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The limits and promise of speed limits

MinnPost photo by Bill Lindeke
Speed limits: more complicated than meets the eye.

If you’ve driven a car for any meaningful length of time, you know the sinking feeling of police lights flashing in your rear-view mirror. It happened to me the other day, in Iowa of all places. I was lost in thought (pondering urban design), but not slowing down quickly enough while driving through a 25 mph small town.

That terrible clenching in the stomach, and the inevitable ticket dance, is probably why speed limits have long been the most visibly hated aspect of road design. Speed limits make people mad! But the more you study them, the more speed limits become devilishly complicated things. While speed limits on highways have become oddly democratic (I'll explain how below), in cities it’s another story. For urban streets, because of an obscure and stubborn state law, Minnesota is falling behind an international trend toward lower speed limits that might finally be improving safety in cities.

The science of speed limits

For a long time, speed limits were misunderstood. Most people assumed that the limit guided people’s behavior, outside of a few lawbreakers who could be reined in by vigilant police. (Basically, it was a "Dukes of Hazard" scenario.)

That narrative really fell apart after the 1974 oil crisis led to a federal 55-mile-per-hour nationwide speed limit. During those years the highly unpopular “55” limit launched a host of studies about how speed limits can fail to improve safety, and repealing the limit became a cause célebrè for libertarian and automobile groups. When it was finally rescinded in the 1990s, it was because many traffic engineers concluded that speed differentials, not speed itself, made driving unsafe. (Differential means the difference between the highest- and lowest-speed vehicle on the road.)

The paradox of speed limits stems from the discrepancies among three engineering concepts: design speed, posted speed and operating speed. Posted speeds are most obvious: those signs you see on the side of the road, and that make you mad when you get pulled over. Design speed is the least obvious: It’s the (often vague) speed at which the road can be driven on safely in most conditions, a combination of pitch, curve, lane width, cross section and many other more fine-grained factors. Finally, the operating speed is how fast people actually drive. The goal of much highway engineering is to bring these three speeds into alignment.

The problem of speed limits is that people vote with their feet (especially the one connected to the gas pedal). Back in the days of the 55 limit, posted and operating speeds were horribly misaligned, so that, as one 1991 report put it

Driver compliance with speed limits is poor. On average, 7 out of 10 motorists exceeded the posted speed in urban areas. Compliance tended to be worse on low-speed roads, better on roads with prima facie limits, or where the speed limit was based on an engineering study.

Basically, back then, almost everyone drove over the speed limit most all the time — which made for a dangerous situation, with arbitrary police enforcement. Since those days, things have improved, at least on highways. Now, most Departments of Transportation use a democratic approach to setting posted limits, aiming them at the 85th percentile operating speed. (i.e. the speed at which only 15 percent of people are driving faster.) In theory, these kinds of speed limits reflect people’s actual behavior.

Source: Transportatoin Research Board
In a 1991 study, almost everyone drove over the speed limit most all the time.

Highways versus urban roads

On the rather hand, the new logic of speed limits starts to break down on urban streets. Because city streets have many intersections with stopped or very slow traffic, speed differentials (the root cause of crashes) only increase as speed goes up. Likewise, because of complex urban terrain, design speeds quickly change as parked cars, old buildings, curves or driveways enter the picture.

Source: Transportatoin Research Board

Pedestrians add the final wrinkle for urban speed limits. Due to a law of physics, the severity of crashes increases exponentially, not linearly, as speed goes up. Someone hit at 20 is far more likely to survive with minimal injuries than someone hit at 35.

The difference between freeways and cities is one of the main drivers behind the recent international trend toward lower urban speed. Places like Paris, Edinburgh and even New York City have begun reducing speed limits down to 20 or 25 miles per hour on city streets (unless posted otherwise). And despite complaints from lead-foot drivers and world-weary cops, early results suggest that it might be working. It invites the question of whether the same kind of thing is possible here in Minnesota, where the state-mandated lower boundary on speed limits is set at 30 miles per hour.

MinnPost photo by Bill Lindeke
The posted speed limit near a school in Stockholm is 30 kilometers per hour, or about 19 miles per hour.

Kathy Lantry is the head of Public Works for St. Paul, and a former City Council member from the city’s East Side. She’d like to see lower speed limits on many of the streets in her neighborhood, especially around schools.

“The No. 1 complaint as a council member was speed of traffic,” Lantry told me last week. “Over the years a desire by neighborhoods to reduce speed limits to 25 mph. The standard response is that to do that you have to change state law.”

