The track should be wide enough to give players a three-step warning (at full speed) that they are approaching a barrier. Their first step onto the track alerts them with a different texture under their feet. Their second and third steps allow players to decelerate before making contact with the barrier. The width of the warning track is determined by the oldest age group using the field.
A minimum of 3″ is enough for most recreational fields if only mowers and turf utility vehicles will be using the track. If vehicles over 2 tons will be using the warning track, it is strongly advised to have a 3″ to 4″ stone road base installed below the 3″ of warning track material — especially for collegiate and professional baseball and softball facilities.
For recreational fields, rely on surface grade and positive surface drainage to move water off the track. A 0.5% to 1% slope will carry the water away from the field. For professional and collegiate fields, stronger tracks should be built that incorporate both subsurface and surface drainage features to quickly remove water.
This is usually a ground aggregate of some kind. Recreational field warning track material should use 3/16″ minus gradation. Professional or collegiate facilities will use a slightly finer 1/8″ minus gradation.