18 A SUMMARY OFACCESSIBILITY STANDARDS FOR OUTDOOR DEVELOPEDAREAS accessibility standards for Federal outdoor developed areas Clear Tread Width [10173] The clear tread width of trails must be a minimum of 36 inches (figure 2). The 36-inch-minimum clear tread width must be maintained for the entire distance of the trail and may not be reduced by gates, barriers, or other obstacles unless a condition for exception does not permit full compliance with the provision. Figure 2—Minimum clear trail tread width. Where gates and barriers require users to make 90-degree or 180-degree turns, sufficient space should be provided for people using mobility devices to make the turns (figure 3). Mobility devices that are used in the outdoors typically have a longer wheel base and are wider than mobility devices that are used indoors. The Access Board and National Institute on Disability and Rehabilitation Research sponsored research to collect anthropometric
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