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Freeway Management and Operations Handbook |
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Chapter 1 – Introduction
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A tollway or toll road is similar to a freeway, except that tolls are collected at designated points along the facility, either electronically, manually, or some combination. Although the collection of tolls may involve interruptions of traffic flow (Figure 1-5), these facilities should generally be treated as "freeways", particularly with respect to strategies and technologies for management and operations. Special attention should be given to the unique characteristics, lane management opportunities, and constraints associated with toll collection facilities. Accordingly, the term "freeway" as used in this Handbook refers to any limited access facility, including the interstate system, expressways, toll roads, and connecting bridges and tunnels.
Figure 1-5: Toll Plaza
The FHWA publication "Managing our Congested Streets & Highways" (10) documents the results of several surveys, with delays caused by traffic congestion topping the list of transportation issues that people reported as affecting their communities. But what is actually meant by the term "congestion"? To truly comprehend freeway management and operations strategies and supporting technologies, and to fully appreciate their potential to deal with congestion problems, it is important to understand both the nature of congestion and the events that occur in the traffic stream as congestion forms. There are multiple definitions and measures of congestion – both quantitative and qualitative, as discussed below.
Figure 1-6: Generalized Relationships Between Speed,
Density, and Flow Rate on Freeways
(Reference 7) D
The form of these curves depends on the prevailing traffic and roadway conditions on the segment under study. Moreover, while the diagrams in Figure 1-6 show continuous curves; in reality there are likely discontinuities, with part of these curves not present. The curves illustrate the following significant points.
Efficient freeway operation depends on the balance between capacity and demand. In the simplest terms, highway congestion results when traffic demand approaches or exceeds the available capacity of the highway system. As vehicle demand approaches highway capacity, traffic flow begins to deteriorate. Flow is interrupted by spots of turbulence and shock waves, which disrupt efficiency. Then, traffic flow begins to break down rapidly, followed by further deterioration of operational efficiency. The result of this spiraling inefficiency can be observed during every weekday commute in almost every metropolitan area: Drivers push their way onto already crowded freeways to join thousands of others already caught in seemingly endless traffic jams. Unfortunately, by joining the already impeded traffic flow, drivers become part of the problem, creating even greater inefficiencies: more stop-and-go traffic conditions, longer delays, and greater potential for accidents.
While this is a simple concept, traffic demand is not constant. It can vary significantly depending on the season of the year, the day of the week, and even the time of day. Also, the capacity is not a constant – it can change (sometimes rapidly) because of weather, work zones, traffic incidents, or other events. It is not necessarily simple, either. The physical fact of finiteness and the principle of conservation underlies traffic stream behavior, as reflected in the smooth curves in Figure 1-6. However, the actual performance of a particular section of freeway at a particular point in time is more ambiguous, resulting from variations in individual human behavior and the mix of vehicle types using the facility. It may be possible to predict the average behavior and average capacity, and the variances about these averages for a traffic stream, but never the precise behavior.
NCHRP Report 398 entitled "Quantifying Congestion" (Reference 16) identifies the following four components that interact in a congested roadway or system:
The report states: "Any definition of congestion, and the congestion measures derived therefrom, should rely on concepts that are understandable by the intended audience. Travel time and its related quantities are widely understood and fundamentally useful in the definition and measurement of congestion. However, the congested reflected in travel times and delays that are acceptable to travelers can vary by city size, location in the urban (or rural) area, and time of day or year. One method that may be used to resolve this issue is to define two quantities, congestion and unacceptable congestion.
Measures of congestion are discussed in Chapter 4.
