《交通工程学》课程教学资源（书籍文献）Traffic Engineering（3rd Edition）.pdf_P16-P20

CHAPTER Introduction to Traffic n 0 0 bngineering 1 .I Trisff ic Engineering as a Profession The Institute of Transportation Engineers defines traffic engineering as a subset of transportation engineering as follows [I]: Transportation engineering is the application of technology and scientific principles to the planning, functional design, operation, and management of facilities for any mode of transportation in order to provide for the safe, rapid, comfortable, convenient, economical, and environmentally compatible movement of people and goods. and: Traffic engineering is that phase of transportation engineering which deals with the planning, geometric design and traffic operations of roads, streets, and highways, their networks, terminals, abutting lands, and relationships with other modes of transportation. These definitions represent a broadening of the profession to include multimodal transportation systems and options, and to include a variety of objectives in addition to the traditional goals of safety and efficiency. 1 .I .1 Safety: The Primary Objective The principal goal of the traffic engineer remains the provision of a safe system for highway traffic. This is no small concern. In recent years, fatalities on U.S. highways have ranged between 40,000 and 43,000 per year. While this is a reduction from the highs experienced in the 1970s, when highway fatalities reached over 55,000 per year, it continues to represent a staggering number. More Americans have been killed on U.S. highways than in all of the wars in which the nation has participated, including the Civil War. While total highway fatalities per year have remained relatively constant over the past two decades, accident rates based on vehicle-miles traveled have consistently declined. That is because U.S. motorists continue to drive more miles each year. With a stable total number of fatalities, the increasing number of annual vehicle-miles traveled produces a declining fatality rate. 1

3 CHAPTER 1 INTRODUCTIONTO TRAFFIC ENGINEERING Improvements in fatality rates reflect a number of and the need to provide safety comfort and convenie trends,many of which traffic enginers have been instru are generic terms and mean different things to different people.Comfort involves the physical characteristics of from the roa cty.D atcdBWehciniucnce(DU tionays,and is infuenced by our percep tion to accommodate all of our tra el needs ily as a result of DUI/DWI convictions,poor accident times.Economy is also relative.There is littenmoe record,and/or poor violations record.Vehicle design has transportation systems that can be termed "cheap. I acts of Congres Highway and other transportation systems involve ma belts with shoulderharnesses.air bags (some vehicles now user taxes and fees.Nevertheless.every eer re have as many as eight),and antilock braking systems gardless of discipline,is called upon to provide the best Highway design has improved through the development possible systems for the money use of er systems for median and roa that ha systemscan alert authorities to ac. portation systems have some negative im acts on the en cidents and breakdowns in the system vironment.All produce air and noise pollution in some forms,and all utilize valuable land resources.In many 8 s,tra systems utili as muc tive environmental impacts and where system architec 1.1.2 Other Objectives with all of th and ob se goals offs to otimize both the transportation systems and the ·Speed use of public funds to build,maintain,and operate them. ·Comfort 1.1.3 ·Convenience ·Economy .Environmental compatibility The traffic engineer has a very special relationship with the public at large.Perhaps m ore thar her type c and in harmony with the environr that who designs a product has this respons All of these objectives are also relative and must be bal- bility anced against cach other and against the primary objec. pro ding upon tive o ced oftravel is much to be desi ed it is lim obligationtoemplGytheaamlabenele5 and state of xisting resources to enhance public safety

