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Accessible Exterior Surfaces

 The results of this pilot study suggest:

  • Surfaces that can be objectively measured as firm and stable generally require less energy, are perceived as less strenuous, allow higher ambulation velocities, and are perceived as less difficult to walk on
  • Ambulation tends to be more difficult (i.e., increased energy consumption, higher ratings of perceived exertion, higher levels of difficulty ratings and decreased velocity) on wood chips (CPBR), engineered wood fiber J (EWFJ), and engineered wood fiber K (EWFK) surfaces.
  • Sand (SAND) is the most difficult surface for all user groups, but the difference is particularly dramatic for those using a manual wheelchair.
  • Ambulatory subjects with mobility limitations (with and without assistive devices) have higher energy costs on the surfaces that are objectively measured as firm and stable, which may explain in part why their energy costs had a relatively smaller increase on the surfaces that were not objectively measured as firm and stable.
  • There is some indication, even with the limited number of subjects in this study, that the softness of a surface can improve the comfort of walking on the surface when using prosthetic technologies. For the same reason that people prefer to run on non-paved surfaces, some users would find walking on non-paved surfaces more comfortable as well.
  • The low energy costs for manual wheelchair users on surfaces that were objectively measured as firm and stable increase dramatically on the surfaces that were not measured as firm or stable.
  • Community mobility of the subjects as measured by total energy expenditure on the ADAAG course, level of fitness as measured by the PWC170and grip strength tests, and the type of mobility limitation were significantly related to the “accessibility” of the surfaces (i.e., subjects who had difficulty ambulating in an ADAAG-compliant environment also found it more difficult to ambulate on surfaces that were not objectively measured as firm and stable).
  • Energy expenditure increases for ambulation with turns relative to ambulation on a straight course.
  • Further investigation would be required to understand the impact of the type of mobility limitation (particularly for the ambulatory groups), the specific type of disability, and the specific type of assistive device used.
  • All subject testing was conducted on surfaces under dry conditions. Conclusions for these types of surfaces under wet conditions could be inferred from the results of objective measurement methods under “wet” conditions.
  • Path fines and dirt were significantly less firm and stable under wet conditions. Increased energy expenditure would be expected on these surfaces in a wet condition for people who have increased levels of energy expenditures on less firm and stable surfaces.
  • The Wheelchair Work Measurement Method provides reliable measures of the work required to propel a wheelchair across a level surface. However, the test equipment is not portable, and measurements are unreliable on sloped surfaces. Therefore, it is not suitable as a standard test method for exterior surfaces.
  • The Rotational Penetrometer test procedure has been proven to provide consistent and repeatable objective measures of both firmness and stability on a full spectrum of surfaces, ranging from very hard and stable to very soft and unstable (i.e., concrete to sand).
  • The Rotational Penetrometer is easy to use, portable, provides repeatable results on surfaces that are level within 5%, and is capable of measuring surfaces under both wet and dry conditions.
  • Measurements obtained with the Rotational Penetrometer correlated with the results of the Wheelchair Work Measurement Method. They also correlated with the level of difficulty ratings and measures of energy consumption for subjects walking on a variety of surfaces.


An objective test procedure for measuring the firmness and stability of surfaces with a portable device and performance specifications need to be accepted in order to enable land managers to determine whether the surface material on a trail (i.e., on site) is considered “accessible.” Surface firmness and stability should be measured using an objective device suitable for use “on trail” or in the field because surface characteristics vary dramatically depending on the installation at the actual site.

  • 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.

Recommendations for Future Research

  • Due to the small and diverse group of subjects, additional research involving persons who are ambulatory with limited mobility would need to be conducted to validate the results of this study, and provide additional information on this subject group regarding which surfaces are accessible or preferred.
  • Direct measures of energy consumption should be recorded in combination with subjective ratings of difficulty to obtain an accurate assessment of surface “accessibility” for the user.
  • Future research should evaluate energy consumption using both linear travel and ambulation with turns. The evaluation of linear travel facilitates comparisons with results from previous research; however it does not accurately represent the energy expenditure of community ambulation (which almost always involves some turning). In order to compare results between studies, a standard test course design which incorporates both 90 and 180 degree turns would have to be agreed to by all researchers working in this area. The proportion of turning and straight travel for such a course should reflect the “typical” patterns of ambulation in the community. (See the Technical Report for details on the test courses used in this research.)
  • The oldest subjects in the published literature were 65 to 70 years of age. Additional testing with individuals over 70 years of age would be required to verify specific surface accessibility requirements for that age group.
  • Children with and without disabilities have higher levels of energy expenditure because they are less “practiced,” and therefore less efficient in their movement. Additional research would be required to verify whether children with disabilities have the capacity to access surfaces that require an increase in energy consumption relative to a paved surface.
  • Additional research would be required to determine the validity and repeatability of the Rotational Penetrometer measurements on surfaces with slopes greater than 5%.
  • The exact specifications for conducting Rotational Penetrometer tests under wet conditions (e.g., specifications for the wetting procedure, the intervals at which measurements should be made after the wetting procedure) or to determine the “wetness” of surfaces tested on-site would need to be developed and validated through future research.
  • Based on the findings of this research, carpet on sloped surfaces may be an accessibility issue. Additional research should examine the existing requirements for carpet and carpet-pad combinations.
  • To evaluate the impact of other surface variables, such as slip resistance and the “levelness” or “evenness” of the surface, further research would be required.
  • Additional research would be required to evaluate the impact of surface firmness and stability in combination with other trail characteristics, such as distance, grade, cross slope or obstacles.