Speed limits seem simple on the surface, but like many details of our transportation system, they get complicated when you dig a little deeper. A few years ago in this column, I described how speed limits have worked in Minnesota. Compared to our neighboring states, the Department of Transportation has long wielded a firm and controlling hand over those rules. This is why, when you travel to cities and towns in Iowa or Wisconsin, you encounter 20 or 25 miles-per-hour streets on a regular basis while you don’t in Minnesota.

But a lot has changed since 2015, and the picture has grown more intriguing. The compromises forged at the end of last year’s legislative session included a new law that allows Minnesota cities to set their own speed limits on city streets.

“During the 2019 session the state of Minnesota passed a state law that allowed municipalities, but not counties and not the state, to change speed limits within some parameters,” explained Kathy Lantry, St. Paul’s director of public works. “[This is only for] city-owned streets, owned and operated by the city. We now have the ability to regulate speeds different from the way we always used to.”

Director Lantry’s distinction about municipalities offers an important caveat. While most streets in St. Paul might appear equal, under the smooth facade they are organized in a complex way, through multiple jurisdictions. On any given trip through town, you could be driving on city-owned, county-owned, or state and federally controlled streets. Each of those roads has a distinct funding source, sets of regulations, and transportation engineers who design and maintain the right-of-way.

The recent rule change, for the first time in generations, allows cities to have more local control over speed limits. In both Minneapolis and St. Paul, the next step is one where cities, on their streets anyway, can begin to set their own pace, and use street regulations to accomplish civic goals. That’s a big reason for the note of caution on the part of Lantry and cities, who remain a bit cautious about carelessly wielding their newfound power.

“We want to make sure that what are doing with speed limits is defensible and replicable,” explained Lantry. “We want to have good engineering principles behind our recommendations for speed limits.”

The perversity of the 85th percentile rule

The best practices for speed limits date back over half a century to something called the Solomon Curve. It’s a principle named for Douglas Solomon, who did a study for the Bureau of Public Roads in the late 1950s of crashes on rural highways.

Solomon’s report showed that most crashes stemmed from speed differential, that when drivers traveled at greatly different rates, it is much harder to predict and judge behavior. This difference in speeds was a primary cause of crashes. And critically, the data in Solomon’s research posited that it was people driving too slowly, rather than too fast, who were more likely to be the root of problem.

As a result, the practice for transportation engineers for over half a century has been to set speed limits at a critical point that balances the risks of speed differential. And thanks to the curve, that point has been the speed where 85% of the people are driving under the limit.

“The 85% speed is the starting point that MnDOT uses to establish speed limits,” explained Brad Estochen, a traffic engineer for Ramsey County Public works. “We go out and measure free flow on the roadway and figure out what the 85th percent speed is, and that’s the starting point to figure out what speed limit should be established.”

With the 85th percentile rule, setting speed limits too low will create more divergence between the slower (law-abiding) drivers and the average drivers. And because speed divergence creates unpredictability, that’s dangerous.

“MnDOT has their process,” said Estochen. “They’re the ones empowered by the state to establish speed limits. We collaborate, and give input to MnDOT on their process.”

Speed limits in an urban context

Especially in cities, the 85th percentile rule can work in perverse ways. At its extremes, the principle rewards drivers who speed by changing the rules. If enough people break the law, the rule simply changes the law to accommodate them. For decades, that’s been how speed limits were set throughout the state.

That’s the rule that explains what happened on St. Paul’s city-owned Shepard Road.

“What used to happen? Shepard Road, after that was constructed, west of Chestnut Street was originally signed for 40 miles per hour,” explained Lantry. “The city went to the state, per the current state law before this change, and said we want to lower speed limits. But you’re required to do a speed study. What came back was that the state said you have to raise [the limit]. That’s the system that we had, prior to the state law change.”

If you know where to look, examples of the 85th speed rule are all around the streets of St. Paul and Minneapolis. There are lots of places where the speed limits have actually been increased because of the frequency of people speeding.

For example, on St. Paul’s Marshall Avenue, a county-owned road, the speed limit suddenly increases from 30 to 35 miles per hour just east of Lexington Parkway, right in the middle of the historic Rondo neighborhood. In an email, a Ramsey County engineer, Joe Lux, admitted the case was “curious,” and should have been corrected by now.

Another example sits on West 7th Street, a state-owned main road running through St. Paul. Just west of Randolph Avenue, the speed limits jump from 30 to 35 miles per hour, despite the street remaining the same width, and being lined with homes and businesses.

Two years ago, because of speeding on West 7th, the city got permission to install temporary bump-outs at the corners of Victoria Street and Otto Street, in the hopes that people could safely cross the street. But while the plastic bump-outs send one message, the speed limit signs continue to send to drivers the opposite signal.

Speeding and crash survival

Those tensions are why transportation officials in Minnesota cities are so excited about having more control over speed limits. Instead of going through the state Department of Transportation for approval of any changes, both Minneapolis and St. Paul are currently floating ordinances to give their own city engineers the power to set the rules.

“There is good empirical data that shows, in crashes between vehicles and pedestrians and vehicles and bicyclists, [reducing speed] saves lives,” explained Lantry. “This is one of those things where there’s no dispute. If a car hits a human being at X miles per hour, what is their likelihood to survive? We know those things. And so our basic entire message is three words: Slower is safer.”

For people worried about pedestrian fatalities, the new law comes just in time. After years of progress on street safety, the last few years have seen troubling increases in pedestrian fatalities. Reducing speeds, especially in urban areas, is one of the few easy solutions to the crisis.

In St. Paul, there’s no clear direction on where and how low to set the new urban limits. The city engineers are looking at a process to think about where and how low the limits should go, and even weighing a collaboratory approach with neighboring Minneapolis to create consistency between cities. In both Minneapolis and St. Paul, the new ordinances codify the ability of city engineers to set limits. But because of the complexity of roadway jurisdiction, it won’t be a simple matter to make changes.

At their recent budget hearing, St. Paul allocated enough money for 1,000 new speed signs in next year’s public works’ budget. At $325 per sign, that’s a lot of money for new infrastructure, but compared to just about any other investment, they hope this one will have sweeping safety benefits.

“The logistics of changing the speed limit, the concept is easy implementation and operations of it more difficult,” admitted Lantry. “I don’t have staff time to put up 1,000 signs in a week. It’s going to take us some time going to make an implementation schedule.”

Because the state law change does not include counties, the potential reach of the new speed limit freedom is limited to city-owned streets. This is a problem because the majority of fatal and injurious crashes occur not on the residential city streets, but on county- or state-owned arterials. For those streets, the old status quo of high-speeds and the 85th percent rule will still hold sway.

Lacking the freedom given to cities, the counties will still have to make a proposal to MnDOT engineers every time they make a change to the road. That doesn’t mean that there’s nothing they can do about speeding, but it does mean that speed limits cannot be the main answer. Instead, engineering changes will have to be the main factor determining driver safety.

“We’re doing things with roundabouts,” said Brad Estochen at Ramsey County. “We’re looking at roadways, using space a little more wisely to promote safety. Four lanes to three lanes, thinking operationally to pursue those as much as we can.”

The public hearings in Minneapolis and St. Paul on their newfound new speed limit power will be held in the coming months. For St. Paul, it’s this afternoon, Oct. 16; in Minneapolis, it’ll be later this winter. After that, if the ordinances pass, for the first time in decades it’ll be up to cities to set their own speed limits.

“This is as much about getting people to comply as it is culture change,” said Lantry. “The more we can get people used to the idea that speed limits are going to go down, the better.”

Look for lots of new signs, and a public campaign to persuade drivers to slow down on the city streets. If it works, it’ll surely save lives.