Given the choice, how would someone living there most often visit a grocery store?

In an urban area walking might be the common mode.

In a suburban area driving is OFTEN going to be the preferred mode, and even in small cities (particularly ones still small enough to have free parking at a nearby grocery store) driving there is still going to be preferred; at least in the US. In my experience Bellevue WA is a small, still suburban like city, while Seattle has crossed the anti-car (and expensive real-estate) line to 'big city'; but is neither old enough (pre-industrial grocery store layouts) nor big enough to be a healthy big urban city.

Offhand, I would define rural as any area where a drive to the third closest typical grocery store is at least a one hour round trip (by car, at legal speeds, including delays at any traffic device or from commute volume).