The Congestion Problem: Causes and Solutions

The root causes of congestion have long been understood, and there is now broad consensus that congestion generally reflects a fundamental imbalance of supply and demand. That is, during hours of peak usage of the transportation facilities most desirable to motorists, the supply of, for example, roadway capacity is insufficient to meet the demand for those facilities. Economists have long understood that such an imbalance stems from inefficient pricing, where the true costs of usage are not reflected in prices paid by the users. For example, travelers are not generally charged for the impact their trip will have on others using the same facility (e.g., increased levels of congestion) or on other members of society (e.g., increased air pollution). In fact, in this country, access to highway travel is, for the most part, rationed by delay.

The imbalance of supply and demand leading to congestion is also impacted by the absolute volume of traffic (e.g., demand) on a given facility relative to its physical capacity (e.g., supply). When we look at traffic congestion from a demand perspective, we are looking at how many vehicles compete for space on a particular facility at a given time. The demand for a facility is a function of individual decisions as to when, where, how, and even if highway travel will take place.

On the supply side, congestion is a function of physical constraints and how the driver interacts with a given facility as well as with other vehicles on the facility. On-the-road congestion is characterized as either recurring (routine) or nonrecurring (random). Recurring congestion happens in roughly the same time and place on the same days of the week. It results when physical capacity is simply not adequate to accommodate short-term demand during peak periods. Recurring congestion may also result from improperly timed traffic signals. As indicated in Figure 1, recurring congestion accounts for close to half of total delay. On the other hand, nonrecurring congestion is caused by transitory events such as construction work zones, traffic incidents, and bad weather. The significance of nonrecurring congestion is that it can temporarily prevent full utilization of the existing physical capacity.

Figure 1. Congestion experienced by highway travelers is caused by many different factors.

Consistent with the paradigm of its causes, there are four distinct, but nonetheless related strategies available to attack congestion. It is important to note that the strategies, while individually of merit, work best when implemented as a coordinated package of tools.

Bring Supply and Demand into Alignment through Congestion Pricing. Congestion pricing or peak-period pricing, entails fees or tolls for road use that vary by level of vehicle demand on the facility. As with market pricing in other sectors, road pricing helps allocate limited supply - in this case that of available road space.

With user charges assessed at the point of use, greater efficiency results through improved response to market forces. Charges are typically assessed electronically to eliminate delays associated with manual toll collection facilities. Road-use charges that vary with the level of vehicle demand provide incentives to shift some trips to off-peak times, less-congested routes, or alternative modes; or to cause some lower-value trips to be combined with other trips or simply to be eliminated.

Congestion pricing has several important objectives. First, it seeks to balance demand with available capacity, i.e., the supply of road space. Second, it seeks to fairly allocate the costs associated with operating, maintaining, and expanding the transportation system to meet growing travel demand. Third, it seeks to improve operation of the highway system. A fourth objective may include revenue generation.

Provide Better Choices as to How, When, Where, and If to Travel. The goal with this strategy is to reduce the number of vehicles on a given road. This may take the form of promoting alternative commute options such as employee telecommuting options or making transit easier and more attractive to use. Also of interest in managing demand are driver incentive programs that, for example, promote ridesharing and off-peak use.

Strategically Invest in New Transportation Capacity. Although there is significant and widespread demand for new highway capacity, concerns about air pollution, noise, and urban sprawl often stand in the way of expanding the system. Equally significant, adding new capacity can be enormously expensive and physically challenging. Despite the barriers, however, new construction that serves critical strategic purposes will go forward in order to preserve or improve system performance.

Although widespread capacity increases are a thing of the past, many of the barriers may be addressed through increased expenditures. Environmental concerns may be mitigated and physical challenges overcome (for example, through tunneling). However, the resources to fund such improvements simply are not available through traditional sources. For this reason, many professionals in the transportation community are enthusiastic about the opportunities potentially afforded by public-private partnerships and road pricing.

Improve the Management and Operation of the System. This area of interest involves better managing the vehicles that are actually on the road, and the road itself. "Smart" roads, traveler information, and improvements to the management and operation of the facility are options available for using the available system more productively and bringing it to peak performance. Management and operations strategies are targeted at managing temporary disruptions (e.g., incidents) in a way that will return the system to full capacity quicker; ensuring more efficient day-to-day operations through coordinating and up-to-date traffic signal timing and operational improvements to relieve bottlenecks; and providing real-time information about the system to that travelers can make immediate decisions about when, where, and how to travel, and transportation agencies can make real-time adjustments to improve system operations.

Effective and efficient management and operation of the system is foundational to all of the above congestion reduction strategies. This is true because as traffic volumes have grown over time and physical capacity has remained relatively constant, the system has become less able to absorb "surprise" - or nonrecurring events. In the realm of managing the highway system, the margin for error is very small and continues to decline. In addition, operational fixes to the system are also helpful in addressing the recurring congestion resulting from bottlenecks and improper traffic signal timing.

To combat the country's growing transportation congestion problem, the U.S. Department of Transportation launched the National Strategy to Reduce Congestion to America's Transportation Network. Not surprisingly, one element of this initiative is focused on applying 21^st century tools to manage and operate the system at peak efficiency.

Additional Information

For more information on the congestion problem and solutions, visit FHWA's Focus on Congestion Relief Web site.