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The past decade has been a strange year for motorists. With the emissions era seemingly long behind us and modern chassis and tire technology far beyond what was possible when the interstate system was first conceived, state governments have taken it upon themselves to push for higher speed limits on limited-access highways. Meanwhile, modern urban planners have advocated for traffic diets and lowered limits in cities, towns and their surrounding suburbs. But whatever the results of individual projects, it is difficult to measure the broader effectiveness of these initiatives without years of hard data. Enter AAA.
Over the past five years (with 2020 ruled out as an outlier due to the pandemic), AAA has conducted a study of several speed limit adjustment projects across the country. The study included instances where speed limits were raised and lowered, and looked at road-based projects in four main categories — interstate highways, major arteries, minor thoroughfares and collectors — from around the US. While your local project may not be on the list, chances are there’s something similar in the mix. Let’s look at some of the main takeaways.
Raising speed limits can lead to more fatal crashes
While deaths increased on three of the roads where limits were raised, they fell on two others; a sixth showed no change. Two of the more fatalities studied were I-70 and I-95; I-84 saw a decline during the same period. While the combined number of fatalities increased between the three highways, it was not to a statistically significant degree, the AAA said. Variations in fatalities on all three highways were small (single digits). Those small differences indicate a small sample population; a longer term study could probably provide a more definitive analysis.
As we move away from highways, the data gets even murkier. Fatalities increased or decreased by only low single-digit amounts on surface streets. In two cases they increased. Two others went down. The remaining two showed no data for either the study period or the control period. AAA also provided data on accidents resulting in non-fatal injuries or property damage only. Their trends followed those of fatalities, lending credence to those figures despite the smaller sample population.
Changing the limit does not necessarily change the driver’s speed
Conventional wisdom might dictate that higher speed limits result in higher speeds. While that’s generally true, AAA found that motorists on interstates where limits were raised don’t necessarily go wild with their newfound freedom. On three highways where the limit was increased by 5 mph (I-70 in Maryland, I-95 in Maine, and I-84 in Oregon), the 85th percentile speed increases were less than 3 mph. I-84 in Oregon showed the largest average speed increase — 4 mph — but even there, 85th percentile speeds only increased by 2.8 mph. AAA says the change did not have a statistically significant impact on travel times on any of the three routes, nor was there a noticeable change in traffic volume due to the higher speed limit.
This also applied to arterial roads. A 5 mph increase on TX-289 (Preston Rd) resulted in an 85th percentile speed increase of only 2.1 mph. An identical change on Victory and Obama Boulevards in California resulted in an 85th percentile speed increase of 2.4 and 1.6 mph, respectively. In all three cases, AAA saw compelling evidence of reduced travel times.
And what about cases where speed limits were lowered? AAA was unable to study interstates with reduced speed limits during the time frame of this study, but six arterial roads and arterial roads were examined, five of which had their limits reduced by 5 mph. The sixth – a stretch of Southwest Capitol Highway in Oregon – was reduced by 10 mph. In only three of these cases, drivers’ perceived average or 85th percentile speed dropped: Division Street in Oregon saw an 85th percentile drop of 2.2 mph despite the average speed of all drivers increasing at 0.8 mph. Northeast Marine Drive (also in Oregon) saw an 85th percentile drop of 1.5 mph against an average decrease of 0.3 mph, while 85th percentile speeds on Southwest Capitol Highway increased by 0.4 mph against an average drop of 1.5 mph – and that’s on a street where the speed limit was reduced from 35 mph to 25.
But before you grab your pitchfork and go after City Hall for lowering the local speed limit, would you like to guess how many of those changes have resulted in an increase in commute times? Yes, the number is zero. Driving 25 mph in the city might be stupid, but it turns out it doesn’t really stop anyone. Overall traffic volume was unaffected, AAA says.
Raising speed limits reduces speeding tickets
Of the conclusions suggested by AAA’s study, the most compelling was its speeding analysis. In all cases where the speed limit was increased, speeding offenses decreased. In the five cases where data was provided for roads whose limits were lowered, those violations were more frequent. And unlike the fatality data we discussed above, AAA says the violation trends were statistically significant. The increase in ticket sales was particularly pronounced right after the speed limit was lowered, which makes sense since many drivers used to their routines were probably unaware of the change. Together with the above, this suggests that motorists tend to choose their speed based on their level of comfort on a given stretch of road, rather than blindly adhering to posted speed limits.
