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4 CHAPTER 1 Introduction The Problem Economic analysis of safety treatments, Treatment selection for short-term programming of safety In order to achieve the greatest return on the investment of improvements, their often limited safety budgets, state and local highway safety Project development to address safety aspects of large engineers must continually make program planning decisions projects, concerning whether to implement a specific safety treatment Design policy development, and/or to determine the costs and benefits of alternative treat- The design exception process, and ments. Making these program planning decisions requires an Public awareness campaigns. accurate measure of safety treatment effectiveness. This meas- ure of effectiveness is referred to as a crash reduction factor In addition to their long-term use by state and local safety (CRF) or accident modification factor (AMF). Both of these engineers, AMFs are key components of the current generation terms reflect the percentage reduction (or increase) in crashes of safety tools and resources being developed for the safety that can be expected after implementing a treatment or field. They are used in FHWA's Interactive Highway Safety program, as derived through research studies and program Design Model (IHSDM) to predict future safety for different evaluations. The level of effectiveness of a treatment is referred alternative roadway designs or rehabilitation designs (1). to in much of the current safety literature as a CRF or an AMF. AMFs are being incorporated into FHWA's SafetyAnalyst, The two terms are simply different ways of expressing treat- where they are applied to estimate the safety effectiveness ment effectiveness levels. An AMF provides the expected measures within the economic appraisal tool (2). AMFs will be proportional reduction in crash frequency and is developed a key component of the Highway Safety Manual (HSM), which by dividing the CRF by 100 and subtracting the result from is now being produced by a TRB task force and NCHRP (3). 1.00. Thus, a treatment shown to reduce crashes by 15 percent Finally, better AMFs will allow AASHTO and NCHRP to (CRF = 15%) would have an AMF of 0.85 (1.00 - 15/100). An update guides already developed or guides to be developed in AMF of 1.00 represents no effect on safety, while AMFs above the future for assisting states and local users with the imple- 1.00 indicate that the treatment can be expected to result in an mentation of AASHTO's Strategic Highway Safety Plan (4). increase in crashes. The term AMF will be used in this report Even though accurate AMFs are critically important to states for consistency with other NCHRP efforts. and municipalities and to the safety tools previously mentioned, The importance of AMFs to state department of trans- there is no accepted standard set of AMFs. This situation is due portation (DOT) safety engineers was documented by a to the fact that the accuracy and reliability of many published survey conducted as part of this study. The survey was sent to AMFs is questionable and that no AMFs exist for many impor- the identified state safety engineer (usually in the traffic engi- tant safety treatments. The lack of AMF reliability, accuracy, neering office) and to a state staff person involved with the and comprehensiveness has been documented in this study, in intelligent transportation systems (ITS) program. All states prior work, and in ongoing HSM development efforts. The were surveyed, and 34 provided responses. Of the 34 states sources of the problem include the following: responding, all but two indicated that they use AMFs, and most states use them for multiple purposes. AMFs are used Origins/Transferability. The origins of AMFs are not always for the following, listed from most frequent use to least clear to the end user. Some states have developed AMFs using frequent use: their own crash data. Other states have simply adopted AMFs