2 CHAPTER 1 INTRODUCTION TO TRAFFIC ENGINEERING Improvements in fatality rates reflect a number of trends, many of which traffic engineers have been instrumental in implementing. Stronger efforts to remove dangerous drivers from the road have yielded significant dividends in safety. Driving under the influence (DUI) and driving while intoxicated (DWI) offenses are more strictly enforced, and licenses are suspended or revoked more easily as a result of DWWI convictions, poor accident record, and/or poor violations record. Vehicle design has greatly improved (encouraged by several acts of Congress requiring certain improvements). Today’s vehicles feature padded dashboards, collapsible steering columns, seat belts with shoulder harnesses, air bags (some vehicles now have as many as eight), and antilock braking systems. Highway design has improved through the development and use of advanced barrier systems for medians and roadside areas. Traffic control systems communicate better and faster, and surveillance systems can alert authorities to accidents and breakdowns in the system. Despite this, however, over 40,000 people per year still die in traffic accidents. The objective of safe travel is always number one and is never finished for the traffic engineer. 1 .I .2 Other Objectives The definitions of transportation and traffic engineering highlight additional objectives: 0 Speed Comfort 0 Convenience * Economy * Environmental compatibility Most of these are self-evident desires of the traveler. Most of us want our trips to be fast, comfortable, convenient, cheap, and in harmony with the environment. All of these objectives are also relative and must be balanced against each other and against the primary objective of safety. While speed of travel is much to be desired, it is limited by transportation technology, human characteristics, and the need to provide safety. Comfort and convenience are generic terms and mean different things to different people. Comfort involves the physical characteristics of vehicles and roadways, and is influenced by our perception of safety. Convenience relates more to the ease with which trips are made and the ability of transport systems to accommodate all of our travel needs at appropriate times. Economy is also relative. There is little in modern transportation systems that can be termed “cheap.” Highway and other transportation systems involve massive construction, maintenance, and operating expenditures, most of which are provided through general and user taxes and fees. Nevertheless, every engineer, regardless of discipline, is called upon to provide the best possible systems for the money. Harmony with the environment is a complex issue that has become more important over time. All transportation systems have some negative impacts on the environment. All produce air and noise pollution in some forms, and all utilize valuable land resources. In many modern cities, transportation systems utilize as much as 25% of the total land area. “Harmony” is achieved when transportation systems are designed to minimize negative environmental impacts, and where system architecture provides for aesthetically pleasing facilities that “fit in” with their surroundings. The traffic engineer is tasked with all of these goals and objectives and with making the appropriate tradeoffs to optimize both the transportation systems and the use of public funds to build, maintain, and operate them. 1 .I .3 Responsibility, Ethics, and Liability in Traffic Engineering The traffic engineer has a very special relationshp with the public at large. Perhaps more than any other type of engineer, the traffic engineer deals with the daily safety of a large segment of the public. Although it can be argued that any engineer who designs a product has ths responsibility, few engineers have so many people using their product so routinely and frequently and depending upon it so totally. Therefore, the traffic engineer also has a special obligation to employ the available knowledge and state of the art within existing resources to enhance public safety

1.2 TRANSPORTATION SYSTEMS AND THEIR FUNCTION 3 neer also fimctions in a world in project.In the case of the Westway Project proposed ir her ofke participants do not u nderstand the the 1970s for the west side of Manhattan.one of the traffic and transportation issues or how they truly affect a bases for legal challenge was that the impact of project Da decision-ma been proper nsibility ffort Bec all ofs protect the community from liabilit b practice face regularlywith the transportation system.man There are many areas in which agencies charged with estimate their understanding of transportation and traffic can be held li must deal productively with t are not limited to) ve assumptions,plans oversimplificd and understated impacts. nent Like all engineers.traffic engineers must under sare tho theire ntain devices in the f this i a "dark"traffic signal in which no indication is o-date given due to bulb or other device failure versions of each are available on-line.In general,gooc Failure to apply the most current standards and professional ethics requires that traffic engineers worl guidelines in making decisions on traffic control in the ugh arce do all wo developing a facility plan or design,or conduct nd ce ing an investigation ble codes and standards and work to the best of thei Implementing traffic regulations (and placing ap ability.In traffic engineering.the pressure to understate Wrialceviecs)wmihouthepropcrlegalauthor ity to sometimes brought to bea A historic standard has heen that"due are"he ex n a projec ercised in the of nlans and that determina As in all enginee the re to mini tions made in the process he reasonable and "no mize costs must give way to basic needs for safety and arbitrary."It is gene ally recognized that professional reliability must mak alue judgments, d th erm "due care Exp has tha the greatest risk to ntal cor oked or analysi is to provide for the public safety through positive pro oversimplified.Sophisticated developers and experienced erams,good practice,knowledge,and proper procedure a fa m h impacts 1.2 Transportation Systems and complete disclost re of impacts so that policy makers and their Function can make inlorme suchdis agreements are not a valid basis for a legal challenge to a shape of the the e of the econ m

