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21st Century Operations Using 21st Century Technologies

Incorporating Travel-Time Reliability into the Congestion Management Process (CMP): A Primer

Chapter 1. Introduction

Recurring and Non-Recurring Congestion

Recurring congestion, or expected congestion, occurs when too many people routinely attempt to drive on a roadway or enter a transit vehicle at the same time. Recurring congestion typically happens during commute hours for most regions.

Non-recurring congestion happens when there are disruptions to the flow of traffic or transit. Disruptions include crashes, disabled vehicles, large special events (concerts, sports games, etc.), inclement weather, and construction work zones.

This primer identifies opportunities for incorporating travel-time reliability into the Congestion Management Process (CMP). It is intended for use by state and Metropolitan Planning Organization (MPO) planners as well as by operations managers and analysts who are planning and programming transportation investments to better manage congestion in urban areas.

Travel-time reliability, which considers the effects that non-recurring events have on delay over time, is an important element of congestion that historically has not been considered as part of the CMP. Taken on average, and as a whole across the United States, non-recurring congestion accounts for more delay than recurring congestion. Non-recurring congestion is caused by disruptions, such as traffic incidents, weather, road construction and maintenance, and/or special events, as shown in Figure 1. This is particularly true in areas with smaller population sizes that do not experience much recurring congestion.

Figure 1. Illustration. Causes of Congestion. This figure is a pie chart that illustrates the various causes of congestion. They are divided up into ones that cause recurring congestion versus those that cause non-recurring congestion. Causes of non-recurring congestion include poor signal timing, special events, work zones, inclement weather, and incidents. Causes of recurring congestion include bottlenecks or capacity constraints. The pie chart indicates that non-recurring congestion account for slightly more overall delay to travelers than recurring congestion.
Figure 1. Chart. Causes of Congestion
Source: Adapted from Traffic Congestion and Reliability: Trends and Advanced Strategies for Congestion Mitigation, Figure ES.2, Federal Highway Administration

To date, most CMPs assess congestion using traditional proxy measures such as level of service and volume-to-capacity ratios. Traffic counts are reduced to average annual daily traffic, leading to viewing the system on a "typical" day. This approach is attributed to the historical costs of continuous data collection and to the lack of analysis tools available for assessing travel-time reliability.

Advances in probe data and system detection technologies have significantly reduced data collection costs. Travel times can now be monitored continuously and cost effectively at the individual facility and network levels, enabling the calculation of reliability. Software programs have been developed to translate these data directly into performance measures.

Several research projects on reliability were conducted as part of the second Strategic Highway Research Program (SHRP 2) [1]. They developed relationships, methods, and analytical tools and techniques to define, measure, monitor, and improve the reliability of roadway networks.

Travel-time reliability is important to the traveling public and shippers, and they often factor in how predictable their trip time is when making decisions about route choice, departure times, or mode of travel. Given this importance, along with the advent of data sources and analytical tools that make measuring and assessing reliability viable, additional guidance is needed to enable MPOs to incorporate reliability into their planning processes.

1.1 What Is Travel-Time Reliability?

Same Trip

A trip taken on a regular basis with identical characteristics, including its purpose, origin, destination, time-of-day, mode, and route.

Travel-time reliability is defined as consistent travel times for the same trip as measured day-to-day or across different times of the day. If trip times are inconsistent, the travel time is considered to be unreliable, because it is difficult to generate consistent and accurate estimates for it. Travel-time reliability is a metric that is important to and innately understood by travelers and shippers. Variable or unpredictable travel times make it more difficult for travelers and shippers to plan their travel, often forcing them to add extra time to protect themselves against the uncertainty of arrival times. This uncertainty may lead to ineffective or even counterproductive travel decisions that waste time and money.

Frequent disruptions to normal flows lead to uncertainty for the traveling public and freight shippers. Figure 2 provides an illustrative example of what travelers experience over a year's time and how they are most likely to recall the highest travel-time delays. As this figure shows, major delays due to non-recurring incidents may be infrequent, but their effects can be large when they happen. In addition, those exceptionally bad days are the ones best remembered by the traveling public.

