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According to this interpretation letter issued by the Occupational Safety and Health Administration (OSHA) in 2009,

https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=27155

all highway workers and other construction workers that come in contact with either road or construction traffic must wear high-visibility equipment, which includes safety vests. The decision was reinforced by figures released by the Bureau of Labor Statistics which showed that from 2003 to 2007, 639 road-construction work-zone fatalities occurred. Safety vests have become an important, legally required part of the personal protection equipment of a construction worker.

Read more at: http://www.ehow.com/info_8330078_importance-safety-vests-construction.html