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Here are some limited policy and regulatory examples:
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U.S. OSHA, Construction, Subpart O, 1926.601(b)(4): No employer shall use any motor vehicle equipment having an obstructed view to the rear unless: (i) The vehicle has a reverse signal alarm audible above the surrounding noise level or: (ii) The vehicle is backed up only when an observer signals that it is safe to do so.
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Jefferson Lab ESH&Q Manual - Rev. 8.8 - 21 MAR 2007
Forklifts must not be operated on public highways unless the forklift is accompanied by an "escort vehicle" directly behind the forklift. The escort vehicle must have its emergency (yellow) flashers operating.
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Ontario Health and Safety Regulation 851 (for Ontario Provincial regulated employer). 56. Where the operator of a vehicle, mobile equipment, crane or similar material handling equipment does not have a full view of the intended path of travel of the vehicle, mobile equipment, crane or similar material handling equipment or its load, the vehicle, mobile equipment, crane or similar material handling equipment shall only be operated as directed by a signaller who is a competent person and who is stationed, (a) in full view of the operator; (b) with a full view of the intended path of travel of the vehicle, mobile equipment, crane or similar material handling equipment and its load; and (c) clear of the intended path of travel of the vehicle, mobile equipment, crane or similar material handling equipment and its load. R.R.O. 1990, Reg. 851, s. 56.
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U.S. Department of Energy, DOE-RL-92-36, Hanford Site Hoisting and Rigging Manual
Spotter - An assigned person whose sole responsibility is to provide a warning or stop signal during vehicle or equipment operation prior to violation of proximity restrictions or pre-determined distance limitations to structures or hazards such as power and communication lines, overhead obstructions, buildings, telephone poles, ground penetrations and etc. (Some operations require the use of a qualified signalman/flagman as a spotter). Signal Person/Flagman - A qualified person who's responsibility is to provide direction for equipment movements to the operator through use of voice signals or standardized hand signals.