Mr. Go Versus the Marsh: A Traffic Parable
Once upon a time, there was a large fellow named Mister Go, and he lived in the swampy lowlands of Louisiana's Cajun Country. Every day, he'd sit and watch the big oceangoing ships as they departed New Orleans, bearing Midwestern grain and made-in-America manufactured goods destined for ports all over the world.
This individual was not, in fact, a person. Mr. Go is the nickname for MRGO, the Mississippi River Gulf Outlet, which was completed by the Army Corps of Engineers in 1965. For decades it provided ship traffic from the port of New Orleans a shortcut through the river’s labyrinthine, muddy delta and into the Gulf of Mexico.
The drawback to MRGO became all too clear when storms dropped colossal amounts of rain on the Gulf Coast region in a short time. The original marshes south of New Orleans were capable of absorbing the resulting huge influx of water, distributing it across hundreds if not thousands of tiny channels, and slowly but steadily filtering it through. These marshes were New Orleans’s best protection against cataclysmic flooding.
The deep, dredged channel of MRGO, on the other hand, would, in flood conditions, become a fire hose aimed squarely at New Orleans. It was a significant factor in the destruction wrought by Hurricane Katrina in 2005. Our nemesis Mr. Go amplified Katrina's storm surge dramatically and funneled all that water straight intro the city.
In 2009, the channel was closed for good.
What Does This Have to Do With Urban Traffic?
A connected network of narrow, calm, slow streets, in which there are many equally attractive routes from A to B for drivers to choose from, is like the myriad of channels water can take through a marsh. The grid does a great job of dispersing traffic and preventing congestion.
A wide, fast highway through an urban area, on the other hand, is a lot like Mr. Go. The same goes for an arterial stroad: a multi-lane thoroughfare that is a street/road hybrid. A stroad is designed for high speed and a large volume of cars (like a road), yet has frequent intersections and is lined with homes and businesses (like an urban street).
At uncongested times, the highway or stroad is the fastest way through the area, and so most drivers use it. At rush hour, on the other hand, it floods with cars, quickly becoming congested and a huge nuisance to those in the surrounding area.
There are a lot of reasons to want to get rid of urban stroads. They're ugly. They're frequently congested. They depress nearby property values. Most importantly, they're deadly by design, because they inject high-speed traffic into an environment where people are likely to be present—on foot, in wheelchairs, on bikes or scooters. A wide road in an otherwise walkable area functions like a moat, a barrier which deters people from crossing.
But when a road diet—removing lanes and shrinking the stroad to the more appropriate dimensions and design of an urban street—is proposed, the same objection universally arises from concerned citizens:
“What will happen to all the traffic?”
In fact, the answer reveals a fundamental paradox of traffic flow: removing the highest-capacity leg in a network may actually make the network as a whole function better, with less congestion and smoother travel.
How is this possible? The Marsh vs. Mr. Go metaphor helps us understand.
Here's a grid of city streets with a four-lane stroad running east-west through the middle. As drivers enter the area from the east and west, the stroad is the widest, fastest, and most direct route through town, so most of them use it, with the exception of the small handful of drivers whose actual final destinations are on the side streets:
At rush hour, there are more drivers present, and the stroad starts to get congested. Eventually, it slows down to the point where it's no longer any faster than the side streets. At this point, additional drivers will use those side streets.
Now let's look at what happens if we shrink the stroad to a two-lane street. This is the urban equivalent of closing Mr. Go and restoring the natural wetlands.
Drivers entering the area see several routes that are all of roughly equivalent speed and convenience. So they naturally spread out. This is the same number of cars as shown above (the peak, rush-hour level of traffic) distributed across the grid:
Every street ends up handling a moderate amount of traffic. No street is terribly congested even at the busiest time of day.
The Ewing Theory: How Having a Dominant Player Can Hurt a Team
There is a large body of academic research to support this counterintuitive finding about how removing capacity from a network might not make traffic worse—and adding capacity might not make traffic any better. Engineers refer to Braess's Paradox, a mathematical explanation for the phenomenon. Laypeople, though—especially those who follow basketball—may know it better as the Ewing Theory.
The Ewing Theory is based on the observation that when Patrick Ewing played for the New York Knicks, the Knicks inexplicably seemed to do better whenever Ewing was not on the court. This despite that he was the best player on the team and one of the all-time greats of the NBA.
In fact, the Knicks played better without Ewing because he was the best player on the team—a dominant "big man." When he was out, the rest of the team had to cooperate better. They had to share the ball more. This threw the defense off its game, and gave other players—not as purely talented as Ewing—more shooting opportunities.
The big, wide, fast stroad—or full-fledged highway—in the middle of your downtown grid is like Patrick Ewing to the Knicks. Or like Mr. Go to the Mississippi River Delta. Is it actually helping?
The Resilience of a Connected Network
A fine-grained street grid offers other advantages besides the more even distribution of traffic:
It's more resilient. If a crash or construction blocks one street for a while, the overall flow through the area is not nearly as disrupted.
It can remove the need for signalized intersections. Smaller, low-volume streets can have roundabouts instead of stoplights, which are more efficient, safer, less expensive to operate, and will still function if the power goes out.
It's safer. Here's the most important thing of all. A huge proportion of serious crashes and traffic fatalities in our cities occur on stroads. If the speed limit is 35 to 50 miles per hour and the roadway is wide, it is only a matter of time before someone gets hurt.
Small, urban streets, on the other hand, move traffic through an area slowly and safety, but smoothly. The result, like a coastal marsh teeming with birds and fish, is an environment where life can flourish.
A note on credit where credit is due: I am not the first person to make the analogy between Mr. Go and an urban street grid. I read a similar argument years ago on a blog, and have been unable to find the source. If it was you or someone you know, please drop me a line, so I can give proper attribution!
(Clip art used to create street grid graphics is from The Noun Project. Users: walkerstudio13, Fahmihorizon, Pamerat, iconsphere, Iris Li, Lluisa Iborra, Made, Bakunetsu Kaito, Vladimir Belochkin, Guilherme Furtado, Jacqueline Fernandes, Ladalle CS)