Perhaps "congestion" lies in the eye (and experience) of the beholder (Figure 1-7). As an example, in 2002, the ITS Forum of the "National Associations Working Group for ITS" (http://www.nawgits.com/itsforum [Link no longer active]) posted as the question of the month: "What should be our common definition of Congestion?" There were numerous replies and opinions, including:
Figure 1-7: Another View of Congestion
With respect to the last point, congestion is typically viewed by travelers relative to their normal day-to-day experiences. Travelers accustomed to low speeds and congestion delays for 12 hours each day may not consider 10 minutes of delay per trip a problem. These travelers have learned to budget extra time or find other ways to cope with the delay. Travelers accustomed to light traffic and reliable trips might consider 5 minutes of delay per trip unacceptable and a problem worth noting at the next City Council meeting. A key aspect of a congestion management strategy is identifying the level of "acceptable" congestion and developing plans and programs to achieve that target. (2)
A major goal of freeway management and operations is to keep freeway capacity and the vehicular demand on a freeway in balance. The most effective way to combat congestion is to take action before traffic flow deteriorates and congestion forms. It would be ideal to manage the demand on the freeway to prevent traffic flow from ever breaking down and congestion from forming. This is usually not possible, and the best result is to delay the onset of congestion and speed the recovery from congestion, therefore minimizing the inefficiencies that congestion causes.
Congestion is often classified as either recurrent or non-recurrent. The type of congestion depends on whether the capacity or the demand factor is out of balance.
Whereas recurrent and non-recurrent congestion have different causes, their solutions have many elements in common.
An individual highway crash is a rare, random, multifactor event, preceded by a situation in which one or more persons failed to cope with their environment. In the aggregate, however, traffic crashes are quite numerous and often follow certain patterns that can be identified. Crashes reflect a shortcoming in one or more components of the driver-vehicle-roadway system. It is therefore very important for freeway practitioners to monitor traffic collision experience, and to use this information to identify, plan, implement, and evaluate corrective actions. Numerous approaches exist for improving safety and reducing crashes on highways. Many of these are beyond the scope of freeway management and operations, per se (e.g., enforcing seat belt laws, in-vehicle crash-avoidance technologies, geometric realignment); but others – such as improved signing and lighting, skid resistance pavement, adding shoulders and auxiliary lanes, and removing obstacles – are well within the realm of "operations".
As previously noted, a major goal of freeway management and operations is to reduce congestion; and a reduction in congestion may also enhance safety. But how does congestion affect highway safety? The basic theory behind the interaction is that congestion leads to higher vehicle densities (i.e., more closely spaced vehicles on a roadway), which provides more opportunities for conflict. Congestion also reduces vehicle speeds, which implies that when vehicles are engaged in a crash, the collision forces are lower, thus reducing the injury to occupants. Another aspect of the model is the concept of "secondary" crashes—crashes that occur due to conditions produced by an existing crash. Some of these conditions—which wouldn't exist without the occurrence of the first crash—include rapid backward queue formation (as vehicles suddenly stop to avoid the first crash), rubbernecking by drivers, and the maneuvers of emergency vehicles. Finally, the flow restrictions produced by crashes worsen existing congestion (2).
The details of the relationship between congestion and safety are not well understood (with the exception of lower crash severities, which have been documented in a general way for congested conditions, and the associated lower speeds). Based on the limited work that has been performed, a few tentative conclusions may be drawn:
In general, it can be assumed that any operational improvement that reduces congestion will lead to fewer crashes. The severity of crashes that occur will be higher, however, and it is likely that a greater proportion will be single vehicle crashes. Knowing these facts can target mitigation strategies to single vehicle crashes and higher severities—such as wider roadside recovery zones, protection of highway "furniture," and coordination with emergency medical services. Moreover, an operations philosophy must take a systems-oriented view, where the consequences of a specific action (e.g., flow improvements) consider linked impacts such as safety (2).
Freeway traffic management and operations is the implementation of policies, strategies and technologies to improve freeway performance. The over-riding objectives of freeway management programs include minimizing congestion (and its side effects), improving safety, and enhancing overall mobility. The TRB Freeway Operations Committee's Millennium Paper (3) states: "Freeway operations, in its broadest context, entails a program to combat congestion and its damaging effects: driver delay, incon and frustration, reduced safety, and deteriorated air quality." Freeway traffic management entails:
Freeway traffic management is all of this and more. Its components are aimed at providing some level of relief from congestion, improving safety and mobility for the traveling public, and meeting other related objectives. Strategies and technologies associated with freeway management and operations are summarized below.
