DOT and EPA have been jointly developing a number of rules for the past few years addressing (1) fuel economy and GHG emissions in automobiles and light trucks (they are on their second rulemaking on this subject), (2) heavy- and medium-duty trucks (they issued a final rule on this in July of 2011), and (3) labels for automobiles and light trucks providing data on fuel efficiency and emissions (they issued a final rule in the Spring of 2011).
Because the fuel economy of gasoline-powered vehicles is measured by GHG emissions of the vehicle, if the two agencies had acted independently under their respective statutes, they could have caused significant duplication and potentially conflicting requirements. They have also worked with the State of California (which has authority for both emissions and labeling requirements) and the FTC (which has some authority over labeling) to lessen or eliminate any duplication or conflict with their requirements. Although there are other examples of joint rulemakings, these efforts are especially noteworthy for the significant achievements so far and the complex differences in the statutes involved, the substantial costs and benefits, environmental effects, and international implications.
By the actual development of joint or consistent standards despite significant obstacles, savings in research, elimination of duplication, agencies can document their results and scale their efforts for other agencies.