Traffic Congestion and Reliability: Linking Solutions to Problems
The Traffic Congestion and Reliability: Linking Solutions to Problems Report provides a snapshot of congestion in the United States by summarizing recent trends in congestion, highlighting the role of unreliable travel times in the effects of congestion, and describing efforts to curb congestion. In particular, the Report develops a framework for understanding the various sources of congestion, the ways to address congestion by targeting these sources, and performance measures for monitoring trends in congestion.
Much of the Report is devoted to measuring recent trends in congestion. One of the key principles that the Federal Highway Administration (FHWA) has promoted is that the metrics used to track congestion should be based on the travel time experienced by users of the highway system. While the transportation profession has used many other types of metrics to measure congestion (such as "level of service"), travel time is a more direct measure of how congestion affects users. Travel time is understood by a wide variety of audiences — both technical and nontechnical — as a way to describe the performance of the highway system. All of the congestion metrics used in the Report are based on this concept.
The different aspects of congestion are discussed using a variety of data sources, with perhaps the newest aspect being the role of reliability in the congestion problem. The variation in travel times is now understood as a separate component of public and business sector frustration with congestion problems. Average travel times have increased and the Report discusses ways to reduce them. But the day-to-day variations in travel conditions pose their own challenges and the problem requires a different set of solution strategies.
Is congestion getting worse? Yes. There are several statistics that point to worsening congestion levels. Congestion extends to more time of the day, more roads, affects more of the travel, and creates more extra travel time than in the past. And congestion levels have risen in cities of all sizes since 1982, indicating that even the smaller areas are not able to keep pace with rising demand.
Figure ES.1 illustrates trends for 75 major urban areas tracked in the Texas Transportation Institute's Annual Mobility Report.1 Congestion levels have risen to levels experienced by the next largest population group every 10 years — in 2001, cities between 500,000 and one million people experienced the congestion of cities between one and three million in 1992.
Figure ES.1 Peak-Period Congestion (Travel Time Index) Trends by U.S. Population Group
Source: Reference (1). The Travel Time Index is a measure of the total amount of congestion. It is the ratio of the weekday peak-period travel time to the travel time under ideal conditions. A Travel Time Index value of 1.3 indicates that peak-period travel takes 30 percent longer than under ideal conditions. Population groups are: Very Large ( greater than three million); Large (one to three million); Medium (500 thousand to one million); Small (less than 500 thousand).
Congestion has clearly grown. Congestion used to mean it took longer to get to/from work in the "rush hour." But congestion now affects more trips, more hours of the day and more of the transportation system. Figure ES.2 shows the growth in several key dimensions of the congestion problem in cities of more than one million persons.
- The average weekday peak-period trip takes almost 40 percent longer than the same trip in the middle of the day, compared to 13 percent longer in 1982.
- Sixty-seven percent of the peak-period travel is congested compared to 33 percent in 1982. Travelers in 75 urban areas spent 3.5 billion hours stuck in traffic in 2001, up from 0.72 billion in 1982.
Figure ES.2 Weekday Peak-Period Congestion Has Grown in Several Ways in the Past 20 Years in Our Largest Cities
Source: Analysis of data used in 2003 Annual Urban Mobility Report, Texas Transportation Institute.
- Fifty-nine percent of the major road system is congested during peak hours compared to 34 percent in 1982.
- The number of hours of the day when weekday travelers might encounter congestion has grown from 4.5 hours to 7 hours.
These are just the average conditions. Many cities have a few places where any daylight hour might see "stop-and-go" traffic. Weekend traffic delays have become a problem in recreational areas, near major shopping centers or sports arenas, and on some constrained roadways (for example, bridges).
Travel time reliability is also a growing problem. The variation in travel time from day to day is a significant characteristic of the congestion problem (Figure ES.3). The extra travel time and amount of the day and system affected by travel delays is not the same every day. It affects not only commute trips, but any trip during the peak travel periods, and is a significant concern of large and small businesses in all parts of the economy. Very detailed data from some urban freeways allow agencies to identify the extra travel time that must be budgeted — or buffer time — above the average travel time. The time that shippers, carriers, business travelers, commuters, and households have to plan for is a real consequence of congestion.
As an example of how travel time reliability affects highway users, consider the following (Figure ES.3).
Figure ES.3 Travel Time Reliability Illustration
Source: Reference (1).
