The Critical Ten


Visual perception at different speeds.

Compromise is the hallmark of politics, so when it comes to street design, it’s tempting to dole out design features like treats to unruly children: a new crosswalk for you, a drive-thru for you, a bike lane for you, a parking lot for you. There, now everyone’s happy!

And if you’re not satisfied with the outcome of the project, well, the city tried to do the best it could. Everyone got a little bit of something that they wanted, so quit complaining! So many street design conversations end up this way, with nobody really satisfied but nobody getting officially screwed.

The problem is that for a good urban street, this muddy “middle ground” between ‘walkable’ and ‘driver’s paradise’ can sometimes be the worst of both worlds. And as I’ve been traveling around Saint Paul lately, I’ve noticed one particular aspect of street design where the city is about 80% of the way to designing a successful street. But much of the time, it’s the last 20% of the design is absolutely crucial to the outcome. I think of it as “the critical ten.”

The Critical Difference Between 30 and 20

I’m talking about traffic speed. If you look at the average speed of traffic on a urban commercial streets, there are a lot of things that begin to change when you slow down cars from the 30 to 35 mile per hour range into the 20 to 25 mile per hour range. Most importantly, perception, reaction time, and crash outcomes are far better at 20 than at 30 mph, while traffic flow doesn’t seem to change very much.

The perception angle is perhaps the most interesting. [See above chart.] Driving speed has a dramatic effect on the driver’s “cone of vision.” You can see a lot more detail at 20: people on the sidewalk, a bicyclist in the periphery, or the ‘open’ sign on a storefront. At 30 mph, the window shrinks dramatically.

ped safety speed graphThe same is true for what you might call ‘reaction time.  [See below chart.] I’ll often talk to drivers about urban bicycling, and they’ll respond with a terrified story about the time that they “almost hit” a bicyclist that “jumped out” at them. And “I didn’t see them” is a common refrain heard by any police officer investigating a crash. The problem is that once you hit 30+ speeds, it’s a lot more difficult to stop in time to make any difference on a potential crash. 

These three factors are the big reasons that crash outcomes vary so dramatically on either side of “the critical ten.” It’s no exaggeration to say that lives depend on getting speeds right.

stopping distance speed graph

Slow Speeds and Traffic Flow

At the same time, the big concern for many engineers, drivers and civic leaders is how lower speeds will impact traffic flow. They amazing thing is that it doesn’t have to make a huge difference. When you’re talking about traffic flow on these urban commercial streets, speed is far less important than delay at intersections. A great example (from the UK) is the main street in the small suburban town of Poynton, which carries more than 26,000 cars a day. They recently dramatically re-designed the street to create “slow speed continuous traffic movement” by removing stop lights. 

The key to the Poynton example is creating a street with low speeds, slowing traffic down below 30 miles per hour. That’s the key to creating streets and spaces where cars and pedestrians, through traffic and local businesses, can really interact. 

stp selby avenue

The stretch of Selby Avenue where speeds stay below 30 miles per hour. Not a stop light in sight.

Here in Saint Paul, the best example is probably Selby Avenue (at least the part running a mile West from the Cathedral). To my mind, this stretch of Selby is the best designed commercial street in the city.

Cars travel slowly, but there are few stoplights. Sidewalks are wide and bumpouts are common. It’s easy to walk, cross the street, and even (relatively) easy to park if you’re willing to walk a few blocks (which many people are because of the nice sidewalks).

And not everyone will agree, but I enjoy biking down Selby, mostly because of the slow speeds and careful drivers. Twenty years ago, if you had told people that Selby and Dale would be one of the most thriving commercial spots in Saint Paul, few would have believed you.

gsp Grand Ave. Spaces

Cars drive very slowly around Grand and Victoria.

Selby’s slow speeds don’t seem to affect its overall traffic volume, and the street carries around 6000 cars a day through this stretch, as well as a high-frequency bus line. Similarly, the Eastern portion of Grand Avenue (which also works well, in my opinion) carries around 11,000 cars a day. Both of these streets are comfortably in the 20 mile per hour range, and are thriving economically.

For comparison sake, Smith Avenue (from the High Bridge to Annapolis) and Rice Street (from Como to Maryland) carry about 10,000 and 15,000 cars per day (respectively), are both struggling economically. Meanwhile, the Poynton UK example has combined sub 20 mile per hour design speeds with 26,000 cars per day (about the same as Snelling Avenue). Believe it or not, designing a street with slow speeds doesn’t have to have huge impacts on volume.

Why 20 is so Critical

Bikram Phuyel accident

Speeds are too high on Rice Street.

But doing the little things to push the design speed of a commercial street into the “critical ten” range can revolutionize our cities. Both Smith and Rice Streets are long struggling commercial corridors with lots of historic buildings and vacant storefronts. Both streets are cases where the city has tried to design a street that would work for drivers, businesses, and pedestrians alike, but hasn’t reached a design solution that encourages foot traffic that would support small businesses. For both streets, it remains difficult to cross the street, and both places cars are still traveling at dangerous speeds.


The intersection of Grand and Ayd Mill Road is about the spot where speeds increase past 25 miles per hour and become dangerous for pedestrians.

Grand Avenue is the perfect case study for why the critical ten is so important. For key stretches of the street (e.g. East of Lexington), Grand is a street that works very well for everyone. Cars travel slowly, you can parallel park, cross the street, and there are few accidents.

As you begin to travel West, speeds on Grand Avenue increase. There are large stretches where the street “tips over the threshold” and loses the virtuous cycle of pedestrian safety, street life, and thriving local businesses. Anytime you see those (oft-mocked) orange pedestrian flags, it’s a sign that we haven’t quite reached the design speed sweet spot.

