Being on private property does not imply you can break the law, however, many road vehicle/traffic regulations explicitly apply only to roads open to public tra

Being on private property does not imply you can break the law, however, many road vehicle/traffic regulations explicitly apply only to roads open to public traffic (which may include some private property e.g. parking lots). If the law says "it's forbidden to do X on public roads", you're not breaking the law if you do that thing on a private track. Details vary between locations, but seatbelts, roadworthiness, insurance, speed limits, etc generally are things that you are allowed to ignore outside of public traffic. In your example of construction workers, other regulations (e.g. OSHA) would cover your concerns; but, for example, a truck driving up the ramp in a strip-mine would generally not be bound by traffic laws; construction sometimes uses various vehicles that simply can't ever legally participate in road traffic, they just get delivered as "cargo" or assembled/disassembled on-site.