- 1982 - If your midday trip took 20 minutes, it would take you 23 minutes in the peak. Although no reliability statistics exist from that long ago, analysis of recent data suggest that you would have had to add an additional nine minutes to that trip to guarantee on-time arrival at your destination; a total of 32 minutes would be planned for that trip.
- 2001 - By 2001, that 20-minute free-flow trip took 28 minutes.
- 2001 (Planning Time) - And if on-time arrival was important you should allow 40 minutes for that trip.
The future holds more of the same. Population and employment growth in America's large cities are expected to continue rising by around two percent each year, resulting in longer periods of congestion on more of the transportation system. Forecasts of population and economic activity — strong determinants of transportation activity — along with forecasts of system-level transportation activity indicate that compared to year 2000 2:
- By 2025, the U.S. population will grow by 26 percent;
- By 2025, the Gross National Product will double;
- By 2025, passenger-miles (all modes, including highway, air, and transit) will grow by 72 percent; and
- By 2020, intercity truck tonnage will grow by 75 percent.
Congestion is a lot more complex than simply "too many vehicles trying to use the road at the same time," although that is certainly a major part of the problem. Congestion results from the interaction of many different factors — or sources of congestion. Congestion has several root causes that can be broken down into two main categories:
Too much traffic for the available physical capacity to handle – Just like a pipe carrying water supply or the electrical grid, there are only so many vehicles that can be moved on a roadway for a given time or so many transit patrons that can be accommodated in a given number of buses or trains. Transportation engineers refer to this as the physical capacity of the highway system. Physical bottlenecks are locations where the physical capacity is restricted, with flows from upstream sections (with higher capacities) being funneled into smaller downstream segments. This is roughly the same as a storm pipe that can carry only so much water — during heavy rains the excess water floods the streets and houses behind the pipe. However, the situation is even worse for traffic. Once traffic flow breaks down to stop-and-go conditions, capacity is actually reduced — fewer cars can get through the bottleneck because of the extra turbulence. Bottlenecks can be very specific chokepoints in the system, such as a poorly functioning freeway-to-freeway interchange, or an entire highway corridor where a "system" of bottlenecks exists, such as a closely spaced series of interchanges with local streets. Physical capacity can be reduced by the addition of "intentional" bottlenecks, such as traffic signals and toll booths. Bottlenecks can also exist on long upgrades and can be created by "surges" in traffic, as experienced around resort areas.
Traffic-influencing events – In addition to the physical capacity, external events can have a major effect on traffic flow. These include traffic incidents such as crashes and vehicle breakdowns; work zones; bad weather; special events; and poorly timed traffic signals. When these events occur, their main impact is to "steal" physical capacity from the roadway. Events also may cause changes in traffic demand by causing travelers to rethink their trips (e.g., snow and other types of severe weather).
The level of congestion on a roadway is determined by the interaction of physical capacity with events that are taking place at a given time. For example, the effect of a traffic incident depends on how much physical capacity is present. Consider a traffic crash that blocks a single lane on a freeway. That incident has a much greater impact on traffic flow if only two normal lanes of travel are present than if three lanes are present. Therefore, strategies that improve the physical capacity of bottlenecks also lessen the impacts of roadway events such as traffic incidents, weather, and work zones.
Only recently has the transportation profession started to think of congestion in these terms. Yet it is critical to do so because strategies must be tailored to address each of the sources of congestion, and they can vary significantly from one highway to another. Nationally, a composite estimate of how much each of these sources contribute to total congestion is depicted in Figure ES.4.3
Figure ES.4 The Sources of Congestion
What Causes Travel Times to be Unreliable? The interaction of all the sources of congestion produce unreliable travel times. Travel time reliability can be defined in terms of how travel times vary over time (e.g., hour-to-hour, day-to-day). The event-related sources (e.g., traffic incidents, weather, and work zones) that contribute to total congestion also conspire to produce unreliable travel times, since events and demand volumes vary day to day. The problem is worse when events are added on top of existing capacity-related congestion. When traffic flow has already broken down to stop-and-go conditions, any additional disturbance causes a large increase in congestion.
