January 8, 2010

Reducing Traffic Congestion and Improving Travel Options in Los Angeles

by Paul Sorensen

While traffic congestion plagues many cities, Los Angeles stands apart. The Texas Transportation Institute tracks congestion statistics for U.S. metropolitan areas on an annual basis, and Los Angeles routinely ranks first for both total and per-capita congestion delays. Considering the value of wasted time and fuel, TTI estimates the annual cost of traffic congestion in greater Los Angeles at close to $10 billion.

The map in Figure 1, based on 2006 Caltrans sensor data, illustrates the weekday pattern of traffic congestion on the LA freeway network. Congestion is pervasive throughout much of the county; most freeways have segments on which traffic averages less than 35 mph at least two hours per day, and many bottlenecks are congested at least four hours per day.

Figure 1. Traffic Conditions on the LA Freeway Network (see figure at

Conditions on the surface streets are not much better. The map in Figure 2, based on 2004 volume-to-capacity (V/C) estimates from SCAG, depicts the pattern of afternoon traffic congestion on the county's largest arterials. Here again it is evident that traffic congestion is broadly dispersed, yet the pattern is particularly intense between downtown Los Angeles and the Westside.

Figure 2. Traffic Conditions on LA Surface Streets (see figure at

With the recent run-up in fuel prices followed by a severe recession, total travel in Los Angeles has declined over the past two years, and congestion has correspondingly eased. Yet if past trends hold, this reprieve is likely to be fleeting. Should the region's economy and population grow in the coming decades, as some forecasts predict, the probable outcome is even more vehicle travel and in turn more intense congestion.

Controversial Solutions for a Daunting Problem
Researchers at the RAND Corporation were asked to recommend strategies capable of reducing LA traffic within five years or less (the short timeframe rules out land-use reforms along with major infrastructure investments). The resulting report, Moving Los Angeles: Short-Term Policy Options for Improving Transportation, offers recommendations at once controversial and likely inescapable. To achieve lasting traffic relief, it will be necessary to manage the demand for travel through pricing reforms (e.g., congestion tolls) that increase the cost of driving and parking in the busiest corridors and areas during peak travel hours. Other measures—better transit service, ridesharing programs, traffic signal synchronization, and the like—can complement pricing, but are not on their own sufficient to stem current and projected future traffic congestion.

Few Strategies Offer Much Promise
Just why should this be so? Consider, first, that traffic congestion results from an imbalance between the supply of roads and peak-hour automotive travel. In fact, congestion can be viewed as a solution (though an unpleasant one) to this imbalance; when demand exceeds supply, congestion makes us wait our turn for available road space to balance the equation. Over the past several decades, the gap between supply and demand has widened considerably; population growth, economic expansion, and rising incomes have fueled the demand for more vehicle travel, while road construction has stagnated. We have therefore been relying, more and more, on congestion to resolve this imbalance.

One response would be to build or expand more roads to accommodate additional vehicle travel. Setting aside policy concerns related to greenhouse gas emissions and energy security, the prospects for "building our way out of congestion” are limited. To begin with, there simply isn't much space to build new roads in Los Angeles, particularly in the most densely developed urban areas. As shown in Figure 3, the density of the road network in the greater Los Angeles region, measured in lane miles per square mile, is already far greater than in any other large metropolitan area in the country.

Figure 3. Road Network Density in Major Metropolitan Areas (see figure at

We also lack the resources to engage in an extensive road building spree. In recent decades, federal and state elected officials have failed to increase fuel taxes enough to offset the effects of inflation and improved fuel economy, thus hobbling the major source of funding for road construction and repairs.

Even if we could expand the road network, though, the benefits would be limited by a phenomenon described as "triple convergence.” Congestion has been a problem for years, and many individuals deliberately alter their travel patterns to avoid severe traffic. When an investment in road capacity reduces peak-hour congestion, many will conclude that they no longer need to go out of their way to avoid congestion delays and will thus "converge" on the improvement from (a) other times, (b) other routes, or (c) other modes of travel. The net effect is that the initial traffic-reduction benefits will usually not last over time. This is why we often see, for instance, that the improved traffic flow resulting from a new freeway lane does not last for more than a couple of years.

If supply-side remedies do not create sustainable reductions in traffic, it becomes necessary to examine ways of reducing peak-hour travel demand. Commonly employed options include improved transit service, voluntary ridesharing programs, flexible work hours, and telecommuting. Unfortunately, the congestion-reduction benefits of these strategies are likewise undermined by triple convergence. If a new subway line induces some peak-hour drivers to switch to transit, other drivers will soon converge on more freely flowing roads to take their place. Indeed, the effects of triple convergence explain why traffic congestion has grown steadily worse despite considerable state and local investment in a broad range of congestion-reduction strategies.

