Sorry, IMHO it's not about aerodynamics, it's simply about the pilot's responsibility of a safe flight path.
What? And nothing to do with vehicles on an airfield giving way to aircraft, regardless of flightpath?
The poor sod was on a first solo fer chrissakes, that why we have rules like "Vehicles give way to aircraft. Always. No exceptions."
The idiot in the chelsea tractor (is there any other kind of occupant of those things?) was 100% to blame, crossing a runway like that without looking or stopping. "I didn't see him". How many car drivers have said those criminal and self-condemnatory words over a dead motorcyclist, I wonder? (Euphemism for "I didn't look" of course). That stude was real lucky.