In flight, the answer really shouldn't ever be "less than 500 feet". During landing, the answer almost certainly will be "less than 500 feet"; a plane's queued up to enter the runway after you land, a ground vehicle is working on something near but not on the end of the runway, etc.
It's a surprisingly tough challenge to solve a) reliably and b) in a way that doesn't cause a whole bunch of false go-arounds wreaking havoc on the busy airport.
It's a much easier challenge when every moving vehicle in the airport environment is issued a relatively exact clearance.
The ATC clearance gives each vehicle a movement contract.
It would be great if avionics actually took those into account. Avionics should help pilots ensure their vehicles don't break the contract and should also alert immediately if other vehicles have, or their velocity vector is such that they will violate the contract. IE braking rate is insufficient to stop before the hold short line. ATC computers should ensure no conflicting clearances are issued.
In flight, the answer really shouldn't ever be "less than 500 feet". During landing, the answer almost certainly will be "less than 500 feet"; a plane's queued up to enter the runway after you land, a ground vehicle is working on something near but not on the end of the runway, etc.
It's a surprisingly tough challenge to solve a) reliably and b) in a way that doesn't cause a whole bunch of false go-arounds wreaking havoc on the busy airport.