Recommended Objective Test Method and Performance Specifications
The objective test method using the Rotational Penetrometer is recommended because it provides valid and reliable measurements of surface firmness and stability on slopes up to 5%, can be used on a wide variety of surface materials, and is suitable for conducting “on trail/on site” tests.
The following are recommended performance specifications for firmness and stability under dry conditions based on the Rotational Penetrometer test results and the human subject results from this research.
| Firmness | Penetration Depth | Firmness Rating |
| Firm | 0.3 inch or less | 3 or lower |
| Moderately Firm | >0.3 to 0.5 inch | >3 to 5 |
| Not Firm | >0.5 inch | >5 |
| Stability | Penetration Depth | Stability Rating |
| Stable | 0.5 inch or less | 5 or lower |
| Moderately Stable | >0.5 to 1.0 inch | >5 to 10 |
| Not Stable | >1.0 inch | >10 |
The proposed cut-off values for firm and stable surfaces generally correspond to the work required to propel a wheelchair up a 3% slope. The proposed cut-off values for moderately firm and stable surfaces generally correspond to the wheelchair work per meter values for a 7% slope.
Objective ratings of firmness and stability should be disclosed to all users through trail signage.
Surfaces that are firm, but only moderately stable should be allowed on linear trails less than 0.5 mile in length (i.e., do not require a lot of turning or pivoting).
Surfaces that are both moderately firm and moderately stable should be allowed on level trails (less than 3% slope in any direction to allow drainage) less than 0.1 mile in length (distances that are similar to those found in indoor environments), or in confined outdoor areas that are level (less than 3% slope in any direction to allow drainage) and where the distances traveled are less than 50 ft (e.g., around a picnic table, on a campsite).
Based on these proposed specifications, the asphalt (ASPP), unpaved road mix (RDMX), path fines (PAFN), path fines with stabilizer (RDOL), and native soil (DIRP) surfaces tested in this research would be considered firm and stable under dry conditions. The wood chips (CPBR) and engineered wood fiber K (EWFK) would be considered moderately firm and stable and would potentially be allowed for limited distances. Engineered wood fiber J (EWFJ) would not be considered firm nor stable. Some carpeted surfaces currently considered “accessible” (i.e., maximum pile thickness of 0.5 in.) would not comply with the proposed specifications.
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