Freeway management and operations include "low-cost" (relative to constructing new facilities) improvements to the freeway infrastructure. Examples of such activities include adding auxiliary lanes, ramp widening, restriping to add an additional lane within the existing pavement, and similar improvements to eliminate bottlenecks. Enhancements to other attributes of the freeway infrastructure (e.g., signing, pavement markings, illumination) to increase safety and driver convenience / comfort are other possible improvements.
Traffic incident management is an operational approach designed to quickly detect, respond, and clear disabled vehicles and other "events" (such as debris) from the roadway that would detract from facility performance. Traffic incidents, such as crashes and disabled vehicles, reduce capacity (i.e., non-recurrent congestion) and decrease safety (along with the life-safety issues associated with responding to a crash scene with the proper medical personnel and equipment as soon as possible). Such incidents are the cause of 40 to 60 percent of all congestion in urban areas.
The primary intent of traffic incident management is to prevent incidents from reducing capacity; but when they do, the focus is to restore capacity as quickly as possible. This prevents backups and significantly decreases the occurrence and severity of congestion, and the possibility of secondary crashes. Traffic incident management programs vary from location to location. They can include: technologies and communications for rapidly detecting incidents and identifying their location and the appropriate response needs (e.g., networks of closed circuit television cameras, vehicle detection sensors, incoming 911 reports, incoming media reports, and mobile reports), systems and procedures for dispatching the appropriate emergency response personnel and equipment; service patrols (vehicles specifically intended to look for and help disabled motorists), incident management teams (interagency working groups formed to develop faster and more efficient responses to accidents and other major incidents) traveler information systems.
The term "managed lanes" covers a variety of strategies and techniques, many of which have been used for years. The basic concept behind lane management is to employ operational tools to maximize the productivity of the available roadway. Lane management concepts can include the following:
Ramp management, is the application of control devices, such as traffic signals, signing, and gates to regulate the number of vehicles entering or leaving the freeway, in order to achieve operational objectives. Most ramp management strategies lead to improved traffic flow and safety by 1) regulating the number of vehicles entering or exiting a freeway, or 2) smoothing out the rate at which vehicles enter the freeway facility. By employing either of these approaches, ramp management helps to balance freeway demand and capacity and maintain optimum freeway operation by reducing incidents that produce traffic delays, improve safety on adjacent freeways or arterial streets, or giving special treatment to a specific class of vehicles. Ramp management strategies are also often implemented with elements of other freeway management programs to create operational efficiencies and to assist in the delivery of overall transportation management goals and objectives.
Ramp management strategies may be used to control access to selected ramps, thus limiting the periods when vehicles may access the ramp or possibly restricting access to the ramp permanently. This significantly reduces, or may even eliminate, the potential for collisions that occur as a result of traffic entering or exiting the ramp facility and in turn smoothes the flow of traffic on segments of roadway where these collisions have occurred in the past.
Ramp management may also control the manner in which vehicles enter and exit a freeway. For instance, vehicles that enter the freeway in platoons introduce turbulence, which causes vehicles on both the mainline and ramp to slow down to safely merge. This causes congestion around and upstream of ramp/freeway merge points. Ramp management strategies may be used to control the flow of vehicles entering a freeway, thus smoothing the rate at which vehicles are allowed to enter the freeway. Similarly, ramp management strategies and treatments may be implemented at the ramp-arterial intersection to better manage the flow of traffic exiting the freeway. Such treatments may reduce queues on exit ramps that extend out onto the freeway, helping to improve safety and mobility on the freeway.
This provides the information travelers need to effectively plan their trip prior to departure; and when en-route, the information may be used by drivers to avoid congestion and / or to adjust their driving behavior (e.g., in response to unsafe conditions). Traveler information provides choices for travelers – a key attribute of mobility. They can select different routes, different modes, different times, or even different destinations. They can avoid congested or unsafe routes (e.g., due to adverse weather conditions). Their ability to choose improves their trip. Such knowledge also reduces stress and limits risk taking behavior, thus producing better travel conditions. Pre-trip information is typically disseminated to the public via websites, media broadcasts, and mobile communication devices (e.g., personal digital assistants, pagers, and cell phones). En-route traveler information has traditionally been disseminated via commercial radio, changeable message signs (CMS) and highway advisory radio (HAR). With the emergence of wireless communication technologies, en-route traveler information can also be disseminated through wireless phones, web-enabled wireless phones, and a variety of personal digital assistants (PDA) equipped with wireless communication capabilities.
