Sight distance
A driver must see far enough ahead to stop for an object in the road. Stopping sight distance is the sum of two parts: the distance travelled during the perception and reaction time, before the brakes act, and the braking distance.
Indian practice (IRC) commonly uses a reaction time of 2.5 seconds and a design longitudinal friction coefficient that falls with speed. Gradients change the braking distance: going downhill lengthens it.
Where overtaking is allowed on two-lane roads, a much longer overtaking sight distance is needed, and intermediate sight distance, commonly taken as twice the stopping sight distance, is provided where full overtaking distance cannot be.
Sight distance controls the length of vertical curves over crests, the clearance needed on the inside of horizontal curves, and the visibility splays at junctions.
On horizontal curves, buildings, cut slopes or trees on the inside of the curve can block the view, so a set-back distance is cleared. At night, headlight sight distance governs on sag vertical curves, because the beam meets the road ahead before the driver's line of sight does.
Convert km/h to m/s by multiplying by 0.278 before using the lag-distance term.
SSD = v t + v² / (2 g f) (v in m/s)In km/h form: SSD = 0.278 V t + V² / (254 f)On a gradient: braking term V² / (254 (f ± 0.01 n)), n = gradient in percent
Ignoring the gradient correction on long downgrades.
Problem. Design speed 80 km/h, t = 2.5 s, f = 0.35. Find SSD on level ground.
- Lag = 0.278 × 80 × 2.5 = 55.6 m.
- Braking = 80² / (254 × 0.35) = 6 400 / 88.9 = 72.0 m.
- SSD = 127.6 m, say 128 m.
Answer. About 128 m
Quick quiz
1Two components of SSD?
Lag (reaction) distance and braking distance.
2Does a downgrade increase or decrease SSD?
Increase.
3Intermediate sight distance is usually?
Twice the SSD.
- IRC:73 (India): Geometric design standards for rural (non-urban) highways.
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References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- S. K. Khanna, C. E. G. Justo, A. Veeraragavan, Highway Engineering, Nem Chand and Bros, latest edition.
- F. L. Mannering, S. S. Washburn, Principles of Highway Engineering and Traffic Analysis, Wiley, latest edition.
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