Figure 2. Illustration. Travelers' Experience of Travel-Time Reliability. This illustration shows two line graphs side-by-side. Both graphs compare travel time (generic) on the y-axis and the time of year on the bottom (January through December) along the x-axis. The left-side graph shows how traffic conditions have been communicated to the public with a straight data line indicating the annual average. The right-side graph shows what travelers experience with data lines that indicate how travel times vary greatly day-to-day. The right-side graph also highlights data points where travel times are the highest and indicates that these are the instances that travelers remember.
Figure 2. Graph. An Illustrative Example of Typical Travel Time Reporting versus What Travelers Experience
Source: Travel-Time Reliability: Making it There On Time, All The Time, Federal Highway Administration, FHWA-HOP-06-070

The basic causes of unreliable travel times are (1) an imbalance between demand and capacity and (2) the congestion that can result from such an imbalance. Once congestion occurs, travel times become more variable (less reliable and thus less predictable). Moreover, congested facilities lack the resilience to accommodate unexpected travel interruptions, which leads to flow breakdowns and serious degradation of reliability. Travel times vary from one day to the next because conditions influencing traffic differ each day.

The SHRP 2 L02 Guidebook [2] identifies seven sources of congestion that influence travel-time reliability. They are: fluctuations in normal travel, inadequate base capacity, special events, traffic incidents, weather, traffic-control devices, and work zones. Actions responding to these factors fall into two broad categories. The first category of actions addresses the demand for travel, including the use of travel information and pricing and incentive-based strategies to influence when, where, how, and how much travel (both personal travel and freight movement) occurs. Emerging smart phone applications such as for dynamic ridesharing and real-time parking information can also be effective tools for managing real-time travel demand.

The second category of actions includes actions to increase roadway capacity or otherwise maximize throughput, such as the following:

  • Expansions of, or additions to, highway or transit facilities;

  • Application of better operational and technical systems, such as active traffic management and intelligent transportation systems, to maximize the performance of existing infrastructure;

  • Advances in technology and procedures that more-quickly restore capacity that has been lost as a result of disruptions (incidents, weather conditions, work zones); and

  • Optimal use of existing transportation system capacity controlled by other transportation agencies, firms, or individuals. (This can be accomplished by providing incentives for mode shifts from single-occupant vehicles to multi-occupant vehicles and more-effective use of alternative rights-of-way).

The types of solutions that can address demand and capacity imbalances depend on whether congestion can be anticipated or whether congestion results from unexpected events. When volume routinely approaches or exceeds capacity (recurring congestion), demand management and capacity increases are likely to be effective in improving reliability. In cases in which unexpected disruptions cause the bulk of congestion (non-recurring congestion), techniques that detect disruptions and facilitate rapid recovery from those events are more likely to be effective. Even for situations in which unexpected disruptions cause the majority of congestion, however, strategies for demand management and capacity increases will also warrant consideration. These strategies create a capacity margin that helps to ensure the system's resilience in effectively responding to unexpected events.

1.2 Why Does Reliability Matter Now?

Why Focus on Reliability Now?

Constraints on expansion of the nation's transportation system, expectations of the traveling public, and Federal legislation are driving interest in travel-time reliability.

Improved monitoring technology and the availability of tools through the SHRP 2 program now make it feasible to evaluate reliability.

Traffic congestion due to non-recurring events such as traffic incidents, weather, road work zones, and special events accounts for a majority of total traffic congestion-related delay in the United States. Up until recently, there were few options for cost-effectively collecting data for non-recurring events, particularly unplanned events such as crashes. MPOs and states that recognized the importance of travel-time reliability early on developed proxy objectives and performance measures, such as reducing collisions. Most agencies, however, have focused their CMPs on recurring congestion.

The focus on recurring congestion in CMPs has been easier to quantify from a monitoring standpoint, but has led to improvement strategies that focus on capacity expansion. Capacity expansion is increasingly difficult and expensive to implement. Expanding the scope of CMPs to address non-recurring congestion would mean more data collection and analysis. However, it would also lead to an expanded toolbox of improvement strategies that would incorporate transportation system management and operations (TSM&O). TSM&O strategies are generally easier and less expensive to implement.

With the growing field of inexpensive travel-time monitoring technologies and new prediction tools through the SHRP 2 program, it is now feasible to develop reliability performance measures. Analysis tools have been developed to identify current reliability problems and to predict reliability problems in the future.

Three major factors have also contributed to driving the focus on travel-time reliability by Federal, state, and metropolitan planning organizations: constraints on further expansion of the nation's transportation system, expectations by the traveling public, and recent Federal legislation.

Constraints on Expansion of the Transportation System

Vehicle Miles Traveled (VMT) is a measurement of miles traveled by vehicles in an area over for a specified time frame. For example, the Federal Highway Administration summarizes monthly and yearly VMT for the nation and each state.

Highway Lane Miles is calculated by the number of lanes multiplied by the length in miles. It can be used to track capacity expansion for both existing and new roadways.

