They should operate in a manner that poses no danger to either the vehicle or the aircraft. You can't say "don't drive within 100 feet of a plane ever" because there are lots of operationally justifiable reasons to drive right up to an aircraft - maintenance and fuel trucks, baggage trucks and airstairs all have to pull close enough to almost touch the plane.
vehicles should never be driven within _____ feet of an aircraft
10
10 ft
10 feet
within 25 feet
10
maximum 10 mph
5 mph
10 ft
25, 200
5
5 mph