United States Department of Transportation
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Abstract:Many freeway users complain that new freeway segments immediately fill up with traffic after they are constructed. This
diminishes the advantages of reduced costs and reduced driving time that would make freeways theoretically superior to
arterial streets. According to previous literature, however, this phenomenon is to be expected and is not an indicator of the
efficiency or inefficiency of having new freeways. Due to Downs’s Law of Peak-hour Traffic Congestion, we expect freeways
to immediately fill upon construction, simply because they do offer superior benefits to roadway users compared to the
alternative arterial streets. Rational drivers choose to enjoy these benefits. Another phenomenon cited in the literature review
is that of induced travel, which states that with the reduction in travel time posed by using freeway segments, it can also be
expected that more commuters will choose to travel on them than otherwise would.
When Downs’s Law works synergistically with the phenomenon of induced travel, more vehicles can be accommodated in a
given geographical area, thus increasing the total number of trips taken. This adds to the overall value of our transportation
system, since after all, the value of that system is predicated on its ability to facilitate increased volume of travel.
This report is based on data acquired from the State of Arizona, the City of Phoenix and Maricopa County, Arizona, and other
sources. Through our analysis of Maricopa County traffic count data we are able to show a significant increase in traffic
volume resulting from adding new freeways. This increase in traffic volume accounts for a net benefit of over $18 million
dollars per year for a given mile-long stretch of roadway. Over a freeway design-life of 20 years this is far in excess of the
average of $72 million needed to construct that mile of freeway.
Ultimately, any evaluation of the freeway system must take into consideration the explicit and implicit benefits of the system.
We know that congestion is going to be present whether new freeways are constructed or not. Before freeway segments are
constructed, the existing arterial streets are congested. After the completion of freeway segments, some drivers shift from
arterial streets to the new freeway. This lessens traffic on the arterials, leading to more drivers taking trips they previously
avoided (i.e. induced travel). Even though congestion is an inevitable condition, even on freeway segments postconstruction, freeways still offer a clear net benefit.
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