Transportation

Beyond 'Common Sense': How Traffic Networks Work

By Ryan McGreal
Published July 16, 2008

With all the talk recently about the Downtown Transportation Master Plan and some councillors' objections to spending money to convert downtown streets to two-way, it seems instructive to study transportation networks a bit more closely.

One one level, the debate boils down to a conflict between a) people who want to be able to drive across the city as quickly as possible; and b) people who want safe, comfortable streets for pedestrians, local businesses, local residents, and so on.

But even within this dichotomy, too many opinions rest on untested "common sense" notions of the dynamics at work - for example, the notion that one-way streets are more efficient for drivers than two-way streets, or that adding more lane capacity to a traffic network improves driving times.

As Stuart Chase famously pointed out, "common sense" is that which tells you the world is flat.

Counterintuitive Network Effects

Network dynamics are actually fairly well understood by mathematicians and systems theorists (i.e. people a lot smarter than me ). It turns out that networks can be remarkably counterintuitive unless you understand their underlying dynamics.

For example, adding another route to a traffic network can often make the system as a whole less rather than more efficient. It's called Braess's Paradox, named after Dietrich Braess, a German mathematician.

He demonstrated that adding another route to a traffic network can often produce a sub-optimal redistribution of traffic flow that actually makes traffic worse, assuming people moving through the network choose their own routes "selfishly". Similarly, removing a route can sometimes improve traffic.

Further, economist Anthony Downs observed that the equilibrium between motorists and transit users (and between peak and off-peak drivers) adjusts when you add lane capacity. The Downs-Thomson Paradox states that adding more peak driving capacity draws a corresponding number of people out of transit and into personal vehicles, causing overall traffic to remain the same or worsen.

Induced Demand

In a more general sense, traffic tends to increase to meet the available supply of road space. Economists call this "induced demand", and it simply means that when the supply of a product goes up, the price goes down and more people demand it.

In the case of traffic, more people choose to drive longer distances more often when it is cheaper and easier to drive. This even extends to decisions on where to live and work; without access to the Red Hill Valley Parkway, not many people would regard the billion-dollar Summit Park subdivision on the east Mountain as a desirable location to buy a house.

Paradoxically, cities that expend the most resources to accommodate traffic tend to suffer the most traffic, longest commutes, highest overall levels of air pollution, and so on.

Induced demand also explains how in a city like Hamilton, more than half the total air pollution comes from vehicles despite a traffic system of wide, one-way streets with timed lights designed to minimize idling at any given intersection.

Optimizing the efficiency of a given road (a subsystem) can pessimize the traffic system as a whole, because more people drive longer distances more frequently. That's why it's wrongheaded to try and maximize traffic flow-through (through timed lights, one-way streets, etc.) in the name of environmental concern.

Reverse Induced Demand

The principle of induced demand is true in reverse, as well. Removing road capacity can leave traffic unchanged or even improve it by triggering shifts in how people choose to get around and where they choose to go.

Certainly the traffic on James and John is not significantly worse than it was when they were one-way, though individual vehicles move more slowly.

Some cities have actually enjoyed considerable success in removing highway capacity. In Portland, Oregon, for example, the 1972 mayoral election became a referendum on building the highly controversial Mount Hood Freeway. It concluded with Portland using the federal funds to build a light rail line instead.

Emboldened, Portland then demolished an existing highway and converted it into a waterfront park. For this and several other reasons, Portland is now regarded as one of the most liveable cities in North America.

Similarly, after a 1989 earthquake severely damaged the Embarcadero Freeway in San Francisco, the city replaced it with a grand tree-lined boulevard that restored access to the waterfront and features art installations, pedestrian plazas, a farmers' market and vibrant streetlife.

Diminishing Returns for Added Lanes

Here's another counterintuitive property of traffic networks: the law of diminishing returns applies to added lanes on a given street. That is, doubling the lane capacity of a single road produces less than double the traffic carrying capacity. As you add more lanes, the marginal vehicle capacity per additional lane declines incrementally.

For example, let's say a one-lane street can carry 800 cars per hour. (Note: these numbers are just examples for demonstration purposes.) If you add a second lane, the second lane will be able to carry only 700 cars per hour. If you added a third lane, it will only carry 600 cars per hour. If you added a fourth lane, it will only carry 500 cars per hour.