Currently in Minnesota, cities that want to reduce speed limits below 30 miles per hour have to apply for permission from the state transportation agency. Unfortunately, it’s rarely granted because, as MnDOT explains in a thorough 2008 report, agency regulators are concerned that lower limits would be unenforceable and ineffective.

That response isn’t good enough for Lantry.

“Around a school, speed limits are the first line of defense,” Lantry told me. “The reason people have been opposed to it is because they think ‘nobody drives 30, so why would anyone drive 25?’ But maybe we might be able to reduce speeding just by lowering the limit. Maybe if the speed limit is 30, people drive 40. If it was 25, who knows? We have to inform the public about why driving the speed limit makes sense.”

The slow-speed movement

It’s long been a kind of truism in the law-enforcement and highway-engineering communities that speed limits, by themselves, cannot solve traffic safety problems. Set them too low and people will increasingly begin to ignore them.

But in neighboring states like North Dakota, Wisconsin and Iowa (as I learned last month), cities and towns can set speed limits below 30. And for some people, especially those on foot or bicycle, it makes a difference.

MinnPost photo by Bill Lindeke
A Wisconsin speed limit set at 25 miles per hour.

“It was quite obvious when I was riding around in Fargo (North Dakota) last year that the law is enforced,” Dorian Grilley, director of the Bike Alliance of Minnesota, told me. “People do drive slower, and it was really noticeable. Wisconsin and North Dakota have 25 mph community speed limits, and for consistency you’d like to see that in Minnesota, too.”

For the past few years, walking and biking advocacy groups have lobbied the state to change the law so that cities can decide their own speed limits, and lower them in certain urban areas. But so far, including this year, speed-limit deregulation has made little progress at the Capitol. Lawmakers often cite concerns about the inability to enforce existing laws, and you can be sure that the specter of the '70s-era “55” debacle lingers in the air.

But as more and more cities around the world and the country begin to experiment with lower speed limits, eventually Minnesota might have to begin looking at making city streets places where cars drive more slowly, and people feel more comfortable crossing walking around urban neighborhoods. Until then, as your mother surely told you, be sure to look both ways before crossing the street.

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Comments (14)

Thanks Bill, good article. I

Thanks Bill, good article. I may suggest using the formula for kinetic energy instead of the one for force to explain how crash energy rises exponentially with speed. Rather than F=ma (force equals mass times acceleration), it might be a little clearer to use the formula E_k=½mv² (kinetic energy equals one-half of mass times velocity squared). The squaring of velocity (speed) gives this exponential growth.

Using that formula, my 3,300-lb. VW Jetta has a kinetic energy of 58 kilojoules when traveling at 20 mph, but 232 kJ at 40 mph, a 4x increase despite a doubling of the speed.

Anyway, it's a little unnerving to realize that many people really regard the speed "limit" sign as a lower bound for a target speed on a highway -- To a certain extent, drivers ignore speed limit signs and go with the flow a lot, though many will take the posted limit and mentally add 10 mph.

Regardless of the origin of the 85th percentile rule (which I've heard is a bit mysterious), there's pretty much always an impetus for people to go even faster, and that gets stronger every year as new models of cars are almost always more powerful than their predecessors.

see? this is a good comment.

just read it.

This kinetic energy equation

This kinetic energy equation is also good to keep in mind when people insist that bikers are a menance that should be subject to exactly the same rules and regulations as a dump truck.

Law of Physics?

Believe it or not, there is more than one "law of physics." You picked the wrong one. F=ma says nothing about speed or injury (change in energy, primarily), and IS a linear relationship. The one you wanted is E=mv^2 (sorry, can't find a delta on this keyboard), which gives you the exponential relationship between energy absorbed in a collision and speed. You got the concept right, but the supporting evidence wrong.

This is also the relationship that explains why difference in speed between vehicles is so important in traffic, while speed itself is not.

and this one leaves a lot to be desired

see the previous comment for actual information for the 99% of people who don't know physics equations, like myself, obviously

Urban/Suburban Speed Limits

A thoughtful and even handed piece on a topic that rarely gets that sort of treatment. One of the (many) complexities about trying to sell the idea of variable/lowered speed limits on low volume streets is that there is no compelling data in my suburban community to support Ms. Lantry's assertion that "maybe we might be able to reduce speeding by just lowering the speed limit". It been a pretty widely held and research support opinion that the major factors affecting driver's behavior in urban/suburban environments is a combination of their perception of what's going on in and around the road corridor and what speed they can "comfortably" navigate the physical constraints of the road. There's very little data (if any) to suggest that merely changing the number on the speed limit sign will accomplish a change in drivers behavior. It recalls the "Slow Children At Play" sign thinking that gives the illusion of public streets becoming safer by virtue of a sign that was largely ignored by motorists.