What Are the Benefits of Making Travel Times More Reliable? If it is possible to reduce the impact of these events on travel, a double benefit is realized: not only are conditions made more "reliable" (that is, less variable), but overall delay is reduced as well. This is because extreme events, especially in combination, lead to high congestion. Making improvements in both the congestion level and reliability is significant for a number of reasons:
- Reducing total congestion saves time and fuel, and leads to decreased vehicle emissions;
- Reducing congestion at international border crossings leads to lower transportation costs and benefits the national economy as a whole. Further, reducing congestion on U.S. highways for freight moving between Canada and Mexico fosters international trade. In essence, congestion on U.S. highways can be thought of as an international problem as well as a national one;
- Improving reliability leads to more predictable and consistent travel, something that all travelers seek: they do not have to budget as much extra time in order to arrive on time at their destinations. This is particularly important for truckers and shippers because many activities (e.g., manufacturing, sales) are now closely timed to the arrival of shipments. Many types of personal travel — such as getting to business appointments and child care pickup on time — are also sensitive to unreliable travel times; and
- Treating three major components of unreliable travel — traffic incidents, bad weather, and work zones — also leads to safer highways. By reducing the duration of these events, we are reducing how long travelers are exposed to less safe conditions.
What Value Does Providing Reliable Travel Times Have? Commuters as well as freight carriers and shippers are all concerned with travel time reliability. Variations in travel time can be highly frustrating and are valued highly by both groups. Previous research 4 indicates that commuters value the variable component of their travel time between one and six times as much as average travel time. Adoption of just-in-time (JIT) manufacturing processes has made a reliable travel time almost as important as an uncongested trip. Significant variations in travel time will decrease the benefits that come from lower inventory space and the use of efficient transportation networks as "the new warehouse." Therefore, in both the passenger and freight realms, evidence suggests that travel time reliability is valued at a significant "premium" by users.
What is Freight's Role in Congestion? Demand for freight transportation in the United States, which is expected to grow substantially over the next 15 years (Figure ES.5), is a major contributing factor to congestion. The expected growth in truck travel is being driven by economic and population growth. The most striking growth is expected to be on rural Interstate highways, indicating the potential for congestion to spread outside of metropolitan areas. Since 1992, traffic has grown substantially on rural highways and at a faster pace than on metropolitan highways. National data shows that between 1992 and 2002, traffic on rural Interstates increased 36 percent compared with an increase of 25 percent on urban Interstates. Further analysis shows that traffic volume per lane (a measure of traffic density) increased by 35 percent on rural Interstates compared with 21 percent on urban Interstates.
Figure ES.5 Percentage of Highway Segments with over 10,000 Trucks
Comparison of 1998 to 2020
Source: Analysis of data from FHWA's Freight Analysis Framework.
What Are the Costs of Congestion? Congestion has real costs for all travelers, including truckers (both long-haul and local pickup and delivery), household and business service providers (such as plumbers, computer technicians, police, and ambulance services), and personal travel (such as commuters, vacationers, and shoppers). Congestion causes more fuel to be used and more emissions to be produced. The extra time spent in congestion causes service providers to make fewer calls per day, leading to higher prices for consumers; this is particularly important for emergency medical, fire, and police services which may be unnecessarily delayed from attending to medical, crime, and disaster situations. Companies with production schedules timed to take advantage of trucks delivering components to an assembly line as they are needed must instead plan for items to arrive early. This consumes space and inventory, expending resources that could otherwise be spent on productive activity. For personal travelers, congestion "steals" time that could be put to better use in the workplace or for social or recreational purposes.
The congestion costs to freight interests are significant. Freight transportation has gone through many changes over the past 20 years as it has adapted to changes in business practices. Within this new operating environment, freight operations and productivity have been optimized to work closely with other aspects of business activity. Deregulation has resulted in excess capacity being eliminated from the highway and rail freight systems. Intermodal services and facilities have revolutionized international trade. Ports and airports have seen services and demand grow rapidly. Freight services are now more efficient and in many cases lower in cost (in constant dollars) than in previous decades. But the elimination of excess capacity has resulted in systems with less redundancy and less ability to withstand shocks or disruptions. Congestion is growing on many key freight segments of the transportation system, and congestion can drastically reduce the productivity of the overall freight network. The delay caused by congestion could vastly increase the costs of those freight movements that are today managed to exacting schedules.
Time is literally money for shippers and trucking interests. A direct linkage exists between transportation investment, travel conditions (congestion and reliability) and economic productivity. For trucking, two key trends identified above will have a substantial impact on the total cost of moving freight:
- As congestion spreads into the midday period, which is the peak travel period for trucks, more direct costs will be incurred; and
- Reliability – For trucks, the ability to hit delivery windows predictably will decrease and will add even more costs as firms struggle to optimize delivery schedules. This is especially a problem for truckers who must meet "just-in-time" delivery schedules set by shippers, manufacturers, and retailers.
