The Sight Distance Issues with Retrofitted Single-Lane HOV Facilities

ABSTRACTIt is well-known that obstruction inside a highway horizontal curve will lead to impaired sight distance. Highway alignment design standards in terms of the minimum horizontal curve radius are specified to allow for adequate stopping sight distance at given design speeds. For a single-lane HOV facility, inside curve obstruction may occur no matter when the facility curves to the left (per travel direction) or right. A unique situation that calls for special attention is that the adjacent mixed-flow lane traffic, once queued, may become sight obstruction. Calculations indicated that such obstruction may govern the minimum curve radius design as long as the left shoulder is not less than 0.92 m, when the HOV lane is contiguous to the mixed-flow lanes. Such governance may necessitate design speed reduction, horizontal and cross-section design adjustment, or both.