If the park is in my neck of the woods, I can just use this definition:
"vehicle" means a device in, on or by which a person or thing is or may be transported or drawn on a highway, but does not include a device designed to be moved by human power, a device used exclusively on stationary rails or tracks, mobile equipment, a motor assisted cycle or a regulated motorized personal mobility device
In a legal context "highway" has a broader meaning than in everyday use. From the link above: highway "includes every road, street, lane or right of way designed or intended for or used by the general public for the passage of vehicles"
A park where vehicles are not forbidden also might not have any highways. Vehicles are defined in terms of being capable of transport on highways, but there are vehicles used off highways.
Just because the definition of vehicles refers to highways doesn't mean the definition is not relevant in a situation in which there are no highways.
"vehicle" means a device in, on or by which a person or thing is or may be transported or drawn on a highway, but does not include a device designed to be moved by human power, a device used exclusively on stationary rails or tracks, mobile equipment, a motor assisted cycle or a regulated motorized personal mobility device
[BC Motor Vehicle Act, 1 Definitions]
https://www.bclaws.gov.bc.ca/civix/document/id/complete/stat...
The words written on some sign on a road or park are not meant to be freely defined by whoever happens to be reading the sign.