All of this adds up to a staggering amount of costs imposed on travelers by congestion. The Texas Transportation Institute estimates that in 75 of the largest U.S. cities in 2001, $69.5 billion dollars are wasted in time and fuel costs. 5 (The costs are a composite of automobile and truck travel costs in urban areas.) The time value costs for trucks are conservative — they include only the cost of truck operating time, primarily the cost of drivers' wages and equipment. The value of the cargo and the response of firms to transportation costs is not included, yet recent work suggests these costs can be significant. These costs include:
- Foregone Investment Opportunities – Higher transportation costs due to congestion reduce a firm's ability to invest in making more products, improve product quality, and introduce new products; and
- Decreases in Regional Employment or Decreases in the Rate of Growth of Regional Income – Higher transportation costs are passed onto other sectors of the economy and hinder general economic efficiency.6
Transportation engineers and planners have developed a variety of strategies to deal with congestion — a toolbox for managing congestion. The strategies can be grouped as follows:
- Adding more capacity for highway, transit and railroads;
- Operating existing capacity more efficiently; and
- Encouraging travelers to use the system in less congestion-producing ways.
Each of these congestion reducing strategies has a role in major cities. More accurately, they all have a role in some locations and corridors within major cities. Implementing the strategies involves consideration of the size and type of problem, funding, and public approval, environmental and social consequences. The decisions resulting from all these factors will be different, diverse and reflect local, state, and national priorities. When used in combination, however, the strategies can have a powerful impact on congestion growth. Also, when applying these strategies, agencies need to think and act regionally about solutions to congestion problems. In fact, FHWA is promoting the concept of regional partnerships as a means to implementing effective operations. These partnerships provide a platform for interagency coordination and joint delivery of operations-based services.
Specifically, each of the three major categories of congestion management strategies entails the following:
Adding More Capacity – Increasing the Number and Size of Highways and Providing More Transit and Freight Rail Service. Adding more lanes to existing highways and building new ones has been the traditional response to congestion. In some metropolitan areas, however, it has become difficult to undertake major highway expansions because of funding constraints, increased right-of-way and construction costs, social effects and environmental constraints and opposition from local and national groups. However, it is clear that adding new physical capacity to highways, transit systems, and railroads is an important strategy for alleviating congestion. This often means that highway designers must find creative ways to incorporate new designs that accommodate all stakeholders' concerns. Since the worst highway bottlenecks tend to be major freeway interchanges, advanced design treatments that spread out turning movements and remove traffic volumes from key merge areas have been developed, often by using multilevel structures that minimize the footprint of the improvement on the surrounding landscape.
Key Strategies to Address Congestion
- Adding travel lanes on major freeways and streets (including truck climbing lanes on grades);
- Adding capacity to the transit system (buses, urban rail or commuter rail systems);
- Closing gaps in the street network;
- Removing bottlenecks;
- Overpasses or underpasses at congested intersections;
- High-occupancy vehicle (HOV) lanes; and
- Increasing intercity freight rail capacity to reduce truck use of highways.
Operating Existing Capacity More Efficiently – Getting More Out of What We Have. In recent years, transportation agencies have embraced strategies that deal with the operation of existing highways, transit systems, and freight services, rather than just building new infrastructure. Collectively referred to as Intelligent Transportation Systems (ITS), real-time control of transportation operations involves making changes from minute to minute and take many forms. In addition to ITS, other Transportation System Management and Operations (TSM&O) strategies that improve the efficiency of the existing road system include minor widening projects, changing the operating methods or the policies that govern the use of the roadway, and monitoring transit vehicles in real-time. There are numerous operations-based congestion mitigation strategies that are enhanced by the use of advanced technologies or ITS.
Key Strategies to Address Congestion
- Metering traffic onto freeways;
- Optimizing the timing of traffic signals;
- Faster and anticipatory responses to traffic incidents;
- Providing travelers with information on travel conditions as well as alternative routes and modes;
- Improved management of work zones;
- Identifying weather and road surface problems and rapidly targeting responses;
- Providing real-time information on transit schedules and arrivals;
- Monitoring the security of transit patrons, stations, and vehicles;
- Anticipating and addressing special events that cause surges in traffic;
- Better freight management, especially reducing delays at border crossings;
- Reversible commuter lanes;
- Movable median barriers to add capacity during peak periods;
- Restricting turns at key intersections;
- Geometric improvements to roads and intersections;
- Converting streets to one-way operations; and
- Access management.
