Of the three roadway functional classes (arterial, collector, and local), the local road is the one intended to provide access. Local street design criteria interact to affect the available sight distance. Urban residential streets of the type often found in newer subdivisions tend not to be laid out in the traditional grid pattern, but rather in a more free-form pattern incorporating elements of discontinuity and curvilinear alignment. In these settings, on-street parking, whether on both sides of the street or only on one side, forces vehicles travelling in opposite directions to operate in the same lane. The presence of vegetation or other objects at the curbside can also limit the available head-on sight distance. Where two lanes of traffic moving in opposite directions operate in one lane, the amount of sight distance needed is greater than under normal conditions. The design needed is analogous to one that permits two locomotives to approach head-on on a single track and to stop before colliding.Roadway designers should recognise situations that require adequate head-on sight distance and provide a sight distance sufficient for the two approaching vehicles to react and stop before colliding.
Abstract