1.5
We live in a complex and rapidly developing world.consequently,the problems that traffic engineers are involved in evolve rapidly.
Urban congestion has been a major iue for many years .given the transportation demand cycle,it is not always poible to solve congestion problems through expansion of capacity .traffic engineers therefore are involve in the development of programs and strategies to manage demand in both time and space and to discourage growth where neceary,a real question is not \"how much capacity is needed to handle demand? \"but rather\"how many vehicles and/or people can be allowed to enter congestedareas within designated time periods?\"
Growth management is a major current iue.A number of states have legislation that ties development permits to level-of-service impacts on the highway and transportiaton system.where development will cause substantial deterioration in the quality of traffic service ,either such development will be disallowed or the developer will be responsible for general highway and traffic improvement that mitigate these negative impacts.such policies are more easily dealt with in good economic times.When the economy is sluggish,the iue will often be a clash between the desire to reduce congestion and the desire to encourage development as a means of increasing the tax base.
Reconstruction of existing highway facilities also causes unique
problems.the entire interstate system has been aging,and many of its facilities have required major reconstruction efforts.part of the problem isthat reconstruction of interstate facilities receives the 90%federal subsidy,while routine maintenance on the same facility is primarily the responsibility of state and local governments.deferring routine maintenance on these facilities in favor of major reconstruction efforts has resulted from federal funding policies over the years.major reconstruction efforts have a substantial major burden not involved in the initial construction of these facilities maintaining traffic.it is easier to bulid a new facility in a dedicated right-of-way than to rebuild it while continuing to serve 100000 or more vehicles per day.thus,is iues of long-term and short-term construction detours as well as the diversion of traffic to alternate routes require major planning by traffic engineers.
Recently,the iue of security of transportation facilities has come to the fore .the creation of facilities and procees for random and systematic inspection of trucks and other vehicles at critical locations is a major challenges ,as is securing major public transportation systems such as railroad ,airport and rapid transit systems.
The list goes on and on .the point is that traffic engineers cannot expect to practice their profeion only in traditional ways on traditional ways on traditional projects.like any profeional,the traffic engineer must be ready to face current problems and to play an important role in any
situation that involves transportation and/or traffic systems.
1.6
In order to remain up to date and aware the traffic engineer must keep up with modern developments through membership and participation in profeional organizations ,regular review or key periodicals,and an awarene of the latest standards and criteria for profeional practice .Key profeional organizations for the traffic engineer include the institute of transportation engineer(ITE),the transportation research board(TRB),the transportation group of the American society of Civi engineers(ASCE),ITS America,and others.all of these provide literature and maintain journals,and have local,regional,and national meetings.TRB is a branch of the national Academy of engineering and is a major source o research papers and reports.
Like many engineering fields the traffic engineering profeion has many manuals and standard references ,most of which will be referred to in the chapter of this text.major references include
Traffic engineering handbook[1]
Uniform vehicle code and model traffic ordinance[2]
Manual on uniform traffic control devices [3]
Highway capacity manual[4]
A policy on geometric design of highways an streets(the AASHTO Green Book)[5]
A few of these have had major update and revisions since 2000,including references ,1,3,4 and 5.most standards such as these are updated frequently,usually on a 5-or10-years cycle,and the traffic engineer must be aware of how changes in standards,criteria,methodology,and other aspect will affect the practice of the profeion.
Other manuals abound and often relate to specific aspects if traffic engineering.these references document the current state of the art in traffic engineering,and those most frequently used should be part of the profeional\'s personal library.
There are also a wide variety of internet sites that are of great value to the traffic engineer.specific sites are not listed here ,as they change rapidly.all of the profeional organizations,as well as equipment manufacturers,maintain web sites .the federal DOT,FHWA,NHTTSA,and private highway-related organizations maintain web sites .the entire manual on uniform traffic control devices is available on -line through the FHWA Web site.
Because traffic engineering is a rapidly changing field,the reader cannot aume that every standard and analysis proce included in this text is current,particularly as the time since publication increases .while the authors will continue to produce periodic updates,the traffic engineer must keep abreast of latest developments as a profeional responsibility .
1.7
In the preface to the second edition of this text,it was indicated that the third edition would be in metric units.at the time ,legislation was in place to require the conversion of all highway agencies to metric units over a short time period.since then ,the government has once again backed off this stance.thus,at the current time,there are states continuing to use US units,states continuing to use metric units(they had already converted),and an increasing number of states moving back to US units after conversion to the metric system.some of the key references,such as the highway capacity manual,have been produced in both metric and US unit versions .others,like the AASHTO Green Book,contain both metric and US standards.
Metric and US standards are not the same.A standard 12-ft lane converts to a standard 3.6-m lane,which is narrower than 12 feet.standards for a 70-mi/h design speed convert to standards for a 120-km/h design speed,which are not numerically equivalent .this is because even units are used in both systems rather than the awkward fractional values that result from numerically equivalent conversions.that is why a metric set of wrenches for use on a foreign car is different from a standard US wrench set .
Because mere states are on the US system than on the metric system(with more moving back to US units)and because the size of the text would be
unwieldy if dual units were included ,this text continues to be written using standard US units.
1.8
The profeion of traffic engineering is a broad and complex one.neverthele,it relies on key concepts andanalyses and basic principles that do not change greatly over time.this text emphasizes both the basic principles and current(in 2003)standards and practices.the reader must keep abreast of changes that influence the latter.