1.2 TRAINSPORTATION SYSTEMS AND THEIR FUNCTION 3 The traffic engineer also functions in a world in which a number of key participants do not understand the traffic and transportation issues or how they truly affect a particular project, These include elected and appointed officials with decision-making power, the general public, and other professionals with whom traffic engineers work on an overall project team effort. Because all of us interface regularly with the transportation system, many overestimate their understanding of transportation and traffic issues. The traffic engineer must deal productively with problems associated with nafve assumptions, plans and designs that are oblivious to transportation and traffic needs, oveirsimplified analyses, and understated impacts. Like all engineers, traffic engineers must understand and comply with professional ethics codes. Primary codes of ethics for traffic engineers are those of the National Society of Professional Engineers and the American Society of Civil Engineers. The most up-to-date versions of each are available on-line. In general, good professional ethics requires that traffic engineers work only in their areas of expertise; do all work completely and thoroughly; be completely honest with the general public, employers, and clients; comply with all applicable codes and standards; and work to the best of their ability. In traffic engineering, the pressure to understate negative impacts of projects, sometimes brought to bear by clients who wish a project to proceed and employers who wish to keep clients happy, is a particular concern. As in all engineering professions, the pressure to minimize costs must give way to basic needs for safety and reliability. Experience has shown that the greatest risk to a project is an incomplete analysis. Major projects have been upset because an impact was overlooked or analysis oversimplijhed. Sophisticated developers and experienced professionals know that the environmental impact process calls for a fair and complete statement of impacts and a policy decision by the reviewers on accepting the impacts, given an overall good analysis report. The process does not require zero impacts; it does, however, call for clear and complete disclosure of impacts so that policy makers can make informed decisions. Successful challenges to major projects are almost always based on flawed analysis, not on disagreements with policy makers. Indeed, such disagreements are not a valid basis for a legal challenge to a project. In the case of the Westway Project proposed in the 1970s for the west side of Manhattan, one of the bases for legal challenge was that the impact of project construction on striped bass in the Hudson River had not been properly identified or disclosed. The traffic engineer also has a responsibility to protect the community from liability by good practice. There are many areas in which agencies charged with traffic and transportation responsibilities can be held liable. These include (but are not limited to): Placing control devices that do not conform to applicable standards for their physical design and placement. Failure to maintain devices in a manner that ensures their effectiveness; the worst case of this is a “dark” traffic signal in which no indication is given due to bulb or other device failure. Failure to apply the most current standards and guidelines in making decisions on traffic control, developing a facility plan or design, or conducting an investigation. Implementing traffic regulations (and placing appropriate devices) without the proper legal authority to do so. A historic standard has been that “due care” be exercised in the preparation of plans, and that determinations made in the process be reasonable and “not arbitrary.” It is generally recognized that professionals must make value judgments, and the terms “due care” and “not arbitrary” are continually under legal test. The fundamental ethical issue for traffic engineers is to provide for the public safety through positive programs, good practice, knowledge, and proper procedure. The negative (albeit important) side of this is the avoidance of liability problems. 1.2 Transportation Systems and their Function Transportation systems are a major component of the U.S. economy and have an enormous impact on the shape of the society and the efficiency of the economy

