This rulemaking would consider upgrades to FMVSS No. 223, Rear impact guards, and FMVSS No. 224, Rear impact protection, for improving rear underride crash protection on trailers and semitrailers. This rulemaking would respond to a petition for rulemaking from the Insurance Institute for Highway Safety (IIHS) that requested a number of modifications to FMVSS Nos. 223 and 224 to improve rear underride protection for trailers and semitrailers. It would also respond, in part, to a petition for rulemaking from Mrs. Karth and the Truck Safety Coalition requesting improving rear impact guards on trailers and semitrailers as outlined in the July 10, 2014, grant of the petition. This rulemaking would also consider harmonizing requirements for rear impact guards and rear impact protection with those specified in the Canadian Standard, CMVSS No. 223, Rear impact guards. The costs associated with this rulemaking are not expected to be significant. This rulemaking action is in accordance with the U.S. DOT's plan, Retrospective Review and Analysis of Existing Rules, for the implementation of Section 6 of the Executive Order 13563 (76 FR 3821, January 21, 2011) that urges agencies to conduct periodic retrospective evaluation of regulations and modify, streamline, expand, or repeal them in accordance with what has been learned from the evaluation.