Sight Distance

Sight distance

Distance along the carriageway over which a driver can see an object.

Safety principle

Drivers need time to DETECT a hazard, THINK of a response, and IMPLEMENT it.

Designer vs driver view

Designer sees whole plan/profile; driver sees small consecutive, distorted parts with magnified discontinuities.

Eye heights

Car 1.1 m; truck 2.4 m; bus 1.8 m.

Object height

0.2 m applies to SSD calculations. OSD uses 1.25 m (approaching vehicle).

Reaction time

2.5 s is the common tutorial/design value, but the appropriate value must be selected for the road, driver and problem conditions.

Stopping Sight Distance (SSD)

Distance for a normally alert driver at design speed on a WET pavement to perceive, react and stop before a hazard.

SSD =

Reaction distance + braking distance.

SSD formula

SSD = Rt·V/3.6 + V²/[254(d + 0.01a)], with Rt in seconds, V in km/h, d = longitudinal deceleration coefficient, a = grade % (positive uphill, negative downhill).

Speed used for SSD

Use the speed specified by the design problem

d values (cars)

0.61 dry; 0.46 mean wet; 0.36 desirable max for SSD (general); 0.26 desirable max on major freeways.

d values (other)

Truck 0.29 (max); bus 0.15 (passenger comfort).

SSD steps

1) Rt 2) V 3) d 4) a 5) check units 6) calculate SSD.

Grade effect

Downhill grade increases SSD; uphill decreases it.

Truck SSD

Higher driver eye height may allow earlier detection, but trucks generally require greater braking distance because of their braking performance and mass.

SSD safety check

Compare the calculated SSD with the available distance; design is OK only if available distance ≥ SSD.

Two approaching vehicles

Calculate each vehicle's SSD separately and add them to obtain the required separation (SSDmin = SSD1 + SSD2).

SSD on horizontal curves

Available sight distance must exceed SSD everywhere on the curve (check obstructions).

Overtaking Sight Distance (OSD)

Distance to safely overtake a slower vehicle without affecting an oncoming vehicle; two-lane two-way roads only. Eye 1.1 m, object 1.25 m.

OSD components

d1 (reaction/initial acceleration) + d2 (in opposing lane) + d3 (clearance) + d4 (oncoming vehicle travel).

Overtaking phases

Establishment (OED) + Continuation (OCD).

OSD calculation workflow

Find d1 (reaction/initial acceleration), then d2 using RELATIVE speed: time in opposing lane t = (vehicle lengths + clearances)/(overtaking speed − slow speed), d2 = overtaking speed × t; add d3 (clearance) and d4 (oncoming vehicle travel = oncoming speed × t). OSD = d1 + d2 + d3 + d4.

Headlight sight distance

Unlit roads at night: max ~120-150 m (= SSD at 80-90 km/h). Road lighting is the ONLY way to match day/night sight distance.

Coordination of H & V alignment

Benefits: safety, appearance, uniform speed, earthwork economies.

Crests & safety

Avoid min-radius curves on crests, lateral shifts, crests at intersections/rail crossings; crest should be INSIDE the horizontal curve (direction change visible first).

Dips

Avoid dips in vertical alignment, especially on long straights.

Long flat grades

Increase overtaking crashes (speed/distance hard to judge).

Ideal curvilinear design

H & V curves coincide, horizontal slightly overlapping vertical; long curves at the end of long straights.