Intersection Sight Distance | AASHTO Green Book 7th Edition (2018)

Sight distance provided to the driver at each relevant position should equal or exceed recommended minimum values.

The AASHTO Green Book (2018) defines seven cases for sight distance:

Case A – Intersections with no traffic control

Case B – Stop control on the minor road
B1 – Left from minor road
B2 – Right from minor road
B3 – Minor road crossing

Case C – Yield control on the minor road
C1 – Minor road crossing
C2 – Left or Right from Minor

Case D – Traffic signal control

Case E – All-way stop control

Case F – Left turns from the major road

Case G – Roundabouts


Intersection Sight Distance from A Policy on Geometric Design of Highways and Streets, 2018 by American Association of State Highway and Transportation Officials (AASHTO)

Drivers must be able to adequately observe traffic control devices and conflicting traffic to take the appropriate action.

Common visual impediments in the sight triangle include terrain, landscaping/vegetation, parked cars, and other roadside objects.

At unsignalized intersections, any movements that conflict with a traffic stream which has the legal right of way should be examined for appropriate sight distance values.

At signalized intersections, specific approaches which allow permitted left-turns or right-turns during the red interval should be the focus of sight distance evaluations.

Medians which are wide enough to store a vehicle in the middle of their crossing movement or left turn can allow the movement to be split into two parts.

Adjustments are made based on deviations from base scenarios:
Grades which exceed +/- 3%
Additional lanes (base case is generally assumed as two-lane road)


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