Encouraging Travel and Land Use Patterns that Use the System in Less Congestion Producing Ways – Travel Demand Management (TDM), Non-Automotive Travel Modes, and Land Use Management. Another key approach to the problem of congestion involves managing the demand for highway travel. These strategies include providing a variety of options that result in more people traveling in fewer vehicles, trips made during less congested times, or trips not made (at least in a physical sense). A major barrier to the success of demand management strategies is that they may require changes in traditional decisions about where, when and how to travel, live and work. Flexible scheduling, for example, is not possible for a large number of American shift schedule workers. Still, when considered as part of an overall program of transportation investments, demand management and non-automotive modes of travel can contribute substantially to a metropolitan area's transportation system.
The historical cycle of suburban growth has led to an ever increasing demand for travel. Suburban growth was originally fueled by downtown workers who moved from city centers to the urban fringe to take advantage of lower land prices and greater social amenities. In the past 20 years, businesses also have moved to the suburbs to be closer to their employees. This in turn allows workers to live even further away from city centers, thereby perpetuating suburban expansion. Strategies that attempt to manage and direct urban growth to influence these processes have been used in several metropolitan areas. The main problem with many of these strategies is that they can be contrary to market trends, burdening consumers with extra costs and dampening economic efficiency, at least in the short term. Unless a truly regional approach is followed — with cooperation of all jurisdictions within the region — and the policies are considered as part of a package of development options, sprawl may simply be attracted into areas not conforming to growth policies.
Key Strategies to Address Congestion
- Programs that encourage transit use and ridesharing;
- Curbside and parking management;
- Flexible work hours;
- Telecommuting programs;
- Bikeways and other strategies that promote non-motorized travel;
- Pricing fees for the use of travel lanes by the number of persons in the vehicle and the time of day;
- Pricing fees for parking spaces by the number of persons in the vehicle, the time of day or location;
- Land use controls or zoning;
- Growth management restrictions such as urban growth boundaries;
- Development policies that support transit-oriented designs for homes, jobsites, and shops; and
- Incentives for high-density development, such as tax incentives.
Is Success Possible Against Congestion? Yes, but past successes tends to be localized. Multiple and systematic strategies for addressing congestion are required, given that demand is increasing on an already stressed highway and transit system. All of the strategies covered in this Report have been successfully implemented — the key to future progress is deploying and using them in a more comprehensive and aggressive manner. It also requires cooperation between transportation agencies, businesses, elected officials, and the public. Since we are all affected by congestion, it is important that we all work together to address the congestion problem. Here are some ways that transportation agencies, businesses, elected officials, and the public can collaborate to mitigate congestion.
Take Ownership – The first step is for all parties to recognize they have a stake in the congestion problem. Public agencies are in the business of serving customers the same way that any private firm is — except that the customers (the public and businesses) are buying efficient and safe travel. The public, elected officials, and businesses are more than just consumers — they are shareholders too. These consumers also should examine their own decisions and policies to identify changes that can improve their quality of life while recognizing that the agencies cannot solve the problem by themselves. The ongoing transportation planning process, which has been successfully used in major metropolitan areas for the past 40 years to address transportation problems, provides an excellent framework for promoting ownership of congestion problems. A major part of the transportation planning process is establishing a Vision that outlines what the future transportation system should look like. The Vision leads to more specific statements of desired actions to achieve these states or characteristics. The Vision is also an opportunity to educate all stakeholders on the nature of congestion in your area and the importance of mitigating it.
Identify the Congestion Problems and Opportunities – Both technical analyses and anecdotal information from the public are useful in identifying where the major congestion problems are, where they will be, and what causes them. The existing transportation planning process in metropolitan areas can be tapped as a resource for this purpose. Thoroughly analyze and provide realistic assessments on what can reasonably be done in each case, and what the expected improvements might be. FHWA supports a wealth of information on expected improvements from operational strategies, such as the ITS Benefits and Cost Database.7 The process should include considerations of:
- Strategies – What types of treatments should be considered?
- Coverage – How much area does the treatment cover?
- Density – How well is congestion treated?
- Congestion Target – What aspect of congestion is treated?
- Effect – What is the delay reduction effect? Are there secondary effects, such as on safety? What are the spillover effects on other facilities and neighborhoods?