4 CHAPTER 1 INTRODUCTION TO TRAFFIC ENGINEERING 221.5 Million Registered Vehicles 191.0 Million Licensed Drivers 2.46 Trillion Vehicle-Miles Traveled 3.92 Million Miles of Paved Highway $61.6 Billion in State Road User Taxes Collected $30.3 Billion in Federal User Taxes in the Highway Trust Fund 41,471 Fatalities in 6.3 Million Police-Reported Accidents 98% of all Person-Trips Made by Highway Figure 1.1: Fundamental Highway Traffic Statistics (2000) in general. Figure 1.1 illustrates some key statistics for the U.S. highway system for the base year 2000. America moves on its highways. While public transportation systems are of major importance in large urban areas such as New York, Boston, Chicago, and San Francisco, it is clear that the vast majority of person-travel as well as a large proportion of freight traffic is entirely dependent on the highway system. The system is a major economic force in its own right: Over $90 billion per year is collected by state and federal governments directly from road users in the form of focused user taxes and fees. Such taxes and fees include excise taxes on gasoline and other fuels, registration fees, commercial vehicles fees, and others. The total, however, does not include state, local, and federal general levies that also affect road users. The general state and local sales taxes on vehicle purchases, fuels, parts and labor, etc. are not included in this total. Further, well over $100 billion per year is expended by all units of government to plan, build, maintain, and operate highways . Moreover, the American love affair with the automobile has grown consistently since the 1920s, when Henry Ford’s Model T made the car accessible to the average wage earner. This growth has survived wars, gasoline embargoes, depressions, recessions, and almost everything else that has happened in society. As seen in Figure 1.2, annual vehicle-miles traveled reached the 1 trillion mark in 1968 and the 2 trillion mark in 1987. If the trend continues, the 3 trillion mark is not too far in our future. This growth pattern is one of the fundamental problems to be faced by traffic engineers. Given the relative maturity of our highway systems and the difficulty 2.5 1 1940 1950 1960 1970 1980 1990 Year Figure 1.2: Annual Vehicle-Miles Traveled in the United States (1 940-2000) faced in trying to add system capacity, particularly in urban areas, the continued growth in vehicle-miles traveled leads directly to increased congestion on our highways. The inability to simply build additional capacity to meet the growing demand creates the need to address alternative modes, fundamental alterations in demand patterns, and management of the system to produce optimal results. 1.2.1 The Nature of Transportation Demand Transportation demand is directly related to land-use patterns and to available transportation systems and facilities. Figure 1.3 illustrates the fundamental relationship, which is circular and ongoing. Transportation demand is generated by the types, amounts, and intensity of land use, as well as its location. The daily journey to work, for example, is dictated by the locations of the worker’s residence and employer and the times that the worker is on duty. Transportation planners and traffic engineers attempt to provide capacity for observed or predicted travel demand by building transportation systems. The improvement of transportation systems, however, makes the adjacent and nearby lands more accessible and, therefore, more attractive for development. Thus, building new

1.2 TRAT\TSPORTATION SYSTEMS AND THEIR FUNCTION 5 Figure 1.3: The Nature of Transportation Demand transportation facilities leads to further increases in landuse development, which (in turn) results in even higher transportation demands. This circular, self-reinforcing characteristic of traffic demand creates a central dilemma: building additional transportation capacity invariably leads to incrementally increased travel demands. In many major cities, this has led to the search for more efficiient transportation systems, such as public transit and car-pooling programs. In some of the largest cities, providing additional system capacity on highways is no longer an objective, as such systems are already substantially choking in congestion. In these places, the emphasis shifts to improvements within existing highway rights-of-way and to the elimination of bottleneck locations (without adding to overall capacity). Other approaches include staggered work hours and work days ito reduce peak hour demands, and even more radical approaches involve development of satellite centers outside of the central business district (CBD) to spatially dj sperse highly directional demands into and out of city centers. On the other hand, demand is not constrained by capacity in all cities, and the normal process of attempting to accommodate demand as it increases is feasible in these areas. At the same time, the circular nature of the travevdemand relationship will lead to congestion if care is not taken to manage both capacity and demand to keep them within tolerable limits. It is important that the traffic engineer understand this process. It is complex and cannot be stopped at any moment in time. Demand-prediction techniques (not covered in this text) must start and stop at arbitrary points in time. The real process is ongoing, and as new or improved facilities are provided, travel demand is constantly changing. Plans and proposals must recognize both this reality and the professional’s inability to precisely predict its impacts. A 10-year trafic demand forecast that comes within approximately 520% of the actual value is considered a signifcant success. The essential truth, however, is that traffic engineers cannot simply build their way out of congestion. If anything, we still tend to underestimate the impact of transportation facilities on land-use development. Often, the increase in demand is hastened by development occurring simply as a result of the planning of a new facility. One of the classic cases occurred on Long Island, New York. As the Long Island Expressway was built, the development of suburban residential communities lurched forward in anticipation. While the expressway’s link to Exit 7 was being constructed, new homes were being built at the anticipated Exit 10, even though the facility would not be open to that point for several years. The result was that as the expressway was completed section by section, the 20-year anticipated demand was being achieved within a few years, or even months. This process has been repeated in many cases throughout the nation. 1.2.2 Concepts of Mobility and Accessibility Transportation systems provide the nation’s population with both mobility and accessibility. The two concepts are strongly interrelated but have distinctly different elements. Mobility refers to the ability to travel to many different destinations, while accessibility refers to the ability to gain entry to a particular site or area. Mobility gives travelers a wide range of choices as to where to go to satisfy particular needs. Mobility allows shoppers to choose from among many competing shopping centers and stores. Similarly, mobility