As the crow flies means measured in a straight line, ignoring the roads. Two miles as the crow flies.
The road distance can be several times greater in hills or along a coast. A distance given as the crow flies is the shortest anybody could possibly travel and rarely the one they do.
1phraseas the crow fliesMeasured in a straight line, ignoring the roads.
“It is four miles as the crow flies and eleven by road.”
as the crow flies is a phrase.
From the idea that a crow flies directly to its food rather than following roads.
From the fact that while crows fly to their source of food, their flight bypasses obstacles like rivers and terrain. Compare beeline.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms