NCHRP 15-14(1) [Completed]
Intersection Sight Distance
| Project Data
||Midwest Research Institute|
||Douglas W. Harwood|
The current AASHTO intersection sight distance procedures were intended to provide adequate sight distance at intersections to promote safe and efficient traffic operations. The basic intersection sight distance (ISD) models for no control, Stop control, and signal control were formulated in 1940. Recently, questions have been raised about the validity of the models, as well as the appropriateness of certain parameter values used to calculate ISD.
Of particular concern, were the Case IIIB and Case IIIC design criteria (i.e., ISDs for a vehicle on a Stop-controlled approach on a minor road to accelerate from a stopped position and turn left or right, respectively, onto a major road), which many considered excessive; and the Case I design criteria (i.e., ISD for vehicles approaching intersections with no control, at which vehicles are not required to stop, but may be required to adjust speed), which were based on assumptions that may not be sufficiently conservative.
Included in the revised ISD concepts used by Midwest Research Institute to introduce a more consistent conceptional basis for ISD models and to set revised values for ISD design based on those models were the use of a revised stopping sight distance (SSD) model in all cases where vehicles approaching an intersection need to slow or stop; the use of actual speed profiles gathered in the field at various types of intersections; the use of the gap-acceptance behavior observed in the field as the basis for the leg of departure triangle along the major road; and the use of observed vehicle-stopping positions as the basis for the leg of departure triangle along the minor road. The research findings and recommendations have been published in NCHRP Report 383: Intersection Sight Distance. The unpublished appendices A through I are available here.