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Intersection design — or how the turning radius is shaping your life

MinnPost photo by Bill Lindeke
A typical turning radius on Grand Avenue, where pedestrian safety has been a problem.

The turning radius: a seemingly obscure street design detail that governs a lot about our cities.

Basically, the turning radius measures the angle of the curve at a street corner. Turning radii (that’s the plural, of course) are such mundane parts of the built environment that they become almost invisible to 99 percent of people using the street.

As it turns out, radii can be a hot-button topic for city engineers because these subtle specifications have a tremendous control over vehicle speeds, pedestrian safety and how trucks navigate our cities. Some recent changes in Minneapolis and St. Paul point to the unsettled debate over turning radii, and how to make streets safer in a world of unruly drivers.

Defining radii

Technically known as the “minimum centerline turning radius,” the best definition for the turning radius is a circular arc formed by the turning path of the front outside tire of a vehicle. For drivers, it controls the vehicle speed. (You have to drive more slowly around tight corners with smaller radii.) In addition, the turning radius can depend on the size of the vehicle; trucks, for example, have different paths for front and back wheels.

For drivers, turning radius controls the vehicle speed.

“There are recommended minimums in the design standards,” K.C. Atkins told me. “However, determining radii is really on a case-by-case basis, depending on where you are on the roadway.”

Atkins is a Minneapolis-based transportation engineer at Toole Design Group, a national consulting firm, and turning radius is a big part of her life. She’s currently working on preliminary designs for St. Paul's downtown bicycle network, where intersection design will play a big role.

Rethinking radii is important because for much of the 20th century the trend in U.S. cities was to increase the turning radii at intersections to try to accommodate higher speeds. These days, however, there’s a change in direction as more cities are trying to improve walkability. Like many street design variables, there’s an unavoidable trade-off between pedestrian safety and traffic speeds, and the turning radius forms the balance point between these priorities.

What is the design vehicle?

For many busier streets, one key variable guiding engineers is the so-called “design vehicle." Are you designing an intersection around personal cars, buses, or semi-trucks? How you answer that question makes a big difference.

City of St. Paul
Turning radius illustrations from Saint Paul's draft street design guide.

“We’re definitely learning a lot more in urban areas about tightening up the turning radii,” Atkins told me. “Before, we used much gentler radii around corners. Agencies looking at what design vehicles they’re designing for have a trend in urban areas of designing for smaller truck types. In many locations, you don’t need quite as large radii as you needed in the past.”

St. Paul’s soon-to-be-adopted new street design manual has a whole section on reducing turning radii. But they take care to mention the importance of emergency vehicles or trucks, which find tight turning radii difficult to negotiate. The St. Paul manual has this to say:

Because emergency vehicles have sirens and flashing lights and other vehicles must pull over, they can typically use the full right-of-way without encountering opposing vehicles. On busier streets, the ability of emergency vehicles to swing wide may be limited by queued traffic that may not be able to pull over.

Another factor for trucks is that they can often be more flexible about how they use an intersection. For example, according to KC Atkins, it’s technically legal at tight corners for trucks to use “both” lanes while turning.

“Say there are two one-way roads,” Atkins told me. “It’s possible a truck could use parts of both lanes to make that turn. Or if there are two lanes on the side they’re turning into, they could possibly turn into the left lane.”

Off-road driving

One persistent problem for engineers: Even if they try to improve safety through decreasing turning radii, some drivers will simply begin crashing into the pavement or streetscape.

“The dangerous factor for turning radius is unruly drivers,” Atkins explained to me. “If cars try to take tight corners too quickly, they often end up driving over the curb or damaging the lighting, pedestrian push button, or anything you have on that corner. From a maintenance perspective, trucks riding up over the curb tend to chip away at that. ... they want to make sure they’re not running over it.”

In other words, there’s literally an everyday war of attrition going on between sidewalks and cars. And because replacing infrastructure can be quite expensive, much of the time the sidewalks seem to be losing. 