Intelligent Transportation Systems (ITS) is the application of advanced electronics, computer, communications, and sensor technologies – in an integrated manner – to increase the increase the efficiency and safety of the surface transportation network. ITS encompasses technologies that can lead to:
Used effectively, ITS opens the door to new ways of managing, operating, expanding, refining, reconfiguring and using the transportation system. ITS has proven itself as a significant enabler of freeway management and operations. Combined with ITS technologies, a Freeway Management System (FMS) consists of a set of resources (e.g., electronic systems, people, objects, and strategies) that are used to accomplish a set of goals to improve the operation of the freeway network. Several of these potential ITS components have already been mentioned above (e.g., CMS, HAR, ramp meters). Other elements of an ITS-based freeway management system that support these functions include:
All of the attributes of a freeway management program – policies, funding mechanisms, strategies, systems and technologies, operational activities, etc. – take place within the institutional framework. This institutional fabric is multi-agency, multi-functional, and multi-modal. Moreover, the authority for transportation decision-making is dispersed among several levels, or "tiers", of government (e.g., national, statewide, regional, agency, individual systems), and often between several agencies and departments within each governmental level. This institutional situation can lead to a fragmented delivery system for transportation services, resulting in an agency-specific and mode-specific focus, rather than an area-wide focus that considers the entire trip. After all, the customer's perspective is that there is only one surface transportation system. The public generally does not care which jurisdiction or agency is responsible for the road or mode on which they are currently traveling. As taxpayers (and in some cases fare / toll payers), they want and deserve a safe, reliable, and predictable trip – one that is safe from physical and mental harm, provides a consistent level-of-service with minimal congestion, and is predictable in terms of travel time.
Achieving the safe, reliable, and secure operation of our Nation's transportation network depends on collaboration and coordination across traditional jurisdictional and organizational boundaries. In other words, there must be "integration" (i.e., a term defined in the dictionary as making into a whole by bringing all parts together). In essence, integration is a bridging function between the various components of the surface transportation, and involves processes that focus on the sharing of information and the combining of resources in a manner that facilitates a more seamless operation. In addition to institutional integration (i.e., coordination between various agencies and jurisdictions to achieve seamless operations), the surface transportation network needs to be integrated in other ways, including:
Freeway management and operations must be addressed within this larger institutional context. Moreover, it is important that the practitioner have an understanding of the many external factors and dependencies that can impact the performance of the freeway network and influence how it should be managed.
In addition to the broad concepts discussed above, the following items also warrant a brief discussion. As with most of the other items addressed in this introductory chapter, additional information and detail is provided in subsequent chapters.
A "program" is a coordinated, inter-related set of strategies, procedures, and activities, all intended to meet the goals and objectives articulated in vision statements and policies. "Projects" are well-defined, individual actions and activities that make up the program. The implementation of projects is how the program is realized. A program has a long-term temporal view, whereas individual projects generally have a shorter implementation period. Managing a program involves trade-offs between budget and timing, and determinations as to appropriate sequence and scope of the associated projects.
Practically every transportation-related program and project involves some sort of "process" – a series of actions – through which ideals and concepts are brought to fruition, implemented, and managed. There exists in every process an underlying structure that shapes and controls events. This framework consists of formal activities (e.g., written or unwritten policies agreed to in a collaborative fashion) and informal ones (e.g., human relationships), all relating to the ways options are created and decisions are made to improve the performance of the transportation network.
