14.10.3 Sight Distance

Drivers approaching or departing an intersection must have an unobstructed view of traffic control devices and sufficient length along the crossroad to safely navigate the intersection. Insufficient sight distance is a significant factor in street crashes and near collisions. As with any other intersection, DDI intersections must provide stopping sight distance (SSD) and intersection sight distance (ISD). Sight distances must be checked for these conflict areas: walls, railings, tall landscaping, or other obstructions that may limit sight distance. Intersection sight distance may also be limited by barriers or other obstacles between the crossovers.
Due to their curvilinear alignment, DDIs have special considerations that need to be reviewed and coordinated closely with the horizontal alignment. Primary considerations for vertical alignment at DDIs include:
  • Crest vertical curve on the overcrossing -
     A crest vertical curve on an overcrossing may potentially mask the turning roadway alignment to the crossover;
  • Skew in an overcrossing -
     A crest vertical curve and skew angle can also result in reduced visibility to the crosswalk across the left-turning roadway; and
  • Crosswalk visibility -
     The relatively sharp horizontal curve to the left and bridge parapet can obscure views to a user waiting at the pedestrian crossing
Refer to
NCHRP Report 959
for additional information regarding sight distance considerations.