A four-lane road in this scenario could carry 800 + 700 + 600 + 500 = 2,600 cars.

However, two two-lane roads could carry (800 + 700) + (800 + 700) = 3,000 cars.

In other words, the downtown traffic network might well work more efficiently overall if all the streets were converted to straightforward two-way, for the simple reason that two streets with two lanes each can carry more cars than one street with four lanes.

Ryan McGreal, the editor of Raise the Hammer, lives in Hamilton with his family and works as a programmer, writer and consultant. Ryan volunteers with Hamilton Light Rail, a citizen group dedicated to bringing light rail transit to Hamilton. Ryan writes a city affairs column in Hamilton Magazine, and several of his articles have been published in the Hamilton Spectator. He also maintains a personal website and has been known to post passing thoughts on Twitter @RyanMcGreal. Recently, he took the plunge and finally joined Facebook.

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By peter (anonymous) | Posted July 17, 2008 at 02:24:37

we need a good earthquake.

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By ventrems (registered) | Posted July 17, 2008 at 07:38:53

or a good mayoral election...

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By jason (registered) | Posted July 17, 2008 at 09:37:32

is it just me, or are the fonts all screwed up on this blog??

we need to toss out council, not the mayor. I realize he doesn't really fit in Hamilton with his common sense ideas and desire to turn the city around, but maybe someday we'll all appreciate someone with a vision for the city.

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By kevlahan (registered) | Posted July 17, 2008 at 10:18:32

Nice article Ryan!

As Eric Miller (transportation engineer at UofT profiled by Christopher Hume in the Star http://www.thestar.com/comment/columnist... ) told me: "we teach traffic engineers about these paradoxes, but they tend to forget about them once they start working in the real world".

Regarding Braess's paradox, it is actually more extreme and relevant than you suggest:

  1. Adding a new 'shortcut' with ZERO travel time (the ultimate shortcut) can increase the travel times for EVERYONE. The converse is also true: removing a "fast" shortcut can decrease travel times for EVERYONE. These results assume that the total volume of traffic is unchanged.

  2. On random networks, roughly 50% of new routes will lead to slow downs for everyone. This suggests that Braess's paradox type slowdowns should be quite common!

When combined with the induced demand effect, it is clear that increasing lane capacity is unlikely to improve traffic in the long run.

The exception is where increasing lane capacity and traffic speed drives out local residents and businesses, which then reduces demand. This is likely what happened to Hamilton downtown!

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By Ryan (registered) - website | Posted July 17, 2008 at 10:35:38

Thanks, kevlahan. I should point out that it was your helpful prodding which got me looking into these effects in the first place. :)

As you suggested recently: far from being a benefit we should celebrate and preserve, the absence of congestion on Hamilton streets is evidence of a serious deficiency in vitality. It's hypotension (low blood pressure) in the civic body.

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By Chris (registered) | Posted July 17, 2008 at 21:14:33

While I applaud the good intentions of this article I do not believe the two way conversion offers any significant improvement to the vitality of downtown Hamilton. I have lived in the lower city for 35 years and patronize small business in the core when ever practical. The deterioration of the core began with urban sprawl and the tendency of people to frequent business convenient to where they live. The key word here is convenient. The late 70's and early 80's saw franchises and specialty chain stores sap the strength of business in the core. Hamilton politicians did precious little to remedy the flight of capital out of the core and in to beautiful Upper James St. Sometimes when I am driving in Mississauga or London I am momentarily disoriented as for a few seconds the landscape of faceless franchise after franchise all blends together and I think I am on Upper James. Or a visit to some shopping mall has me wondering is this the Mapleridge Galleria or have all malls finally become interchangeable; universal in their bland sameness. Too many people with long commutes and multiple incomes have been robbed of the luxury of more relaxed downtown shopping time. When time is in short supply many opt for the convenience of the nearest shopping mall. Two way streets in the core are not going to entice these people back, particularly when there is a view of parking enforcement as a cash cow. I have had people complain of excessive parking fines for example a $50 ticket that turned into more than $80 less than 3 weeks from the infraction date. Note to City of Hamilton Parking Authority; people do not get parking tickets at shopping malls. Reserve the fines for serious traffic matters unless you want to send the more adventurous packing yet again. Two way streets provide a temporary captive audience at best. With James and John both two way I avoid sections of these streets like the plague. It is not relaxing or opening me up to the downtown sights to be stuck in traffic and I avoid placing myself in that situation unless it is unavoidable or it is my destination. I would like to see all sorts of new destination spots open downtown. Destinations and cost effective residential and commercial conversion are the only route to reinvigorating downtown. Encouraging investment in a substantially is the only way to achieve this. This will not happen anytime soon as Hamilton council is not capable of anything more substantive than tinkering. Anyone who doubts this has only to look at their garbage. Specifically their "blue box" and ask themselves; is this a marvel of Zen like simplicity and utility or is it a pathetic and wholly unimaginative attempt to con the public. Until the advent of that technological marvel the "green bin" this was the apex of recycling in this city. I can hear the rush to divert recyclables now. At least I won't worry about being swept away in the rush; it is coming at us down a two way street.

