Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard or its potential threat, select an appropriate speed and path, and initiate and complete the required safety maneuver safely and efficiently. Because decision sight distance gives drivers additional margin for error and affords them sufficient length to maneuver their vehicles at the same or reduced speed rather than to just stop, its values are substantially greater than the stopping sight distance.

Drivers need decision sight distances whenever there is a likelihood for error in either information reception, decision-making, or control actions. The following are examples of critical locations where these kinds of errors are likely to occur, and where it is desirable to provide decision sight distance: interchange and intersection locations where unusual or unexpected maneuvers are required; changes in cross section such as toll plazas and lane drops; and areas of concentrated demand where there is apt to be "visual noise" whenever sources of information compete, as those from roadway elements, traffic, traffic control devices, and advertising signs.