Stopping sight distance: how far ahead a driver must see

Stopping sight distance (SSD) is the clear distance a driver needs to see an obstruction and stop before hitting it. Every highway curve, summit and junction is checked against it.

Two parts

1. Lag (reaction) distance: the vehicle keeps moving at full speed while the driver perceives the hazard and applies the brakes.
2. Braking distance: the vehicle slows to a stop.

SSD = v t + v²2 g f

v in m/s, t the total reaction time, f the longitudinal coefficient of friction.

In km/h units (V):

SSD = 0.278 V t + V²254 f

IRC values

• Reaction time t = 2.5 s.
• f = 0.35 to 0.40, lower at higher speeds (0.35 at 80 km/h and above).

Effect of gradient

On an upgrade gravity helps you stop; on a downgrade it works against you. With gradient n (as a fraction, + for up, − for down):

Braking distance = v²2 g (f ± n)

Worked example

Design speed 80 km/h, so v = 22.2 m/s; f = 0.35; level road.

Lag distance = 22.2 × 2.5 = 55.6 m

Braking distance = 22.2²2 × 9.81 × 0.35 = 71.9 m

SSD = 127.5 m, round up to 130 m

IRC tables give 120 m for 80 km/h, because they use slightly different friction values. Always use the code table for design and the formula to understand it.

Related sight distances

• Intermediate sight distance (ISD) = 2 × SSD, used where overtaking sight distance can't be provided.
• Overtaking sight distance (OSD) is much longer, since a driver must pass and return before meeting oncoming traffic.
• Headlight sight distance at valley curves at night is taken equal to SSD.