The case of the Minneapolis medians

The question of turning radius came to the fore in South Minneapolis last week when Christopher Meyer, a local urbanist, noticed a Facebook photo of a Public Works employee slicing up newly installed pedestrian medians in his neighborhood, at the corner of Park Avenue and 28th Street. Meyer ended up calling his City Council member and quickly headed to the location to stop the employee from cutting up the pavement.

"It didn't actually get all that dramatic," Meyer told me. "The truck drove off while I was standing there, before I was able to confront them about it. I watched them make preparations to saw around the median at Park and 28th, after they had just finished sawing Portland & 28."

MinnPost photo by Bill Lindeke
One of the controversial pedestrian medians in South Minneapolis, slated to be removed because of turning radius concerns.

The medians are part of the city’s newest protected bike lanes, and sit on the intersections of 26th and 28th Streets and Park and Portland Avenues, all of which have traditionally been high-speed one-way streets dangerous for bicyclists and pedestrians. 

So what's going on with the disappearing medians?

“When 26th and 28th were resurfaced earlier this year, we put in a new protected bikeway with pedestrian refuge medians,” Heidi Hamilton told me. Hamilton is the deputy director for Minneapolis’ Public Works department, and doesn’t think the existing design is working.

“Our concerns are about turning radius,” Hamilton told me. “The medians themselves have real corners on edges, and we’ve had some vehicles running over those medians. On the cross streets, some of the drivers are confused about whether they should be turning into the bike lane, and there are some concerns about the visibility of the medians.”

Earlier this year, Minneapolis passed an ambitious plan to build more than 30 miles of protected bikeways. Streets like 26th and 28th, with plenty of excess capacity most hours of the day, are ideal candidates for this kind of bike infrastructure. However, mixing more people on foot and on bicycles with speeding traffic can be a recipe for disaster, and the city made sure that traffic calming was a big part of the picture. These medians, installed earlier this year, were meant to provide needed protection.

“There are always tradeoffs with design,” Hamilton said. “For turning radius, does it slow down traffic as much if you set the median back from the cross street? Does it now make pedestrians invisible? There are a variety of factors that need to be taken into account.”

The city remains firmly committed to removing the medians before winter takes hold, but a City Council member, Lisa Bender, is asking that a replacement plan be in place before any changes are made.

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Comments (11)

Busses and pedestrian bump outs

A couple years ago during the repaving and rebuild of streets in our neighborhood out local corner store had to argue very persistently that the plan for pedestrian friendly bump outs to shrink the crossing at an intersection was incompatible with city busses that made the turn and barely avoided the curb as it was already. I think you should end up designing for the least common denominator and the city design team should do good observational work of the area before they start proposing changes.

New concrete should always tighten the curb

Minneapolis continues to replace curb cuts for corner sidewalks in the interest of ADA compliance. They've been working on most corners in the Central neighborhood for the past few months.

Every time we replace a curb, especially a full corner, we should automatically tighten the turning radius and provide bumpouts -- unless there's a very compelling and immediate reason not to.

We need to significantly tighten the geometry of our streets to #slowthecars and make a safe city.

"But they take care to

"But they take care to mention the importance of emergency vehicles or trucks, which find tight turning radii difficult to negotiate."

This is an argument I hear often. Instead of retrofitting all of our infrastructure (expensive) to accommodate large emergency trucks, we should take the opposite approach. Right-size our infrastructure and have smaller, more-nimble emergency vehicles.

This makes much more sense from both a transportation and financial point-of-view.

Fire trucks doing EMT calls

There's also the whole thing about huge fire trucks increasingly doing a job that a small ambulance could do. Here's a chart marking that trend: http://streets.mn/2015/05/29/53928/

Fire Trucks doing EMT Calls

A very good point, as just this morning I saw a fire truck in Mpls - with the ladder attachment on it - respond to a call for a pedestrian who had fainted. They knew it wasn't fire related, and they certainly didn't need that truck if any fire truck.

I'm not sure if the full message is just not getting from dispatch to the station, or if they are willfully ignoring the "right size" approach to the response. I assume its the former.

Smaller, more nimble emergency vehicles...expensive too

Have you priced them lately? How will you fund their replacement and change the tax code to account for needed to depreciate vehicles before replacing? How will you force the manufacturers to downsize emergency vehicles?

