---
title: "Types of sight distances"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:sight-distance-types"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/sight-distance-types
site: BCT Engineering
---

# Types of sight distances

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2078 Baisakh, 4 marks; 2062 Jestha (old course), 6 marks (asked 2 times)
- Video: [HE_TE-U2.9. Sight Distance and It's 3 Cases, Types of Sight Distances](https://www.youtube.com/watch?v=C0EQK_toGS0) — Er. Venkatesh Koudagani
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/sight-distance-types
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

Briefly explain the types of sight distances (overtaking sight distance and stopping sight distance).

## Short answer

Stopping sight distance (SSD) is the minimum distance to stop before an object (eye 1.2 m, object 0.15 m), given by SSD = 0.278 V t + V²/(254 f). Overtaking sight distance (OSD) enables safe overtaking on two-lane roads and is computed as d1+d2+d3. Intermediate sight distance (2×SSD) is used where OSD cannot be provided.

## Answer

**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.

| Type | Use | Value |
|---|---|---|
| SSD | Every road | Minimum required |
| ISD | Where OSD is impossible | 2 x SSD |
| OSD | Two-way overtaking sections | Largest |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Stopping sight distance is the sum of
   A. lag distance and braking distance
   B. overtaking and braking distance
   C. lag distance and clearance
   D. headlight distance and lag distance
   Answer: A. SSD = distance during reaction (lag) + braking distance. (Key point: SSD components)

2. The IRC eye height and object height for SSD are
   A. 1.2 m and 1.2 m
   B. 0.75 m and 0.15 m
   C. 1.2 m and 0.15 m
   D. 1.08 m and 0
   Answer: C. IRC uses 1.2 m eye height and 0.15 m object height for SSD. (Key point: SSD heights)

3. Intermediate sight distance, where OSD cannot be provided, equals
   A. SSD
   B. 2 x SSD
   C. OSD / 2
   D. 3 x SSD
   Answer: B. ISD = 2 x SSD, giving some chance to overtake. (Key point: ISD)

4. Overtaking sight distance is needed
   A. on two-lane roads for safe overtaking without colliding with oncoming vehicles
   B. only at intersections
   C. only on valley curves at night
   D. only on one-way roads
   Answer: A. OSD is the minimum distance for safely overtaking a slower vehicle on a two-way road. (Key point: OSD)

## Related in Transportation Engineering I

- [Overtaking sight distance derivation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aosd-derive) — Transportation Engineering I, chapter 3, asked 6×
- [Vertical curve for overtaking sight distance](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-81c-vc-overtaking) — Transportation Engineering I, chapter 3
- [Overtaking sight distance and zone, 60 kmph](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-76b-osd-60) — Transportation Engineering I, chapter 3
- [Overtaking sight distance and zone numerical](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-62j-osd-numeric) — Transportation Engineering I, chapter 3
- [Stopping sight distance and derivation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Assd-factors-derive) — Transportation Engineering I, chapter 3, asked 6×
- [Factors affecting stopping sight distance](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Assd-factors-only) — Transportation Engineering I, chapter 3, asked 3×

## Related in other subjects

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- [Correlation vs regression; stopping distance line](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3Acorrelation-regression-plot-stopping-distance) — Probability and Statistics, chapter 5
- [Stopping distance and brake energy](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Abraking-distance-energy-numerical) — Automotive Technology, chapter 7
- [Principles of EDM measuring distances](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aedm-principles) — Surveying I, chapter 2, asked 12×