By Dom Nozzi
November 19, 2002
Road diets occur when travel lanes are removed from a road. Cities and Automobile Dependence, by Peter Newman and Jeff Kenworthy, was the book that first brought my attention to the benefits of road diets.
In the book, by using comparative data from a number of cities throughout the world, the authors make the persuasive case that more compact, congested areas can lead to a REDUCTION in fuel consumption and air pollution on a regional basis (among other beneficial things). The conventional wisdom, of course, says the opposite.
The primary rationale used by the authors is that modest roads and congestion reduce the number of “low value” car trips (the car trips to drive across town on a major arterial at rush hour in order to, say, rent a video). Big roads encourage, attract, or otherwise induce low-value trips. Road diets reverse that process. On more modest roads, or congested roads, some folks decide to drive at non-rush hour times, or choose a different route, or travel by bus/bike/foot.
More information about “low-value” car trips can be found on Todd Litman’s On-Line TDM Encyclopedia at the Victoria Transport Institute.
Newman and Kenworthy make the point that in more compact, congested areas, trips tend to be shorter, and more travel options are available, so more trips tend to be non-car trips.
Over the long term, congestion or modest roads encourage infill and densification as people start relocating to locations that are closer to their daily destinations. By doing so, it would seem that there would be a regional reduction in car volumes. This sort of long-term effect is due, at least in part, to what is called the “travel time budget.” That is, a number of researchers have pointed out that cross-culturally and throughout history, people will, on average, commute for 1.1 hours per day. Increasing the speed of urban roads (or fattening them) will, over the long run, disperse development and increase the amount of auto dependency in the region.
This happens because people are moving back to the 1.1-hour equilibrium (you can live further away from your daily travels and still maintain the 1.1-hour commute time).
Conversely, slowing speeds and putting (or keeping) roads on a diet does the reverse. The 1.1-equilibrium is returned to by retaining (or returning to) a more compact community size that corresponds to a more modest, 1.1-hour commute-shed (again, over the long term).