budcoh:

Several rules from OSHA 1910.178, Powered Industrial Trucks, could apply. Here are a few with the primary rules being 1910.178(a)(4) and 1910.178(q)(6):

"1910.178(a)(4) Modifications and additions which affect capacity and safe operation shall not be performed by the customer or user without manufacturers prior written approval. Capacity, operation, and maintenance instruction plates, tags, or decals shall be changed accordingly.

1910.178(m)(5)(i) When a powered industrial truck is left unattended, load engaging means shall be fully lowered, controls shall be neutralized, power shall be shut off, and brakes set. Wheels shall be blocked if the truck is parked on an incline.

1910.178(m)(5)(ii) A powered industrial truck is unattended when the operator is 25 ft. or more away from the vehicle which remains in his view, or whenever the operator leaves the vehicle and it is not in his view.

1910.178(m)(5)(iii) When the operator of an industrial truck is dismounted and within 25 ft. of the truck still in his view, the load engaging means shall be fully lowered, controls neutralized, and the brakes set to prevent movement.

1910.178(q)(6) Industrial trucks shall not be altered so that the relative positions of the various parts are different from what they were when originally received from the manufacturer, nor shall they be altered either by the addition of extra parts not provided by the manufacturer or by the elimination of any parts, except as provided in paragraph (q)(12) of this section. Additional counterweighting of fork trucks shall not be done unless approved by the truck manufacturer."

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Truck mounted forklift trucks are considered rough terrain forklift trucks. There are numerous sections of the National Consensus Safety Standard for Rough Terrain Forklifts that could apply. Here are a few from ITSDF B56.6-2005/(Reaffirmation of ASME B56.6-2002):

"6.2 General

6.2.1 Before starting to operate the rough terrain forklift truck: (a) be in operating position and fasten seat belt, if so equipped; (b) place directional controls in neutral; (c) disengage clutch on manual transmission equipped rough terrain forklift trucks, or apply brake on power shift or automatic transmission-equipped trucks; (d) start engine.

6.2.2 Do not start or operate the rough terrain forklift truck, any of its functions or attachments, from any place other than from the designated operator's position."