Only Pricing Strategies Promise Sustainable Reductions in Traffic Congestion
This brings us to the rationale for pricing strategies. Among the many possible options for reducing traffic congestion, only pricing resist the effects of triple convergence. By increasing the cost of driving or parking in the busiest areas or corridors during the busiest times of day, pricing measures manage the demand for peak-hour travel, in turn reducing congestion. Once traffic flow improves, the prices remain in place, thus deterring excessive convergence on the newly freed capacity.

Pricing strategies offer two additional benefits. First, pricing generates revenue to support needed transportation investments. And in comparison to sales taxes, a common option for raising local transportation revenue, pricing has been shown to reduce the relative tax burden on lower income groups (though wealthier individuals consume more taxable goods than their less-affluent counterparts, to an even greater extent they (a) drive more, (b) are more likely to travel during peak hours, and (c) are more likely to pay peak-hour tolls rather than alter their travel choices). Second, pricing enables more efficient use of the road capacity that we already have, because roads on which traffic flows smoothly (at roughly 40 mph or higher) can carry far more vehicles per lane per hour than roads snarled in stop-and-go congestion. Paradoxically, then, we see that the introduction of pricing enables roads to accommodate more peak-hour trips. It is therefore useful to think of pricing as a means of managing peak-hour travel demand rather than reducing it.

Pricing Strategies Will Be Particularly Valuable in Los Angeles
Pricing holds promise for most major cities, but the case in Los Angeles is especially compelling. To understand why, it is necessary to consider the interactions between population density and travel behavior, factors that help to explain the severity of LA traffic.

Contrary to its reputation for sprawl, Los Angeles is quite densely populated when viewed at the regional scale. Downtown Los Angeles isn't as dense as, say, Manhattan or central Chicago, but the suburbs surrounding Los Angeles are much denser than the typical suburb, leading to high aggregate density on a regional basis.

As population density increases, individuals tend to drive less on a per-capita basis. Trip origins and destinations are closer together, leading to shorter car trips, and in dense neighborhoods people can rely on alternatives such as walking, biking, or transit for a larger share of trips. Yet this can be overwhelmed by the fact that there are also more drivers competing for the same road space within a given area, thus intensifying traffic congestion. The net effect is that greater population density tends to exacerbate congestion—think downtown Manhattan.

LA traffic congestion is further exacerbated by the fact that Angelinos do not curtail their driving as much as one would expect in response to higher population density. Figure 4 compares per-capita vehicle miles of travel (VMT) and population density for the 14 largest metropolitan regions in the country.

Figure 4. Population Density vs. Per-Capita VMT for the 14 Largest U.S. Metropolitan Regions (see figure at

Looking across the different regions, there is a fairly consistent relationship in which per-capita VMT declines with regional density. Los Angeles, though, bucks this trend. The only other metropolitan regions with higher per-capita VMT (Atlanta, Dallas, Houston, and Detroit) are all much less dense than Los Angeles. For regions in which the level of density approaches that of Los Angeles (San Francisco, Washington D.C., and New York), per-capita VMT is much lower.

In short, we see a confluence of three density-related factors that combine to explain the severity of congestion in Los Angeles: (1) congestion is likely to rise with increased population density; (2) Los Angeles is much denser than its peers at the regional level; and (3) Los Angeles exhibits a surprisingly high level of per-capita VMT relative to its density. The third of these underscores the importance of pricing strategies as a means of managing the demand for automotive travel in Los Angeles.

In the end only pricing strategies promise sustainable reductions in traffic congestion. Other measures – including improvements in alternative transportation modes – can be beneficial, but none will be nearly as effective as pricing. This recommendation will no doubt stir controversy, but pricing offers the only realistic prospects for managing peak-hour travel demand in the most traffic-choked of American metropolises.

Dr. Paul Sorensen is an operations researcher at the RAND Corporation, where he serves as Associate Director of the Transportation, Space, and Technology program. Dr. Sorensen has published peer-reviewed studies in the areas of geographic information analysis, location optimization modeling, emergency response logistics, and transportation finance policy, and he also holds a U.S. patent on a methodology for forecasting the demand for ambulance services. Dr. Sorensen received a BA in computer science from Dartmouth College, an MA in urban planning from UCLA, and a PhD in geography from UCSB.

This op-ed originally appeared with graphics on

This commentary originally appeared on on January 8, 2010. Commentary gives RAND researchers a platform to convey insights based on their professional expertise and often on their peer-reviewed research and analysis.