The clear roadside concept has a direct and obvious application to the selection of slopes and design of drainage features such as ditches, curbs, culverts, and drop inlets. A traversable, unobstructed roadside zone should extend beyond the edge of the driving lane for an appropriate distance so that the motorist can generally stop or slow the vehicle and return to the roadway safely.
The width of the zone depends on the traffic volume, the design speed, and the roadside slope. Vehicles on high-volume, high-speed routes obviously require more room to recover than those on less congested routes. A suggested guide for determining the width of the clear zone is presented in Fig. 6.2. The clear zone distance (width) is given in terms of the range of design average daily traffic (ADT) or vehicles per day (VPD), the design speed, and the roadside slope. Enter the chart from the left with the slope, intersect the appropriate design speed curve, and project down to the appropriate scale at the bottom to read the suggested width. The width should be used as a guide and may be adjusted for site-specific conditions and practicality. The AASHTO guide gives modification factors (1.1 to 1.5) that can be applied to increase the clear distance on horizontal curves where accident histories or site investigations show a need. Increased superelevation may be another option, depending on climatic conditions.