Regardless, no amount of legislation or signage should ever be sought as a replacement for the common sense practice of looking both ways before you cross a street. I hope that isn't really where we're leading people to in this dialog especially if we're doing our jobs as parents.

Speed kills

…is as much sophistry at truth. Logically, if speed is all that dangerous, we should avoid building highways in the first place, or limit highway speeds to some arbitrary low figure, setting a purely artificial balance point beyond which hitting someone or something dramatically increases the odds of serious injury or death. The blue and green chart accompanying the article is useful in that regard, whether Mr. Lindeke is using the right equations or not. Automobiles and trucks could easily and fairly simply be equipped with governors that simply wouldn’t allow the vehicle to exceed a designated speed. Most modern vehicles have computer-controlled engines, so tweaking the onboard computer to control the speed ought to be fairly straightforward.

It would also be counterintuitive and draconian.

If lower speeds are the answer to our automotive-related safety prayers, it would be less expensive and probably less intrusive to simply build roadways and vehicles significantly differently than we do now, and my guess is that those kinds of changes aren’t on the horizon. Europeans have discovered, or perhaps it’s more accurate to say they’ve rediscovered, that narrowing streets and eliminating lane markings both have significant effects in lowering speeds.

Changes in road surfaces will limit speeds as well. The Twin Cities are famous for spring thaw-related potholes. They’re dangerous only because people insist on driving through them at relatively high speeds. One budget-saving solution would simply be to not fix them. When a roadway deteriorates badly enough, people are forced to drive more slowly – not just a little bit, but a lot – to keep from wrecking their cars. Voilá, no need for police officers to pretend they’re working by training radar and laser guns from the windows of their squad cars. We can put them back on the street where they belong, since no one can drive more than about 20 mph anywhere. Adopting such a “no maintenance” policy state-wide, or even nation-wide, would not only save government entities huge sums of money, which could then be devoted to other, more productive uses, it would prevent many thousands of serious injuries, while simultaneously saving many, many lives.

One major beneficial side effect of such a policy change would be to resurrect the country’s barely-surviving interstate passenger rail service. Buses would have the same problems as automobiles, and flying, already too expensive, would become prohibitively so. Interstate highways with posted limits of 70 mph, but practical travel limits of 20 mph because the road isn’t maintained could easily spark a rail travel revival of huge proportions. Rail shipment of freight would once again become the predominant means of getting goods from point A to point B. Sure, truck manufacturers and over-the-road drivers would be out of work, but they have skills that would be valuable to the revival of rail transit, from building locomotives instead of trucks to operating trains, scheduling them, loading and unloading them, and so on.

Of course, adopting a “no maintenance” policy flies in the fact of a century-long trend in demand toward more speed in virtually every field of endeavor, by people in every line of work. The same housewife (it’s rarely males who complain about too much speed on the highway) who complains about speeding drivers in her neighborhood is quite willing to put pedal to the metal in someone else’s neighborhood. Much of the objection to “speed” as a safety issue boils down to something parochial (I want my kids to be able to play in the street), or is a reflection of some other problem (my kids have to play in the street because there’s no nearby park – a common suburban malady that has more to do with land-use and tax policy than automotive safety).

And, while speed per se is sometimes a factor in injury and fatality-related automotive crashes, based on what I read in a variety of publications, it seems to be speed differentials and, increasingly, lack of attention to the task at hand (driving) that play a far greater part than the media generally acknowledge in sober crashes. One issue unaddressed in the article is driving skill. In the land of “We’re all above average,” that’s understandable, but my experience of Minnesota drivers in the north metro is that driving skills in this part of town are, to be polite, minimal. We also downplay, as a society, the number of drivers whose already-minimal attention and driving skills are further impaired by alcohol.