Develop Plans, Programs, Policies, and Projects – Congestion solutions can take a variety of forms. Think broadly — no single tool will be highly effective against the congestion problem. But when used in combination — and tailored to specific circumstances — packages of congestion mitigation strategies can be successful. The strategies should include action elements — things we can accomplish in a short timeframe and at low cost. But longer-term actions also should be developed — consider all types pf strategies including adding new highway and rail capacity, improved operations, and better land use planning. Recognize that many transportation and community plans already exist and should be tapped as mechanisms for carrying out the Vision. In fact, acting on a list of "things we can do now" should help galvanize support for congestion mitigation over the long term.
Plan, Manage, and Operate the Transportation System Proactively and Regionally. Focus on addressing system reliability by targeting capital and operations strategies to specific conditions. Anticipate problems and take corrective actions early. Also, regional and multimodal cooperation is key to the success of deploying effective operations — many different agencies have a stake in the congestion problem. Therefore, a broad perspective should be taken in applying capital and operations strategies — avoid a narrow, facility-oriented view.
Use Performance Measures to Track Progress — One of the main actions that transportation agencies can contribute is the tracking of congestion trends and the effect of improvements over time. Trends provide a basis for determining how well your actions are working and can identify changes in the underlying congestion problem (e.g., traffic crashes may become more important in your area). Use of performance measures also brings an element of accountability to the process — what we are really getting for our investments — just as businesses do.
Travel Time Index (TTI) is a comparison between the travel conditions in the peak period to free-flow conditions. It uses the units of travel rate (the inverse of speed) due to the ease of mathematical calculation and availability of data elements in both traffic surveillance and roadway inventory databases. The equation below presents the calculation of the travel time index for areawide applications.
The index can be applied to various system elements with different free-flow speeds. The travel time index compares measured travel rates to free-flow conditions for any combination of freeways and streets. Index values can be related to the general public as an indicator of the length of extra time spent in the transportation system during a trip.
The Buffer Time Index (BTI) expresses the amount of extra "buffer" time needed to be on-time 95 percent of the time (late one day per month). Indexing the measure provides a time and distance neutral measure, but the actual minute values could be used by an individual traveler for a particular trip length. The index is calculated for each road segment and a weighted average is calculated using vehicle-miles of travel as the weighting factor.
The Planning Time Index (PTI) is simply the 95th percentile travel time index. It is used as a supplemental measure for reliability. Because reliability is related to the distribution of travel rates, the 95th percentile indicates an excessively high travel rate, one that only five percent of all travel rates exceed for the time period under consideration.
Delay is the amount of extra time spent in congestion compared to the time it would take under ideal or free-flow conditions. For example, if a trip takes 10 minutes under ideal conditions, and during the peak it takes 15 minutes, the total amount of delay is five minutes.
To Access the Full Report on Traffic Congestion and Reliability – To gain access to the full report and detailed appendices, go to the FHWA Office of Operations web site at: http://www.ops.fhwa.dot.gov/.
1 Schrank, D. and Lomax, T., 2003 Annual Urban Mobility Report, Texas Transportation Institute. This methodology measures congestion conditions on individual highway segments using roadway-based data. Alternate ways of measuring congestion exist, such as monitoring the travel times of entire trips with household surveys. One such survey, the National Household Travel Survey (NHTS) has been conducted periodically since 1969. Recent data from the NHTS suggest that commute times have not increased as fast compared to roadway-based congestion data, such as is used by the Urban Mobility Report. This may be due to people changing their residential and employment locations. The net result is that for individual travelers, congestion is probably not getting as worse as fast — highway segments may be more congested but the ability to move around the metropolitan landscape provides a "cushion" for some individuals.
2 Sources: (1) U.S. Department of Transportation, Bureau of Transportation Statistics, http://www.eia.doe.gov/ and (2) FHWA Freight Analysis Framework, http://www.ops.fhwa.dot.gov/freight/freight_news/FAF/talkingfreight_faf.htm.
4 Cohen, Harry, and Southworth, Frank, On the Measurement and Valuation of Travel Time Variability Due to Incidents on Freeways, Journal of Transportation Statistics, Volume 2, Number 2, December 1999, http://www.bts.gov/publications/journal_of_transportation_and_statistics/volume_02_number_02/.
5 Schrank, D. and Lomax, T., 2003 Annual Urban Mobility Report, Texas Transportation Institute.
6 ICF Consulting, HLB Decision Economics, and Louis Berger Group, Freight Benefit/Cost Study: Capturing the Full Benefits of Freight Transportation Improvements: A Non-Technical Review of Linkages and the Benefit/Cost Analysis Framework, May 11, 2001.