The era of new roadway construction has largely ended in most of the major metropolitan areas of the country. In addition, the practice of widening existing roadways is also falling out of favor due to high costs, the built out nature of many urbanized areas, and community desires for more multi-modal streets. While vehicle miles traveled (VMT) per-capita has been dropping steadily since 2004, population growth continues, particularly in urban areas, and this has led to more VMT overall. At the same time, transit ridership has increased to its highest level since 1956 [3]. There's growing momentum for making more-efficient use of the entire existing transportation system.

Growth in population, drivers, vehicles, and vehicle miles of travel overall has far outpaced roadway capacity expansion. As shown in Figure 3, the population of the United States increased 36 percent in the 30 years between 1980 and 2010, the number of auto drivers increased by 45 percent, the number of licensed vehicles increased by 50 percent, and the annual VMT increased by 94 percent. Meanwhile, new lane-miles of highways increased by only 8 percent during the same period.

Figure 3. Bar Chart. Growth versus New Highway Construction. This two-dimensional bar chart has the percent increase on the y-axis and indicators along the x-axis that include population, licensed drivers, registered vehicles, vehicle-miles-traveled (VMT), and highway lane miles. It shows that, between 1980 and 2010, the United States population increased by 36.3 percent, licensed drivers increased 44.6 percent, registered vehicles increased by 49.8 percent, and VMT increased by 94.2 percent. For the same time period, highway lane miles increased 8.3 percent.
Figure 3. Chart. 30-Year Growth of Key Metrics in Relation to Highway Lane Miles
Created using data from the United States Census Bureau and the Federal Highway Administration's Office of Highway Policy Information, Highway Statistics Series

As the physical capacity of our roadways is consumed by the growth in traffic, they become more vulnerable to disruptions caused by traffic incidents, inclement weather, special events, and work zones. These non-recurring events can occur at any time and place, and can cause congestion even in areas that don't usually experience recurring congestion. Variability in travel times is increasing on more roadways and for more times of the day, in part because non-recurring congestion has not typically been addressed in CMPs, or in the traditional transportation planning process. The highway transportation system has become more fragile and more susceptible to major disruptions due to traffic incidents.

Expectations of the Traveling Public

Surveys of the traveling public and freight shippers repeatedly show that they value travel-time reliability more than speed. They are aware that technologies have been developed to extract data from mobile devices and to monitor real-time traffic conditions.

They expect that public agencies will use this data to provide real-time information and to alleviate the effects of disruptions on the roadway and transit network.

Federal Surface Transportation Reauthorization Law

On July 6, 2012, President Obama signed into law the Moving Ahead for Progress in the 21st Century Act (MAP-21) [4]. It funds Federal surface transportation programs at an average of about $41 billion per fiscal year and has been extended through May 31, 2015. This was the first long-term highway authorization enacted since 2005. In addition to funding, MAP-21 establishes the policy and programmatic framework for investments to guide the growth and development of nation's transportation infrastructure.

MAP-21 established a new paradigm for states and MPOs called "performance-based planning and programming" (PBPP). This means that MPOs, state departments of transportation (DOT), and transit operators will be involved in performance measurement, target setting, and reporting on the outcomes of their transportation investments. MAP-21 began this process by setting seven national goals, and subsequently prescribing performance measures for each. One of the seven goals is:

"(4) System reliability - To improve the efficiency of the surface transportation system."

MPOs that develop reliability goals and incorporate reliability into their CMPs provide a pathway to developing PBPP elements that help address national system reliability needs that aid this national goal by improving regional travel. 1

Since 1991, Federal transportation law has required that MPOs explicitly address congestion. The Safe Accountable Flexible Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) surface transportation reauthorization law stipulated the requirement for the use of the CMP in Transportation Management Areas (TMAs). This requirement was continued under MAP-21. The CMP consists of actions employing a variety of performance measures and analytic tools to define and identify congestion across all modes, and to develop and select appropriate strategies to reduce congestion. These strategies may include capital projects in the metropolitan transportation plan (MTP) and transportation improvement program (TIP), or operations actions carried out by MPO member agencies. While the CMP establishes a set of prioritized projects and actions, it does not impose upon MPOs a requirement to program them. An individual MPO may, for example, assign a low priority to congestion mitigation in their planning goals and objectives.

Over time, MPOs will need to pay attention to outcomes of the Federal investments in their TIP by reporting on performance. Many have only rudimentary performance measurement and monitoring systems in place. While some of the national goals may be limited to the National Highway System (NHS), reliability is a system-wide issue for most agencies, incorporating freeway, arterial, and transit operations.

