As the crow flies is an idiom. Usually a crow flies in a straightline in short laps.It is also the distance between one post code to another as in UK Postcodes. In other words , it is the shortest rout between points.
That term means if you were able to fly directly from one point to another without using roads. For example the distance from point A to point B would be 10 miles " as the crow flies" but driving it might take 12 miles.
