Types of sight distances

Sight distance is the length of road ahead that is visible to the driver. Adequate sight distance is necessary for safe operation.

1. Stopping Sight Distance (SSD)

The minimum distance needed to see an object on the road and stop before hitting it (eye height 1.2 m and object height 0.15 m, IRC). It is the sum of the lag distance and the braking distance:

$$SSD = 0.278\,V\,t + \frac{V^2}{254\,f}$$

($V$ in km/h, $t = 2.5$ s, $f \approx 0.35$-$0.40$). It must be provided at every point of the road. Example: $V = 60$ km/h gives about 82 m (IRC table: 80 m).

2. Overtaking Sight Distance (OSD)

The minimum distance needed to overtake a slower vehicle safely on a two-lane road without colliding with an oncoming vehicle:

$$OSD = d_1 + d_2 + d_3 = v_b t + (v_b T + 2s) + vT$$

where $v_b$ = speed of the overtaken vehicle (m/s), $T = \sqrt{4s/a}$, and the spacing $s = 0.7v_b + 6$ m (IRC). It is provided on straight sections; for example, 470 m at 80 km/h on a two-way road.

3. Intermediate Sight Distance (ISD)

Where OSD cannot be provided, ISD $= 2\times SSD$ gives some chance to overtake.

4. Other sight distances

• Headlight sight distance for valley curves at night.
• Safe sight distance at intersections (approach sight triangle).
• Decision sight distance at complex locations.