Freeway management programs – consisting of operational strategies, low-cost roadway improvements, and / or ITS-based systems – are funded and implemented as a means to preserve mobility, improve reliability, enhance safety, and meet the public's expectations for efficient and effective travel. It is important to ensure that the funds are spent wisely, that the agency makes the best use of its available resources, and that the full potential of past and current investments is realized. This, in turn, requires that the performance of the freeway be continuously monitored and evaluated, including an assessment of the extent to which the implemented solutions achieve the intended objectives.
Performance measures provide the basis for evaluating the effectiveness of implemented freeway management strategies, as well as for identifying the location and severity of problems (such as congestion and high crash rates). This monitoring information can be used to track changes in system performance over time, identify systems or corridors with poor performance, identify potential causes and associated remedies, identify specific areas of a freeway management program or system that requires improvement, and provide information to decision-makers and the public. In essence, performance measures are used to measure how the transportation system performs with respect to the adopted goals and objectives, both for ongoing management and operations of the system, and for the evaluation of future options.
Usually associated with the development of a freeway management system, regional architecture, and other ITS deployments, a "Concept of Operations" is a document that defines the environment in which the freeway (and other elements of the transportation network) is to operate, and the needs of the users. This environment includes the relationships between the system and the owning agency's policies, procedures and responsibilities; the interagency working relationships and agreements; the physical environment (i.e., the capabilities of the system); and the expectations (performance measures). The Concept of Operations is a tool by which regions, agencies, and traffic management centers – and the associated practitioners – identify (at a high level with few technical details) the optimal solution based on their preferred approach, their capabilities, and their constraints.
Section 1.3 identified the intended audience of the handbook as "transportation professionals that participate in or responsible for any phase in the life cycle of a freeway network." Persons whose responsibilities are primarily policy development will find this introductory chapter, plus chapter 2, most pertinent. Those with the responsibility for program management and the development of freeway-oriented programs and / or projects should also review chapters 3 and 4.
Chapters 5 – 17 address topic-specific aspects of freeway management and operations, and are pertinent to all practitioners with day-to-day responsibilities for managing or operating a freeway facility, depending on their specific interests and needs. The most appropriate strategies, technologies, and services will vary based on the conditions unique to each metropolitan area, including the type and extent of problems, the political structure, the agencies' experience with traffic management, and the level of cooperation between local agencies. Moreover, depending on regional needs, the overall freeway management goals, and the extent to which freeway management and operations are performed, will likely vary from region to region.
It is emphasized that this Freeway Management and Operations Handbook is not a design manual or a detailed technical reference. For many of the technical issues, excellent reference materials exist that provide more detailed information, in more of a "how-to" manner. These include recent handbooks on Traffic Incident Management, Planned Special Event Management, Communications, HOV Lanes, and Detection and Surveillance Sensors. For the chapters covering these areas, the Handbook references these technical documents and provides only a brief summary.
The FHWA White Paper "Freeway Management and Operations State of the Practice" (6) makes a distinction between the "practice" and the "art" – specifically:
Specific distinctions between the state of the practice and the state of the art are not addressed in this Handbook (as they are in Reference 6). The purpose of this Handbook is to provide the practitioner with a wide range of potentially useful alternatives for implementing freeway management programs and projects, thereby improving freeway operations. Some of these may be considered "practice"; while others may be deemed as "art". Emerging trends will also be identified. Moreover, experience, lessons learned, and examples are provided in many instances.
Many of the items addressed in this Handbook are either technology or program-based, and therefore likely to change and/or become outdated at any time following the release of this Handbook. The reader should check the date of the Handbook. At the time of writing this document, the plan is for it to be more dynamic. The Internet is being used as one of the methods for distributing the Freeway Management and Operations Handbook. As technologies, operating practices, or programs change, as additional experience is gained and new lessons are learned, and / or as new reference documents are developed, the affected chapter(s) of the Freeway Management and Operations Handbook will either be modified and posted on the web, or announcements regarding new reference materials will be posted. For now, additional information on Congestion Mitigation activities can be found at the FHWA Congestion Mitigation web site at https://www.fhwa.dot.gov/congestion, the ITS Joint Program Office web site at http://www.its.dot.gov, or the Office of Operations web site at http://www.ops.fhwa.dot.gov.
