• Many motorists base curve speeds on their perception of road features and the apparent curve radius. The apparent curve radius is the radius as seen from the motorists’ perspective.
• Topography and road elements can distort the apparent radius, making it look sharper or flatter. This issue is pronounced on combination curves. For example, a vertical curve superimposed on a horizontal curve. In this case, the horizontal curve looks flatter than it is.
• Pay close attention to sag horizontal curves whose apparent horizontal radius differs from radius specified in plans. Consider lowering posted speeds to encourage speed reductions.
• Figures 6.1 and 6.2 are nomographs that indicate combinations of vertical and horizontal curve radii that produce apparent radii conducive to unsafe curve entry speeds.
• Table 6.1 summarizes the influence of different design elements on curve perception.

Figure 6.2 Acceptable and Unacceptable Combinations of Horizontal – Sag Vertical Curve Radii from Preceding Curve