Posted on June 20, 2012 1:27 PM
Measuring and Mitigating Traffic Congestion, Part 2
Building on a previous Policy Update, this second post in a two-part series surveys various strategies typically employed to address traffic congestion and analyzes their limitations. It also offers broader thoughts on the nature of congestion and its role in metropolitan planning.
While we may “know it when we see it,” congestion remains a difficult phenomenon to measure and mitigate, as are its associated economic and social impacts. This series' first post described several well-read congestion studies and methodological considerations in measuring traffic congestion. These studies estimate substantial delay and resulting costs caused by congestion for commuters and firms in northeastern Illinois. For one, the 2011 Urban Mobility Report estimates a total cost of congestion of $8.2 billion to the Chicago region, or $1,568 per peak-period auto commuter. CMAP estimates a regional travel time index -- a ratio of average travel time to free-flow travel time -- of 1.17 in the morning peak period and 1.22 in the afternoon peak period in 2010. CMAP continually monitors congestion performance data for the region through its Congestion Management Process.
What We Can Do About Congestion
Strategies that address traffic congestion take one of two approaches: They target either the demand for travel or the supply of transportation infrastructure. Supply-side strategies increase the capacity of highway and transit facilities to accommodate more travelers during peak periods. Increased capacity can be achieved not only by adding new physical infrastructure, for example major capital projects, but also through operational improvements that make more efficient use of existing infrastructure. For example, synchronizing traffic signals and installing ramp meters can allow for more efficient traffic flow. Operational strategies can also address the impacts of “non-recurring” congestion events such as traffic crashes, construction, weather, and special events, which the Federal Highway Administration estimates to cause over half of congestion nationally. CMAP works through the Regional Transportation Operations Coalition to promote and coordinate operational strategies among transportation agencies.
In contrast, demand-side strategies seek to reduce the need for travel on congested facilities during peak periods. Many of these strategies take an indirect approach through the promotion of alternative modes of transportation. Initiatives to promote transit use, carpooling, walking, and bicycling -- including those that seek to do so through land use -- can reduce demand for vehicle travel during peak periods. Other policies, such as telecommuting and alternative work schedules, can reduce the need to travel entirely. Demand-side approaches can also target driving directly through pricing. Congestion pricing on highways, to be discussed below, and variable prices for parking can increase the costs to drive and park during peak periods, thereby encouraging travelers to switch to alternative modes, routes, or times of day.
For more information, a report from the Federal Highway Administration gives an overview of common strategies to address traffic congestion. It argues that a variety of strategies implemented in combination, for example physical capacity expansions in concert with improved operational strategies, can effectively address congestion.
What We Can’t Do About Congestion
Although they may provide short-term congestion relief, most of the above strategies are unlikely to effectively address congestion in the long term due to a phenomenon that Anthony Downs of the Brookings Institution refers to as “triple convergence.” Any strategy that improves travel speeds on congested facilities during peak periods will attract travelers who had previously driven at different times of day (or on different routes), chosen an alternative mode of transportation, or not traveled at all.
Triple convergence also refers to the broader concept of induced demand: New transportation capacity encourages new trips to be made, in part because of long-term shifts in land use. For example, a highway improvement may encourage new commercial and residential development, which in turn generates new trips on the highway. A recent study estimates that when new freeway capacity is added, 40 percent is absorbed, or in essence reduced, with 31 percent due to behavioral shifts and nine percent due to changes in land use.
Although they may be insufficient to effectively reduce congestion in the long run, the various strategies described above still have merit. Expansion in highway capacity allows the system to accommodate more trips, and the economic and social activities they enable, even if the new capacity eventually becomes congested. In a similar way, promoting alternative modes of transportation such as transit or carpooling also enhances overall mobility. Moreover, these alternative modes provide travelers with new, valuable options and help play an important social service role by accommodating the travel of those who can’t or prefer not to drive.
New Approaches to Addressing Congestion
Congestion pricing is the only mitigation strategy that can avoid triple convergence and preserve higher travel speeds on the region’s highways. Congestion pricing works by applying economic principles of supply and demand to manage scarce road space. During peak periods when demand for a road is high, congestion pricing would charge drivers a higher toll to access a highway facility. During off-peak hours when demand for a road is lower, drivers would pay a small toll or no toll at all. In this way, congestion pricing acts as a sort of triple divergence during peak periods: The higher tolls encourage drivers to consider alternative routes, times of day, and modes, or to forego a trip entirely.
Despite these advantages, congestion pricing is not widely implemented in the U.S. Many detractors cite concerns about the equity impacts of pricing, and others fear that congestion pricing of freeways may lead to traffic spillover on adjacent arterial roads. Most drivers prefer not to pay tolls, especially for roads that are currently free. These challenges are not insurmountable; a growing number of metropolitan areas across the country have implemented congestion pricing on special express toll lanes, bridges and tunnels, and, in some cases, entire tolled highway facilities. A recent report from the Government Accountability Office provides an overview of the benefits and challenges of congestion pricing, as well as a list of congestion pricing projects in the U.S.
GO TO 2040 supports the implementation of congestion pricing in the region. The plan’s recommended “managed lane” corridors -- along the Stevenson Expressway (I-55) and the Jane Addams Memorial Tollway (I-90) -- are two opportunities to implement congestion pricing on new capacity. These projects envision congestion pricing on new lanes that will be added to existing highway corridors, rather than on existing lanes. Two recent advisory councils have recommended the implementation of congestion pricing along the I-90 corridor and the Central Lake County Corridor. Furthermore, CMAP plans to launch a website about congestion pricing later this summer to provide more information on the policy and to analyze its potential application in the Chicago area.
Reconsidering Traffic Congestion
The types of congestion studies presented in the first Policy Update in this series have both reflected and influenced our understanding of urban traffic congestion -- namely that it is an everyday crisis that imposes dramatic costs on society and drivers alike. This paradigm has motivated a policy response to eliminate or reduce traffic congestion through a variety of strategies, both supply- and demand-side.
However, critiques of the methodologies used by congestion studies suggest that the costs of congestion may be overstated, and new research encourages us to rethink congestion entirely. Some observers argue that, rather than representing a sign of failure, congestion reflects economic and social vitality. Congested cities can still provide a high degree of accessibility and therefore may not be the drain on economic productivity and growth that we have long assumed.
Issues like triple convergence and induced demand force us to rethink our expectations regarding congestion policy. While investments are unlikely to eliminate congestion entirely, they can still provide meaningful value to the region by accommodating greater mobility and offering new travel choices. Triple convergence and induced demand are not arguments against investment, but perhaps they ask us to set different goals.
GO TO 2040 suggests a new paradigm for congestion policy in northeastern Illinois. While it calls for targeted capacity expansion, the plan recognizes the potential impact of triple convergence and induced demand on capacity expansions. The plan does not set a goal to reduce congestion in the region over the next 30 years, but rather to contain it. To help accomplish this goal, GO TO 2040 calls for the implementation of congestion pricing and pricing for parking to better manage the region’s infrastructure. These innovative approaches hold promise to maintain reliable travel speeds on priced facilities (or, in the case of parking, a few available spaces on each block or parking garage) while paying for themselves through sustainable user fees. GO TO 2040 presents an outline for responsible, pragmatic congestion management in the 21st Century.