Grass grows not upon the highway.

Constant traffic prevents grass from taking root on a road. The image therefore contrasts a place exposed to continual passing and disturbance with one left still long enough for growth to appear.

Figuratively, the saying may suggest that persistent movement, activity, or interference leaves little room for quiet accumulation or settled development. It can also imply that an absence of visible growth marks a route as one much used, rather than neglected.

Use it as an observation about busy paths or as a figurative comment on circumstances where continual activity prevents things from settling, growing, or becoming established. Whether it is meant literally or figuratively depends on the surrounding discussion; its inverted wording sounds deliberately proverbial and somewhat old-fashioned.