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.
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.
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.
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.
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.
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