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By seancb (registered) - website | Posted July 19, 2008 at 00:48:22

"With James and John both two way I avoid sections of these streets like the plague."

But, that is exactly the point. You avoid using them as through streets in your car, leavig more space for actual people!

So while you have switched over to using (I'm guessing) Wentworth and Wellington, what you are missing are all of the residents of neighbourhoods surrounding James and John finally being able to comfortably walk the streets again. And these are the people who will be filling the vitality void, not the shoppers at the big box centres who just use the core as a highway to get to and from the free parking...

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By Undustrial (registered) - website | Posted July 20, 2008 at 13:44:53

While I'm a big fan of two-way streets downtown, I have to agree with Chris - suburban chain store expansion is a major cause behind the downtown's decline. The more one increases the ability of the average person to travel along a diverse network (through subsidized automobile-based transportation systems and cheap oil) the laws of geography just start to break down. Why place a store, a workplace, or a housing development anywhere other than the cheapest place to build it (middle of nowhere, by definition), if people can drive across town in as little time as little time as it takes to drive downtown? This engenders massive wastes of land, both by causing the breakdown of all logical traffic flows (mixed-use main streets and traditional neighbourhoods), and requiring it for freeways and parking lots. In this type of system there's no need for density, since no matter how low it is, housing and shopping centres will still be more profitable uses of land than farms.

Economists should recognize the fallacy here - as long as road networks are publicly subsidized and essentially free, people will use them carelessly - sprawling cities out into the horizons, until southern ontario is one big suburb from London to Guelph to St. Catherines. This brings to mind the guy who furnished his apartment with tables, chairs and beds built from free fed-ex packaging - without a reasonable cost attached, there is no reason (other than foresight and wisdom) to use them rationally.

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By A Smith (anonymous) | Posted July 22, 2008 at 23:26:16

Government is a confused little animal. It builds toll free roads, and then it criticizes us for taking advantage of them, since doing so will cause the planet to catch on fire.

I agree with Undustrial, by removing toll free roads, the market would signal to consumers the real costs associated with the suburban lifestyle.

By embracing the market based idea of paying for what we use, the goal of denser communities would become a lot easier to sell to the consumer.

I don't hold out much hope for this idea, however, since most people would rather complain about things, than pay the price to fix them.

I guess that's why Ryan's ideas are so fully embraced by the majority of people on this blog, they perfectly reflect the idea that receiving is more virtuous than giving.

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By Councillor Terry Whitehead (anonymous) | Posted July 30, 2008 at 12:09:39

Editor's Note: this comment has been posted as a blog entry (with Councillor Whitehead's permission):

http://raisethehammer.org/blog/1073

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By jason (registered) | Posted July 30, 2008 at 13:01:36

isn't it funny how all these new businesses chose to locate on James and John as opposed to Cannon or Main? Interesting how Locke is booming around the corner from a barren Main St. Empirical evidence, visual evidence blah, blah, blah. Some common sense makes this a pretty simple issue to figure out.

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By Ryan (registered) - website | Posted August 20, 2009 at 12:27:42

The Braess Paradox turns up in some interesting places:

http://www.technologyreview.com/blog/arx...

"In recent years, researchers have noticed how closing major roads has improved the flow of traffic through a city, a phenomenon called Braess's Paradox.

"That makes for an interesting basketball analogy. Players can be thought of as 'routes' through the network. The implication of Braess's Paradox is that removing the best player can sometimes improve a team's performance".

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