1.3 The CMP and the Value of Incorporating Reliability

The Value of Reliability

Travel-time reliability is a metric that is important to and is innately understood by travelers, freight shippers, and elected officials.

MPOs, by their very nature, must be responsive to the needs of their constituents as they develop plans and investment programs for the future of their regions. These typically address a wide range of issues, including mobility, safety, accessibility for all users, and quality of life. Non-recurring congestion can have a negative impact on all of these factors. Some MPOs are learning to pay more attention to system reliability. It is a metric that has become important to their constituents and elected officials.

Incorporating travel-time reliability into the CMP creates a systematic method to address the issue. The benefits include a more-robust understanding of the regional transportation system and a toolbox of strategies that go beyond capacity expansion to include operations and demand management solutions. A CMP that does not incorporate reliability will be heavily weighted towards traditional capacity improvements, missing out on, or under valuing more cost-effective operations strategies. A CMP that adequately incorporates reliability will tend to include more (or place a greater weight on) operations strategies, such as signal retiming or traveler information. Figure 4 demonstrates a variety of capacity-related and operations-related strategies that may be included in a robust CMP.

Figure 4. Illustration. Typical Capacity and Operations Related Strategies. This illustration shows capacity-related strategies for improving reliability are to build or widen roadways, to build or expand transit systems, to build or widen walkways, to increase transit vehicle fleets, or to build or widen bikeways. It also shows operations-related strategies for improving reliability that include arterial management, incident management, work zone management, traveler information, freeway management, special event management, travel weather management, Travel Demand Management (TDM), freight management, and transit operations and management.
Figure 4. Chart. Typical Capacity and Operations Related Strategies

Another advantage of incorporating reliability into the CMP is that many of the TSM&O strategies for addressing reliability problems can be deployed more quickly, at lower cost, and with a smaller environmental impact than traditional large-scale capacity improvements.

Focusing on system management and operations has been shown to have positive effects not only on reliability, but on safety and the environment as well. Many of the steps to create the CMP provide opportunities to incorporate travel-time reliability.
These include:

  • Developing goals and objectives
  • Identifying performance measures
  • Monitoring
  • Identification of problems
  • Identification of strategies
  • Monitoring effectiveness of implemented strategies

1.4 Purpose of the Primer

This primer is intended to provide advice to staff at MPOs on how to reevaluate their CMPs. It will show the value of integrating travel-time reliability into the CMP to address non-recurrent congestion. It will identify tools and data sources that are available to assist planners in monitoring and predicting travel-time reliability. Incorporating reliability in the CMP will result in a more-robust regional transportation system with objectives, performance measures, and strategies that align more closely with public and freight shippers' concerns.

In particular, this primer provides guidance and potential opportunities for using products developed through the SHRP 2 Reliability focus area. They have been developed, in part, to support and advance the CMP and Planning for Operations [5] as part of the metropolitan transportation planning process.

SHRP 2 was authorized by Congress to address some of the most-pressing needs related to the nation's highway system, including congestion stemming both from inadequate physical capacity and from events that reduce the effective capacity of a highway facility. SHRP 2 reliability products focused on developing basic analytical techniques, design procedures, and institutional approaches to address the events such as crashes, work zones, special events, and inclement weather that often result in the unpredictable congestion that make travel times unreliable. This primer also includes guidance for using the National Performance Monitoring Research Data Set (NPMRDS) and other reliability tools.

1.5 Content of the Primer

The primer includes a "How-To" guide for incorporating travel-time reliability into an agency's next CMP (Chapter 2). It also contains a guide to getting started (Chapter 3) and a model CMP (Chapter 4) that use examples from CMPs that have incorporated travel-time reliability. The appendix contains a CMP refresher (Appendix).

Throughout the primer are examples from MPOs that have included travel-time reliability in their CMPs and Planning for Operations processes. The primer's content is sensitive to metropolitan regions of various sizes and addresses a range of issues from agencies that have very little system performance data to those with access to a large amount of data, recognizing that the field of transportation data collection is resource-intensive but also very dynamic and technology-enabled.

Urbanized areas with populations greater than 200,000 are designated as TMAs by the Federal Highway Administration and the Federal Transit Administration. MPOs located in long-standing TMAs often have many years of experience producing congestion management plans. By taking advantage of the contents of this primer, MPOs can now include non-recurring congestion and travel-time reliability in a CMP that provides more value to their constituents.

1 As of the writing of this primer the notice of proposed rule-making (NPRM) had not been released, thus the specific requirements and where they will apply were not known.


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