As the crow flies and At once

As the crow flies adverb - By the shortest and most direct route.

At once and as the crow flies are semantically related In some cases you can use "At once" instead an adverb phrase "As the crow flies".

At once adverb - Without delay or hesitation; with no time intervening.

As the crow flies and at once are semantically related in immediately topic. Sometimes you can use "As the crow flies" instead an adverb phrase "At once".