Over more than half a century of driving, I’ve learned never to lead the pack of cars on the interstate (a worthy research topic would be *why* drivers/cars tend to travel in packs on interstate highways). Leading the pack simply makes my car the first one hit by the radar beam. On the other hand, if the pack is traveling at 72 mph in a 60 mph zone, I’ll be happy to slip into the middle of the group and simply keep up, letting the leader and tail-end Charlie get the tickets…

The whole point of paved roads is higher speeds. Add wider lanes and design geometry that purposely makes curves gradual and broad, and you’ve created an invitation to go fast. Attempts, then, to limit that going-fast are going to be frustrating for all the parties concerned, since limiting speed on roadways designed for speed is going to seem (and *be*) illogical on several levels. If low speeds are really what we want, there are numerous ways – none of them involving law enforcement personnel or vehicles – that would lower speeds across the board. We simply don’t want to adopt them as a society because, as a society, we want *faster,* not *slower* to be the watchwords.

Speed Limits

Most are bogus... drivers, drive according to conditions. There are always the speedsters and soccer moms in a hurry to get somewhere and the speed limit has no meaning to them. The rest of us pay attention to surroundings in residential areas and commercial corridors. We are defensive drivers and use caution when entering or exiting lanes of traffic. It is a standard bell curve that is set by drivers and highways are no different. Politicians try to control this with artificial limits thus becomes the great debate and conflict of interest. None of this has to do with safety or the reduction of accidents. If limits are to be imposed they should reflect the 85th percentile of the Bell Curve determined by drivers.

Good article, more on the math

A nice article that clarified the complex issue of how to deal with the endless struggle between technological capability and human nature. It reminded me of a conflict I once encountered in Montana between "reasonable and prudent" and a highway's design. The highway won: I slowed down.

When I first read the equation F=MA I was tempted to write a correction similar to those by Hicks and Scholin, but after thinking about it, I'm unsure if you're incorrect. In a head-on collision between a car and human being, there's a tremendous transfer of momentum to the victim. This takes place in a terribly short time and can be referred to as an impulse. The amount of momentum transferred is linearly proportional to the car's speed. However, I have to think that the duration of the impulse (the time it takes the momentum transfer to happen) is inversely related to the car's speed. Put that together and the poor victim is subjected to a peak momentum transfer rate that's proportional to the square of the vehicle speed. And since the momentum transfer rate is essentially the same as acceleration, the force on the victim is given by the equation in question, F=MA. The nonlinearity in question comes from A and the particulars of momentum transfer.

RIP Mr Cannon

I'd like to dedicate this comment thread to my high school physics teacher Larry Cannon (Henry SIbley H. S.) for his fine education skills. May he rest in peace, and I apologize for getting everything wrong.

What speed limits?

Speed limits? What speed limits? Sure, there are signs, but most drivers apparently can't read two-figure numbers. (Nor can many read the sign on I-94 before the Lowry Hill tunnel that says: "Do not cross double white line." Must be another failure of our educational system.) While I'm driving 60 in a 55-mph zone, hoping not to get rear-ended, almost all I see is tail lights getting smaller.

One problem is the typical American attitude of "I'll do whatever I want," coupled with another one endemic among the baby-boomer generation and those since: "I'm special."

A third and very big problem is lack of enforcement. Our understaffed state troopers are so busy racing from one time-consuming metro-area crash to another that I seldom even see any, and our "don't tell me what to do" public, and clever defense attorneys, have prevented useful measures such as unattended radar and red-light-runner cameras. (Seems to me pretty simple: You get a ticket and pay it or you make the person driving your car pay it.)

Until we as a society get over the idea that we have no collective responsibility, that we each can do what we want because the rules only apply to others, we're going to continue to have speed-differential chaos on our highways.

Do You Mean

The red light cameras that have been proven to INCREASE rear end collisions? I think they're a bad deal.

Lowry Tunnel

There is no such sign and there are no double lines in the tunnel. The single solid white line means to pass with caution. You should use more caution when selecting examples.

Interesting comment from Mn-DOT

I received this email from an official at Mn-DOT with further facts on speed limits (and Wisconsin in particular):

1. Wisconsin has 35% more pedestrian crashes and 25% more pedestrian fatalities than Minnesota from 2008-2013.

2. Minnesota cities and townships have the authority to lower the speed limits on their residential roadways to 25 mph (169.14, Subd.2.7.). They also have the authority to set school zone speed limits down to 15 mph (169.14, Subd.5a).

3. The primary cause of drivers going faster comes down to the design of the road and the surrounding environment. No one can design a roadway for capacity (volume and speed), then throw out a 25 mph sign and call it safe. Not only does it promote the disdain for speed limits you mention in your article, but it gives residents and pedestrians a false sense of security.