Induced Traffic

Induced Traffic

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Induced traffic is the phenomenon whereby decreasing the cost of vehicle trips in a particular corridor -- usually by decreasing congestion through a roadway improvement -– induces new vehicle trips in that corridor. Transportation planners generally believe that land uses generate travel demand, and roadway capacity is provided to respond to that demand. In recent years, evidence points to a strong reverse direction in this relationship: the building of roadways encourages land development as well as new trips from existing land uses. This is “induced traffic”. vancouver.protests.highway.jpg

Induced traffic may consume much of a roadway’s added capacity within a few years, although highway construction proponents dispute this. Induced traffic is added to the system in both the short-term (new trips induced immediately by the reduced congestion) and the long-term (trips added from new development that was itself encouraged by the added roadway capacity). Historically, decisions to build a new highway or widen a highway or arterial ignored the induced traffic effect of the capacity expansion.

Quantifying Induced Traffic

Todd Litman at the Victoria Transport Institute concluded from several major studies that half of increased roadway capacity is consumed by added traffic in about five years, and 80% of increased capacity is eventually consumed by induced traffic.[1] In a typical example, a study of California’s freeway system found that 60-90% of increased urban road capacity is filled with new traffic within five years.[2] However, the extent of this effect is controversial,[2] and is not easily incorporated by current transportation models.[1] Since the mid-1990’s, traffic and economic models have become available to include induced traffic as an important factor in deciding whether a roadway improvement is economically justified.

Generated Traffic = Diverted + Induced

Induced traffic is a subset of generated traffic, which includes induced traffic plus traffic diverted from other roads, times of day, and travel modes (“diverted traffic”). As additional roadway capacity is consumed by generated traffic, the economic benefits to travelers may be small or large, depending on the nature of the trip. Some trips are high-value trips such as commuting. Those making such trips do not consider them optional. However, most generated trips tend to be of low value, at least in the short-term. That is, they would be shifted or foregone completely if the costs of travel returned to pre-project levels. Thus, not all trips are created equal, although the benefits assigned to them by models may assume as much.

When a roadway improvement encourages development on the outskirts of an urban area, the development pattern tends to be automobile-oriented. Thus, over the long-term, the portion of generated traffic made up by induced traffic tends to increase significantly, and the costs to society of this type of development are far higher, particularly in an era of concern over excessive use of fossil fuels. Such costs are rarely if ever considered when new roadway capacity is being planned.

Highway Removal : The Road Not Taken

A number of cities worldwide have sought to repair the severed fabric of their downtowns by removing highways built through the middle of the urban core. U.S. examples include San Francisco and Milwaukee. (Boston moved its Interstate-95 Central Artery underground, at a cost of $15 billion.) For these projects, fears of increased congestion on other roadways following the removal generally proved misplaced. In the same way that new roadway capacity induces new trips otherwise not taken, studies show that when a portion of a highway is removed, much of the traffic from it does not reappear elsewhere on the road system. Instead, drivers tend to travel at off-peak times, use other modes such as public transit, meet their needs locally, or forego the trip altogether. A study of highway removals in 11 different countries found that 14-25% of the traffic disappeared completely, and that few if any of the removals resulted in the “traffic chaos” warned of by opponents.[3] The best results occurred where there were numerous travel alternatives available. A number of U.S. cities are considering a Highway Removal treatment as decaying elevated structures in need of expensive repairs must be evaluated for possible rebuilds or removal.

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REFERENCES

[1] Litman, Todd. 2001. Generated traffic: implications for transport planning. ITE Journal, April 2001: 38-47.

[2] Hansen, M. and Y. Huang. 1997. Road supply and traffic in California urban areas. Transportation Research Volume A, 31(3):205-218.

[3] S Cairns, C Hass-Klau and PB Goodwin. Traffic Impact of Highway Capacity Reductions: Assessment of the Evidence.