By Dom Nozzi
August 15, 2013
Most American cities are a long way away from where they need to be on an over-arching position on transportation and land use. In particular, it is a nearly universal belief that it is essential to promote free-flowing car travel (by adding turn lanes, generally opposing road diets, establishing continuous left-turn lanes, synchronizing traffic signals for cars, etc.) in order to reduce emissions and fuel use. By striking contrast, I am of the opposite view. I am firmly convinced of two things on this issue:
- Car travel is a zero-sum game. Nearly always, when car travel is made more convenient or free-flowing (usually by giving cars more space), bicycling, walking and transit decline (due to “barrier effect” problems). Promoting free-flowing traffic shifts many trips from non-car travel to car travel as a result.
- Low-value car trips. When roads and nearly all car parking is free, we are begging people to drive a car with those big subsidies. After all, there are so many rational reasons to drive a car: cargo carrying, convenience, security, speed, status, flexibility, protection from weather, etc.). Since motoring appears to be “free” when roads and parking are free, there are little if any disincentives to driving. First-year economists know quite well what the inevitable result will be: Over-use of roads and parking due to the INDUCEMENT of new car trips (particularly “low-value” trips such as driving a car to rent a video at rush hour) that were formerly discouraged from happening at rush hour or on certain roads. In other words, if a road is carrying, say, 20,000 vehicles per day, and the City or State opts to make the road more free-flowing, that road will NOT continue to carry 20,000 ADT. It will now carry, say, 25,000. The 5,000 new (latent, induced) trips were formerly discouraged by the less free-flowing conditions in the past.
An example of latent demand: I hear friends say over and over again that they are not going to drive at such and such a time or on such and such a road because those times or roads are too crowded with cars. It does not take rocket science to know what will happen if we make those times or roads more free-flowing…
The combination of the above two factors therefore means that free-flowing cars produce MORE car emissions and gasoline consumption, not less (as the advocates of free-flowing traffic would have us believe). Yes, in free-flow conditions, an INDIVIDUAL CAR produces less emissions and uses less fuel, but on a community-wide basis, the induced new trips result in more emissions and fuel use overall. In effect, engineers in this case fail to understand simple economics and changes in human behavior (economics and psychology, again) when they strive for free-flowing conditions. They fail, in other words, to realize that free-flow induces new car trips that would not have occurred had the free-flow conditions not been promoted. This inducement, again, is inevitable when we have free-to-use roads and (mostly) free parking. Largely because there is a great deal of latent demand for more car travel if conditions are made more convenient or pleasant for car travel.
By the way, the concept of induced car travel has been quite well-established for a number of decades. A great many traffic engineers know it exists. But almost no conventional traffic engineer will act on their awareness of induced demand – probably because they have not been given permission to do so.
The above also illustrates why, even in theory, it is impossible to build (widen) our way out of congestion. Induced demand for car trips is the bugaboo elephant in the bedroom that nearly all American cities ignore.