Who speaks for "the city?"

Hamilton? And who is responsible for installing and then attempting to remove these without telling anyone a couple of months later? Who has decided to remain resolute in making these intersections more dangerous for pedestrians?

On an entirely different topic, when I took driver's ed, I was told that the driver in the lefthand illustration from St. Paul's design guide is turning from the wrong lane. Shouldn't the driver move into the right/parking lane before executing their turn, thus also no longer being in position to hook a cyclist that's continuing straight?

Frustrated drivers, frustrated,.....

walkers and frustrated bicyclists. And mostly not enough money to do "it right!" Until the entire rebuild to create our dream working world the nightmares and disagreements and slap to together gerryrigging of all the possibilities will continue. But dream away. There are plenty of street corners that have in my experience have become less safe for one of the modal travelers after redesign. One of the things that erks me in the rebuilds are the loss of sight lines for any off the modes used in travel. I can adjust to a lower speed to make a corner as a biker or a driver. But if the build is to the sidewalk I cannot see around the corner unless I am walking. Yes what I want is sight lines for any of the three. And of course enough money to do it all right.

Pedestrian Safety

This post only mentions pedestrians in passing, but my personal experience would argue for tighter corners as a rule, primarily for the benefit of people NOT in cars.

My brush with turning radius came at 46th and Nicollet. My 6-year-old son and I were at the BP station (NE corner), about to cross Nicollet. We had the green light, and the walk signal. As we approached the crosswalk, my son ran up from behind me, and was about to step off the curb ahead of me, when a car traveling westbound on 46th turned northbound onto Nicollet at nearly full speed.

The driver also had the green light, of course, albeit with the expectation that he would yield to any pedestrians either in or about to enter the crosswalk. But with a very generous turning radius, there was no reason for him to slow down, and he did not. (It's worth noting that there is a five-foot-tall utility box built in to the sidewalk near that corner, making it harder for drivers to see whether a pedestrian is about to enter the crosswalk, and I think that was a contributing factor. I encourage readers to look at this intersection on Google's StreetView. Among other things, you will see that that the corners are cut far away from where the crosswalks converge, making pedestrians highly vulnerable to turning cars.)

Luckily, I was swiveling my head, worried about just such a possibility, and saw the car coming toward us in my peripheral vision. I threw a hand into my son's chest and knocked him backwards onto the sidewalk just as the car reached us. My elbow hit the rear quarter of the car as it zipped past. It is not exaggerating to say that we both would have been killed or horribly injured had my head been turned the other way at that moment.

Obviously, the driver is to blame here. Pedestrians are wise to remain fully aware as they step into any street. But the more I thought about it, the more I realized that the turning radius of the corner facilitated the entire episode. A tighter radius would have made what he did -- turning at nearly full speed -- impossible. Forcing cars to slow down in a turn like that gives them a greater chance to notice a pedestrian, and the pedestrians a greater chance to get out of the way. Our near miss was caused by the speed of the vehicle, something which can easily be limited by adjusting the turning radius of the corner.

We were both seriously shaken. We walked in silence, hand in hand, for about 10 minutes, then sat down on the steps of a house in the neighborhood to talk about what had happened. We talked about how dangerous it is when people on foot get close to cars on the street. We talked about how drivers don't always follow the rules, and how they sometimes don't see important things like children. We talked about how sad our family would be if something had happened to us. We talked about why Dads get mad when kids don't look both ways before crossing streets -- even when they have a green light.

I hugged him tightly. It had been that close.

Being a pedestrian in the city is dangerous. Being a parent with a small child at an intersection is a call for extra vigilance -- which is what saved us here. But there have been several recent pedestrian deaths in St. Paul which give me pause. It's hard to tell from news reports, but I wonder if one or more of those deaths could have been prevented simply by using better sidewalk, crosswalk, and corner design.

snow?

Bill does the potential for large snowfallls factor into this work here? Thinking of the winters where we have lost substantial road width for long spells. Do they just assume intersections can be completely cleared at all times?

sneckdowns

Interesting angle on that question here: https://en.wikipedia.org/wiki/Sneckdown

I'll be sure to cover this once the snow falls!