Preliminary information obtained through a small research project on accessible exterior surfaces conducted for the Access Board suggests that surfaces considered “firm” (i.e., does not give way significantly under foot) can range from very firm to moderately firm (defined in table A).  Similarly, surfaces considered stable (i.e., do not shift from side-to-side or when turning) can range from very stable to moderately stable.

The degree of firmness and stability desired or most appropriate is related to the intended use of the trail, the predominant direction of travel, and the overall length of the trail.  For example, a surface which is both very firm and very stable, is recommended for trails of more than .5 mile in length due to the duration of travel for a person with a disability. However, it may be acceptable for the surface to be moderately firm (rather than very firm) (using calculations and classifications in Table A below) for trails less than .5 mile but greater than .1 mile in length, and where the travel pattern is primarily linear.  It may also be acceptable for the surface to be both moderately firm and moderately stable for trails less than .1 mile in length, and where the trail is moderately level (< 3% slope).

Table A Calculation and Classification

Firmness Classification - Amount of Penetration

Very Firm

0.3 inches or less

Moderately Firm

Greater than 0.3 inches and less than 0.5 inches

Not Firm

Greater than 0.5 inches

Stability Classification - Amount of Penetration

Very Stable

0.5 inches or less

Stable

Greater than 0.5 inches and less than 1.0 inch

Not Stable

Greater than 1.0 inch

Surfaces that are moderately firm or stable may be appropriate in areas where a cushioned surface is preferred (e.g., for a multi use trail that includes equestrians). Surfaces that are moderately firm and stable may also be appropriate on trails for winter use only because most trail surfaces are firm and stable when frozen.