warning track

Short definition from Wiktionary

Noun

1. 1
   The dirt or other material on the edge of a baseball or softball field that warns a player that he or she is approaching the fence, especially the portion in the outfield.
2. 2
   A non-racing surface at the margin of a racing course, representing the edge of the race track.
3. 3
   A non-driving surface at the edge of a road, with a different surfacing warning drivers of the approach to the edge of the road, as part of the shoulder of a road.