Transportation Design
Intersection Sight Distance
AASHTO Case B intersection sight distance calculator for stop-controlled minor roads — left turn, right turn, and crossing maneuvers.
Intersection Sight Distance
This calculator determines the sight distance a driver stopped on a minor road needs along the major road to safely complete a left turn, right turn, or crossing maneuver before conflicting traffic arrives. It applies AASHTO Case B — the stop-controlled minor-road condition — using the time-gap method rather than a fixed sight-distance table.
The required time gap starts from a base value for passenger cars and maneuver type, then adds time for heavier vehicles that accelerate more slowly, for minor-road approach grades over 3% that slow the departing vehicle, and for each additional major-road lane the vehicle must cross. Multiplying the resulting time gap by 1.47 × the major-road design speed gives the required sight distance in feet.
ISD = 1.47 · V_major · t_g [ft, mph, seconds]
t_g = t_base + t_vehicle + t_grade + t_lanes
t_base: left turn 7.5 s · right turn/cross 6.5 s (passenger car)
A passenger car turning left onto a 45 mph major road, on a minor-road approach with less than 3% grade and crossing a single lane, needs a base time gap of 7.5 seconds with no additional adjustments. The required intersection sight distance is 1.47 × 45 × 7.5 = 496 ft — the clear line of sight the departing driver needs along the major road in both directions before completing the turn.
Intersection Sight Distance Calculator
AASHTO Case B — stop-controlled minor road