There is no statute law that sets this. It is up to the administration of the airfield to set these rules.
It depends on local legislation. Many countries have no restrictions others impose arbitrary rules.
too many crashes . When I was young, there was only the 30mph limit in towns, and a few 40mph limits on urban through ways. Apart from that, no limit on the open road. Now we have 70mph on motorways...
45 MPH
When another speed limit sign is reached that changes the limit from the previously posted one.