The U.S. Federal Highway Administration approved the use of the Clearview font for highway signage back in 2004, because testing showed that it contributed to increased readability. The approval has now been rescinded, so future signage will be in good old Highway Gothic. According to the FHWA, the legibility claims for Clearview have been disproven, though the agency has yet to reveal any scientific basis for their change.
More background at the New York Times: The Road to Clarity
Thanks to one of our Typographic Irregulars, MrMild.