"The intent of this synthesis is to collect information on the types of best management practices (BMPs) currently being used by state departments of transportation (DOTs) for meeting total maximum daily load (TMDL) water quality goals for stormwater runoff. The study approach includes two major components: interviews with 12 state DOTs to identify the existing state of the practice as it relates to TMDL implementation, and a review of selected literature sources based on the criteria of highways, TMDLs, BMP performance, and BMP cost to stay consistent with the goals of this synthesis. In particular, detailed quantitative BMP performance and cost data, including life-cycle costs, are presented, which builds significantly on previous studies of this nature. The impetus for this study was to help fill in a significant information gap on what types of BMPs are cost-effective for specific use in linear highway applications for TMDL implementation purposes. Even with the advent of new low-impact development/green infrastructure practices, there remain a lack of effective BMP technologies and nonstructural controls (e.g., source control and water quality credit trading) for DOTs to implement for National Pollutant Discharge Elimination System permit compliance. This problem will only grow larger as new TMDLs are continually being developed, and many DOTs are unprepared both technically and economically to cope with the additional requirements (some states already have 60+ TMDLs in which they are a named stakeholder). In an effort to help state DOTs with TMDL implementation, a simple user-friendly BMP matrix/toolbox with quantitative performance and, where available, life-cycle cost data for various structural and nonstructural BMPs is presented. Some of the more common TMDL pollutants of concern (sediment, nutrients, fecal coliform, and metals) are focused to maximize applicability for state DOTs. The performance and cost data were derived from numerous literature sources including the International Stormwater BMP Database, which currently consists of more than 400 studies. This study is designed to help promote information exchange and technology transfer among DOTs for the mutual benefit of all highway managers faced with TMDL implementation. Conclusions from this synthesis are briefly highlighted here by general topic area, with more details provided in chapters four and five. Performance for structural BMPs varied by pollutant and BMP type; however, certain trends did emerge from the literature review. In general, total suspended solids (TSS) appear to be relatively easy to treat with a broad range of BMPs, including infiltration basins, sand filters, and bioretention. Nutrients (especially total nitrogen) can be more challenging to remove; nonetheless, some BMPs (e.g., Austin sand filters for total nitrogen and infiltration basins for total phosphorus) showed some promise. Fecal coliform data were limited; however, several BMPs were documented as being effective, including infiltration basins, and infiltration trenches, among others. Additional BMP performance data from the International Stormwater BMP Database support the view that media filters and retention ponds are consistently effective for a wide variety of TMDL pollutants, including TSS, nutrients, fecal coliform, and total metals. This conclusion is based on statistics that show that median concentrations of these pollutants were statistically lower in effluent concentrations compared with influent concentrations based on a large number of studies from around the country (although not all highway related). Overall, while these BMPs may be generally effective across a range of environmental conditions, obtaining local site-specific BMP monitoring data would be preferable for developing individual state DOT TMDL programs. Performance data are also presented for nonstructural practices such as street sweeping, catch basin cleaning, and tree planting. Quantitative performance data are generally lacking in the literature for these types of BMPs. The limited information found suggests that street sweeping and catch basin cleaning may potentially be effective strategies for reducing TSS, nutrients, and metals provided they are performed frequently enough and the right technology is used (in the case of sweeping). Tree planting and stream restoration were documented as having some water quality benefits for nutrients. Notably, anti-icing management has been successfully demonstrated in New Hampshire, where a 20% reduction in chlorides was achieved by upgrading the technology on snow plows in response to a chloride TMDL. In addition to performance, life-cycle cost data are presented where available. However, the cost information could not be adequately synthesized owing to differences in cost estimating approaches, reporting units, variability in costs among states and regions, and inconsistencies in BMP naming conventions. This also prevented a true cost-benefit analysis. However, numerous sources of life-cycle cost data, as well as sources for individual cost elements such as design, construction, and operation and maintenance, are provided where the interested reader may obtain more detailed information. Given the differences in cost from one region to another, the reader is encouraged to obtain cost data that are most relevant to their state. Hyperlinks are provided in the BMP matrix/toolbox where one may access examples of reports with detailed life-cycle cost data, and numerous additional cost sources are cited throughout the section on Highway Best Management Practices in chapter three. There appear to be several common elements to developing an effective TMDL implementation program, all of which have the potential to benefit DOTs by helping them receive a more equitable waste load allocation and developing a more manageable TMDL program. The key elements are listed here (although not all may apply to every DOT): Increase awareness and training within the DOT on TMDL issues, especially in cases where the DOT is named a stakeholder in only a few TMDLs (or none). Develop off-site watershed partnerships and collaborate with other stakeholders to ensure cost-effective approaches based on economies of scale and to promote information sharing and technology transfer among stakeholders. Collaborate with the state regulatory agency during the TMDL development process, especially early in the process. Estimate pollutant loads generated within the DOT right-of-way (either through water quality monitoring or modeling) and predict potential load reductions from various BMP implementation scenarios. Although some DOTs had relatively successful TMDL programs, others clearly faced a number of challenges. The primary challenges were limited financial resources, a lack of effective BMP technologies for linear highway applications, and difficulties in navigating complex regulatory environments where TMDL-related requirements were either inconsistently enforced or restricted the flexibility of the DOT in implementing BMPs of their choice. Further research is suggested on the following topics: long-term adverse environmental and cultural aspects of BMP implementation; new and innovative BMP technologies suitable for the highway environment; more studies on BMP longevity, life-cycle costs, and maintenance costs and standards; and alternative and creative solutions to addressing emerging TMDLs for less traditional pollutants such as biological integrity, sediment toxicity, and organic compounds (e.g., vehicle source control, water quality trading)"--Pages 1-2.