• Preview sight distance measures the motorist sight distance required under the assumption they base their behaviors on a preview of the road surface and other cues that lie downstream. The concept applies to locations where a horizontal curve is initially beyond the driver’s line of sight (e.g., horizontal curves near the top of crest vertical curves, the bottom of sag vertical curves).
• Design values in Table 7.3 specify required preview sight distances based on horizontal curve radius and curve type (i.e., simple, spiraled). After converting original values from meters, numbers were rounded to the nearest foot.
• In Table 7.3, A is calculated using the following equation:

S_T = Motorist perception-reaction time that falls on the roadway upstream of the point of curvature of the horizontal curve

S_C = Amount of horizontal curvature needed for motorist to detect a curve

• To calculate preview sight distance for daytime conditions, use the line of sight from the motorist’s eye to the pavement marking. At night, headlamp illumination of pavement markings limits sight distance. As such, line of sight must reference headlamp height.