The Event Safety Alliance (ESA) recommends that all personnel working in the vicinity of moving motorized vehicles–including fork lifts and elevated platforms–meet all local, state/provincial, and federal requirements in a work zone. If there is doubt regarding whether wearing HVA is required in a particular location, and especially if work operations will be conducted in low light conditions, the ESA recommends that that the nearest appropriate standards be used, which can be determined through the use of a risk assessment (http://www.ccohs.ca/oshanswers/hsprograms/risk_assessment.html). If no relevant standards exist, the American National Standard for High-Visibility Safety Apparel and Headwear (ANSI 107-2010) standard for Performance Class 1 risk exposures may be a good option, but should be based on a risk assessment.
 
Some helpful documents on this topic for the US and Canada include: