Passenger car, case B1
At 60 km/h on a two-lane major road, with no median and zero approach grade, tg = 7.5 s. ISD = 0.278 × 60 × 7.5 = 125.10 m; display 130 m after rounding.
Calculate time-gap sight distances for left turns, right turns and crossing manoeuvres at an intersection.
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, B3) or +0.1 s per percent (B2).
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.
Calculate time-gap sight distances for left turns, right turns and crossing manoeuvres at an intersection.
Speed km/h; time gap s; sight distance m; approach grade %.
ISD = 0.278Vtg. The implemented cases are B1, B2, B3 and F. A passenger-car left turn from stop uses a base 7.5 s gap; lane crossings and approach grades can increase it. Confirm the selected vehicle and control case.
Illustrative inputs for checking the method; these are not project records.
At 60 km/h on a two-lane major road, with no median and zero approach grade, tg = 7.5 s. ISD = 0.278 × 60 × 7.5 = 125.10 m; display 130 m after rounding.
At 80 km/h on a two-lane major road, with no median and zero approach grade, tg = 6.5 s. ISD = 0.278 × 80 × 6.5 = 144.56 m; display 145 m after rounding.
Calculate time-gap sight distances for left turns, right turns and crossing manoeuvres at an intersection.
Speed km/h; time gap s; sight distance m; approach grade %. ISD = 0.278Vtg. The implemented cases are B1, B2, B3 and F. A passenger-car left turn from stop uses a base 7.5 s gap; lane crossings and approach grades can increase it. Confirm the selected vehicle and control case.
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