Ratsnakes occur almost anywhere in their range, but to find the highest density, and therefore the most likely spot to encounter one, you have to search wher the food is.
Ratsnakes do eat lots of birds and their eggs, but it's hard to hang out in a tree waiting for a snake to slither by, SO, think "Where do the mice and rats hang out?"
Barns are good. Deserted barns, houses and silos are better. Fields are good, too, but a trafficked field with livestock usually is too busy for ratsnakes, especially during the day.
Mice like edges of things. Edges of forests where they meet fields. Edges of lawns where tall grass meets short grass. Edges of streams. Here, there be serpents, tracking, dining, digesting, ...
All this said, your best bet is none of these spots, but on or crossing a road at night in the summer. Drive back roads that are little traveled with flashlight and hi-beams ready, and look for snakes soaking up the last of the day's warmth.
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...and I think to myself, "What a wonderful world."