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It depends if there is an offence that is defined as occurring anywhere, or on a public road.
Purposefully killing someone with your car would be murder wherever.
Drink driving on private land is not an offence if it is not accessible to the public / not a public place. Similarly dangerous driving, IIRC. The legislation would be specific about it, and there are probably cases that have occurred that define it further.
Killing someone while drunk in your car on private land? I don't know - manslaughter?
I expect that any signage could be used as evidence.
^^^Thinking aloud...
Would consequences of any incidents/collisions be a civil matter also? (especially if they involved following/not following the traffic instructions on those signs)