What a horrible title but par for the course around here.
From the horses mouth:
Traditional laminated automotive safety glass would add significant weight to the Transition® and could fracture in such a way as to obscure the vision of the pilot in the event of a bird impact. This exemption allows the use of polycarbonate materials that provide comparable protection to the occupants at significant weight-saving without shattering or crazing – improving the safety of the Transition®. In the exemption text, NHTSA states: “We further conclude that the granting of an exemption from these requirements would be in the public interest and consistent with the objectives of traffic safety.” In 2010, the Transition® was granted an additional 110 pounds allowance by the Federal Aviation Administration (FAA) in a prior exemption action by the DOT.
Also the full text of the exemption is here: http://www.federalregister.gov/articles/2011/06/29/2011-16222/terrafugia-inc-grant-of-application-for-temporary-exemption-from-certain-requirements-of-fmvss-no