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7 CHAPTER TWO LITERATURE REVIEW The literature review involved a search of major sources includ- Recent and more detailed approaches recognize that, in ing transit agency websites, TRB proceedings and publications, practice, the need to replace an asset is related not only to age, and APTA publications. The literature search was conducted but to other factors as well, such as intensity of use (e.g., using web and Transportation Research Information Services miles), level of preventive maintenance, and climate. There- (TRIS) search engines. fore, two identical assets may be scheduled for replacement at different ages based on the intensities of their use and their The literature search focused on identifying technical respective levels of maintenance. methods used for effective transit asset management. As mentioned in chapter one, these methods can be used to The approach used by the FTA in its Transit Economic produce an estimate of the funding required to address asset Requirement Model (TERM) is an excellent example of this replacement and rehabilitation needs. The methods can approach (Laver 2009). It simulates the full life and decay of also help set and prioritize the programming of replace- all transit assets based on factors such as asset use (e.g., ment and rehabilitation projects when available funding is miles), annual maintenance, and age. Empirically derived constrained. decay curves are used to determine when assets should be replaced. These curves are based on detailed asset condition The literature review found a large number of publications inventories that used a five-point scale (1 = poor condition, that discussed the importance of maintaining transit assets in 5 = excellent condition). An asset is replaced when its condi- SGR. Generally, the major focus of these articles was that the tion falls below 2.5. condition of transit assets has been declining through under- Another advance is suggested by the approach developed in investment and that increased funding is needed to reverse this Illinois for determining when buses should be replaced (Booz decline. These publications, however, did not discuss or Allen Hamilton 2003). A minimum cost replacement strategy describe the technical methods that might be used for more was used to minimize total life-cycle costs. These costs were effective transit asset management. allocated over the life of a vehicle on a per mile basis. The pur- chase and rehabilitation costs per mile decline over the life of The literature on technical methods used for transit asset the vehicle. In contrast, operating and maintenance costs per management is limited. Eight articles, papers, or reports were mile tend to increase as a vehicle ages. When these divergent identified and judged important to this synthesis project. The unit cost trends are combined to produce a total life-cycle cost publications are cited in the References and summarized in the curve, a minimum unit cost and its corresponding lifetime Annotated Bibliography found in Appendix D. mileage can be determined and used as the replacement point. Although the literature is limited, the review of the relevant sources suggests how the technical methods may advance ASSET COSTS CONSIDERED in the coming years and become more comprehensive and sophisticated. The key advances may come in several areas A common public view is that good transit asset management including the definition used for SGR, the asset costs consid- will be achieved when there is sufficient funding to replace or ered in the analyses, and the use of scenario testing. rehabilitate assets at the end of their useful lives. Therefore, the task is to determine the needed replacement and renewal funding. MEASURE OF THE STATE OF GOOD REPAIR This approach ignores the issue that mid-life renewals often The conventional approach for defining assets being in are needed to ensure that assets reach their useful life. Exam- SGR is that the assets are replaced before the end of their ples of these mid-life renewals include the replacement of useful life. Examples of these definitions are 12 years for engines and transmissions for buses, heating and roofs for sta- buses, 25 years for rail cars, and 50 years for stations. tions, and traction motors and assemblies for rail cars. Often, these definitions are based on federal grant require- ments that only permit federal funding to be used for asset More comprehensive approaches recognize the need to replacements when the assets have reached minimum ages. include these mid-life renewals (Yoder and Delaurentis 2003;