Regardless of which chapter(s) the practitioner is perusing, there are a few key concepts that should always be kept in mind. Some of these have already been mentioned, while others will be discussed in subsequent chapters. They nevertheless apply to all aspects and processes involving the management and operation of a freeway facility. These key themes are highlighted in Table 1-4, and discussed in the following bullets.
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Each of the 17 chapters that comprise the Freeway Management and Operations Handbook has been developed to "stand alone" within its specific topic area. Not lost on this, though, are the relationships and dependencies between various freeway management activities and other elements that comprise the surface transportation network. Thus, just as the freeway practitioners must view transportation as a whole, this Handbook should be looked at in a similar fashion. The chapters are not completely independent – for example, performance measures (discussed in Chapter 4) should be an integral part of the various processes (discussed in both chapters 2 and 3) by which transportation improvements are planned, developed, and implemented. Moreover, a freeway management program will encompass strategies and technologies from multiple chapters.
It is emphasized that the chapters only provide an "introduction" to their respective subjects. For additional details and design guidelines, the practitioner should consult a variety of references, many of which are identified (including web addresses) in the text of the chapter, along with the references section at the end of each chapter.
Chapters 7 through 17 – the ones that address specific freeway management strategies and technologies – utilize a common basic structure, starting with an introduction of the topic, the purpose of the chapter, and its relationship to other freeway management activities and Handbook chapters. The next section addresses "Current Practices, Methods, Strategies, and Technologies", including an overview of the subject, benefits, key considerations during freeway management program development, the relationship to the National ITS architecture, specific technologies and strategies, and emerging trends. This is followed by a section on "Implementation and Operational Considerations". The chapter concludes with "Examples" and "References". This format is not rigid. Depending on the chapter and its subject matter, these sections may have a different order, additional sections may be included, examples may be included throughout instead of in a separate section, and design considerations may be located in different sections; all to keep an appropriate flow of thought.
A brief summary of the material covered in the remaining chapters is provided below:
1. Meyer, M.D. A Toolbox for Alleviating Traffic Congestion and Enhancing Mobility. Institute of Transportation Engineers, Washington D.C. 1997
2. Lomax, Turner, Hallenback, et al; "Traffic Congestion and Travel Reliability—How Bad is the Situation and What is Being Done About It?"; September 2002
3. Freeway Operations: Year 2000 and Beyond; Committee on Freeway Operations; Chairman: Peter M. Briglia Jr.,
4. ITS National Architecture The ITS National Architecture, Documentation – Version 4.0, April 2002
5. Testimony Before the Subcommittee on Highways and Transit, Committee on Transportation and Infrastructure, House of Representatives; United States General Accounting Office; "Highway Infrastructure – Physical Conditions of the Interstate Highway System Have Improved, but Congestion and Other Pressures Continue"; Statement of Katherine Siggerud, Acting Director, Physical Infrastructure Issues
6. "Freeway Management and Operations: State-of-the-Practice White Paper"; Prepared for Federal Highway Administration, Office of Travel Management; March 2003
7. Highway Capacity Manual, Transportation Research Board, National Research Council, Washington D.C: 2000
8. "Integrated Surface Transportation Systems: The Role of Transportation Management Centers", Jon Obenberger & Walter H. Kraft, October, 2001;
9. FHWA "Operating the Highway System for Safety, Reliability and Security: TEA-21 Reauthorization Proposal".
10. FHWA; "Finding Out What America Thinks", Publication No. FHWA-OP-01-018
12. Sussman, Joseph; "Transportation Operations: An Organizational and Institutional Perspective"; ITE Journal, December 2002.
13. A Policy on Geometric Design of Highways and Streets, American Association of State Highway and Transportation Officials, Washington, D.C. 2001
14. ITS America 10 year vision
15. Intelligent Transportation Systems Benefits and Costs – 2003 Update; Mitretek Systems; Washington D.C.; May 2003
16. Lomax, T., Turner, S, & Shunk, G.; NCHRP Report 398 – Quantifying Congestion; National Academy Press; Washington D.C.; 1997