Cities like Saint Paul have been trying for years to design streets that move cars while creating thriving spaces that support small businesses. It’s frustrating to see so many places that are almost well designed. These streets are victims of compromise, almost-but-not-quite safe for pedestrians and almost-but-not-quite good for small businesses. Sometimes, small differences in design can pay big rewards. I believe “the critical ten” is a key missing piece for our main streets, and that if we reduce speeds down to the 20 miles per hour range, our cities might find the renaissance they’ve been looking for.


Saint Paul’s Smith Avenue, where it’s still difficult and dangerous to cross the street.

16 thoughts on “The Critical Ten

  1. Walker AngellWalker Angell

    Selby between Western and Dale is a great example of this. People drive much slower on the end near Dale where it’s narrower and ‘feels more dangerous’ than on the end near Western. It’s not unusual to see someone heading east drive slow until just about the Happy Gnome and then speed up.

  2. Dana DeMasterDanaD

    I don’t know if this is fact or just planning urban legend, but I really like it. Seattle timed its downtown street lights so that the optimal speed is 20 mph. Drive faster and motorized vehicles get stopped every block. Although most drivers wouldn’t know to drive at 20 to hit all the lights, it is self-regulating and traffic adjusts itself to that speed.

    I have mixed feelings about Selby Avenue. On a Sunday morning I am happy to take my kids on the bike with me on Selby. Rush hour? Saturday evening? Not so much. We take Dayton.

    1. Keith Morris

      I’m still not sold on Selby being all that ideal: it’s only two lanes with low traffic but enough on each side that a cyclist will have a number of motorists approaching from behind queuing up and hardly any break in oncoming traffic to pass, safely that is. And then stops aren’t frequent enough to slow traffic down to a cyclist’s speed.

      I actually prefer parts of Grand because the high level of traffic and stops mean that maintaining the same speed as motorists is a breeze over there because the amoubt of motorists are alread slowing traffic speeds way down. It sounds counterintuitive, but rush hour is the best time to ride on streets you’re probably averse to using: Hennepin is one example where I will maybe ride it for a couple blocks max while during rush hour I feel fine going severaI blocks since I don’t have to bust my ass nearly as much to match motorists’ speed.

  3. Aaron Berger

    Yes. It is such a disaster that the only streets in Minneapolis with speed limits below 30 are the parkways. No residential street should have 30 mile per hour traffic. Twenty is plenty.

      1. Keith Morris

        It’s actually quite shameful that NYC has lowered their speed limit citywide to 25 while Minneapolis and St Paul, both just a tiny fraction of the area of NYC, are still clinging to the outdated notion that big cities need fast roads. There’s no good reason why 20 MPH isn’t the limit. It’s actually a very pro-suburban stance for a city that essentially says mitigating the extra seconds suburbanites are delayed are worth more than city residents’ lives.

    1. Rosa

      I’d be pretty happy if the speed limits we have were just enforced. I try to drive at or just below the speed limit whenever I’m driving a car on residential streets, and it makes it pretty clear that nobody really does that. It’s suprisingly hard to not just drift up to “speed of traffic” even when (rarely) nobody’s tailgating or honking at me for it.

      Or “enforced” with better design. The slowdown on Park & Portland with the not-so-new wider bike lanes & striping is amazing.

      1. Nathanael

        It’s much easier to “enforce” with road design which makes drivers feel like the road is narrow. Much harder to enforce with cops.

        When the road appears wide and straight it’s very hard for motorists to slow down — some sort of psychological thing. When it looks narrow and twisty most motorists slow down automatically.

  4. Matty LangMatty Lang

    That part of Grand by Kowalski’s is a nightmare. Their parking lot has three (!) curb cuts onto Grand. Add to that people racing to and from the Ayd Mill road ramps and it’s the perfect recipe for a hostile environment.

  5. jeffk

    This is awesome work, Bill. I drive sometimes,and I’d happily take a steady 25 over stop and go 40.

    But you do touch on an important issue, and one that is devisive among urbanist types. It’s the idea that in American politics, we all win by getting our due. But in the case of Selby, it’s not what’s there, it’s what’s not there. The simplicity is what makes it work. I’d take Selby as my ideal street any day over a “complete street” with separate spaces for everyone. And creating Selbys has the added advantage of being cheap, quick, and easy.

  6. Brian Quarstadbq

    Matty, I’d never thought about that before (multiple curb cuts at Kowalski’s) plus two exists and one entrance to Ayd Mill and Syndicate intersection all within a block. I travel past there often as well as traveling to Kowalski’s fairly regularly. How could we simplify that area?

    When Buffalo Wild Wings went in MnDOT insisted on changing the curb cuts on Snelling at the strip mall from two to one I thought there would be an issue with southbound cars trying to turn into the lot. But the single curb cut has greatly simplified things and there are now less accidents, less close calls and it’s easier to navigate on foot when walking down Snelling. It may also help crossing Snelling but if it’s helped it’s been negligible because of so many other factors that make that a dangerous cross.

  7. Matty LangMatty Lang

    A few years ago there was some attempt at getting a median installed and shutting down 1 or 2 of the curb cuts to the Kowalski’s lot. Of course, there was a lot of complaining about how that would put Kowalski’s out of business just like the proposed median at the liquor store with multiple curb cuts on Marshall at Cleveland and it didn’t happen.

    I can’t recall all of the details. Maybe Bill can tell the story?

  8. Tim

    I agree with the central point, but Poynton is not a good example. It doesn’t have lower limits or infrastructure to slow motorised traffic, except for non-standard “shared space” layouts, designed to confuse people into proceeding more carefully.

    Shared space doesn’t work well in busy areas (regardless of which modes are sharing). Poynton doesn’t deserve to have such high levels of traffic through the centre and I don’t enjoy driving, cycling or walking across the “roundels” (roundabouts) there.

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