Characteristics of Emerging Road and Trail Users and Their Safety

Horizontal Alignment

For horizontal alignment, the critical factor is the point at which users will instinctively decelerate to maintain a comfortable degree of lateral acceleration while traversing a curve. This point is represented by the coefficient of friction used in the minimum design radius equations of AASHTO. When evaluating the coefficient of friction, it is important to remember that it does not truly represent the point at which a tire will slide out from under the path user, rather, it is a comfort factor. This is not to suggest that the coefficient of friction should be regarded lightly. Indeed, if a curve is so sharp as to make a path user uncomfortable, the user may understeer and run off the side of the path.

As shown in table 19, users of manual wheelchairs tolerated the lowest degree of lateral acceleration, "coefficients of friction." However, given that their 85^th percentile speed is 8 km/h (5 mi/h), the minimum radius required for manual wheelchair users would be 12.2 m (40 ft). Recumbent bicycles, though having the highest calculated tolerance for lateral acceleration (coefficients of friction) for 3.1-m and 9.2-m (10- and 30-ft) radii, also have the highest 85^th percentile speeds. The minimum radius calculated for the 85^th percentile recumbent bicycle is 26.8 m (88 ft). As a result, recumbent bicycles would be the critical design user for turning radii.

Most users did not appear to reduce their speeds when entering radii greater than 15.8 m (50 ft). The exception is recumbent bicyclists, who may have been constrained by even the 27.5-m (90ft) radius.

AASHTO (p. 38) currently recommends a minimum radius of 27 m (90 ft) for cyclists traveling at 30 km/h (20 mi/h) around a curve with a 2 percent superelevation.⁽²⁾

^2. American Association of State Highway and Transportation Officials. Guide for the Development of Bicycle Facilities. American Association of State Highway and Transportation Officials, Washington, DC, 1999.