed on
About Ken Zino
Ken Zino, editor and publisher of AutoInformed, is a versatile auto industry participant with global experience spanning decades in print and broadcast journalism, as well as social media. He has automobile testing, marketing, public relations and communications experience. He is past president of The International Motor Press Assn, the Detroit Press Club, founding member and first President of the Automotive Press Assn. He is a member of APA, IMPA and the Midwest Automotive Press Assn. He also brings an historical perspective while citing their contemporary relevance of the work of legendary auto writers such as Ken Purdy, Jim Dunne or Jerry Flint, or writers such as Red Smith, Mark Twain, Thomas Jefferson – all to bring perspective to a chaotic automotive universe. Above all, decades after he first drove a car, Zino still revels in the sound of the exhaust as the throttle is blipped during a downshift and the driver’s rush that occurs when the entry, apex and exit points of a turn are smoothly and swiftly crossed. It’s the beginning of a perfect lap. AutoInformed has an editorial philosophy that loves transportation machines of all kinds while promoting critical thinking about the future use of cars and trucks. Zino builds AutoInformed from his background in automotive journalism starting at Hearst Publishing in New York City on Motor and MotorTech Magazines and car testing where he reviewed hundreds of vehicles in his decade-long stint as the Detroit Bureau Chief of Road & Track magazine. Zino has also worked in Europe, and Asia – now the largest automotive market in the world with China at its center.