Choosing between driving and flying for a domestic trip is often treated as settled in favor of the car. The honest comparison depends on a few variables that change the answer.
Both modes are measured per person
A car's emissions are produced by the vehicle regardless of how many people are in it, so a solo driver carries the entire load and four passengers each carry a quarter.
Aviation works the same way, with the aircraft's fuel burn divided among the passengers actually aboard on that flight.
Comparing a full car to a full plane is a different question from comparing a solo driver to a full plane, and the two produce different conclusions.
Short flights carry a heavy fixed penalty
Takeoff and the climb to cruising altitude consume a disproportionate share of a flight's fuel, and that cost is the same on a short hop as on a long haul.
On a brief route there is little efficient cruising to spread that penalty across, so the per-mile figure is much worse than for a long flight.
This is where a well-occupied car most often comes out ahead, particularly on trips of a few hundred miles.
Long distances favor the aircraft's efficiency
Across a continent, an aircraft at cruise is moving a large number of people at high speed with a per-mile fuel burn that is genuinely competitive.
The car meanwhile accumulates every mile at its own rate, and adds overnight stops that carry their own energy use in lodging.
Which means the mode advantage flips somewhere in the middle distances rather than holding across all trips.
The vehicle itself is a large variable
Fuel economy differs enormously between a small efficient car and a large truck, and a single comparison figure for driving hides that spread.
An electric vehicle shifts the emissions to the grid serving the charging points along the route, so the same trip differs by region.
Terrain, speed and load also matter, since sustained highway speed and a roof box both raise consumption noticeably.
What actually decides most trips
Trip length and party size are the two factors that dominate, and both are known before booking, which makes the comparison unusually easy to make honestly.
Where a route is served by rail, that option frequently beats both, and where it is not, the choice returns to occupancy.
The least defensible option in nearly every case is a lightly loaded car on a very long drive, which combines the worst of both patterns.