Sight distance along the major road that a driver needs to turn left, turn right or cross from a stop on the minor road, or to turn left from the major road, with the departure sight triangle to keep clear, per the AASHTO Green Book. Completes the stopping, decision and passing sight distance tools.
Inputs: the boxes under Manoeuvre & Vehicle, Major Road and Minor Road Approach — change them to suit your intersection. Calculated: the Calculation box and the results below. Hover over or tap i for help.
ISD = 0.278 · Vmajor · tg (m), V in kph, tg in s, design value rounded up to 5 m.
Time gaps (P / SU / combination truck): B1 left turn from stop 7.5 / 9.5 / 11.5 s ·
B2 right turn and B3 crossing from stop 6.5 / 8.5 / 10.5 s · F left turn from major road 5.5 / 6.5 / 7.5 s.
Extra lanes: +0.5 s (car) or +0.7 s (trucks) for each lane crossed beyond one (B1, F) or beyond two (B3), including narrow medians.
Approach upgrade steeper than 3 %: +0.2 s per percent (B1) or +0.1 s per percent (B2, B3).
The sight line runs from a 1.08 m eye height to a 1.08 m object (the approaching vehicle): keep the triangle clear of anything taller.
Base time gaps for the design vehicle, two-lane undivided major road, level approach, rounded up to 5 m. highlighted = current design speed; outlined = current case.
The departure sight triangle is bounded by the minor-road leg, from the decision point (driver's eye, in the middle of the minor-road lane, typically 4.4 m back from the edge of the major-road travelled way) and the major-road leg, the intersection sight distance measured to the middle of the lane used by the approaching vehicle. Keep it clear of buildings, parked vehicles, signs, hedges, crops and cut slopes higher than about 1.08 m. For a left or crossing manoeuvre check traffic from both directions; for a right turn only traffic from the left needs checking (right-hand traffic — mirror the sketch for left-hand traffic countries). Where the sight distance cannot be provided, consider relocating obstructions, lowering the major-road speed, adding turn lanes or signal control.
It is the length of major road a driver must be able to see to turn onto or cross it safely without forcing approaching vehicles to slow down much. It is based on the time gap drivers accept, not on stopping, so it is usually longer than the stopping sight distance.
AASHTO gives ISD = 0.278 · Vmajor · tg, where Vmajor is the design speed of the major road in kph and tg is the time gap in seconds for the manoeuvre and design vehicle, adjusted for extra lanes crossed and steep approach upgrades.
Use B1 for a left turn from a stop-controlled minor road (usually the governing case), B2 for a right turn, B3 for crossing straight over, and F for a vehicle turning left from the major road across oncoming traffic.
Each extra lane to be crossed adds 0.5 s for a passenger car or 0.7 s for a truck. A narrow median that cannot store the design vehicle counts as extra lanes (median width ÷ lane width). Where the median can store the vehicle, check each half of the crossing separately.
The method follows A Policy on Geometric Design of Highways and Streets, 7th Edition (2018), intersection sight distance for stop control on the minor road and left turns from the major road. Always confirm against the edition adopted by your road authority.