---
title: "Stopping sight distance at 50 kmph"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:n-66m-ssd-50"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/n-66m-ssd-50
site: BCT Engineering
---

# Stopping sight distance at 50 kmph

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2066 Magh (old course), 3+3 marks
- Video: [Calculate the safe stopping sight distance for a design speed of 50 Kmph. (Highway Engineering) 2](https://www.youtube.com/watch?v=A5tZ0dueUjg) — Civil Engineering YouTube Channel 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/n-66m-ssd-50
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

Calculate safe stopping sight distance for the design speed of 50 kmph for: (i) two-way traffic on a two lane road (ii) two-way traffic on a single lane road. Assume appropriate data for calculation.

## Short answer

SSD = 0.278Vt + V^2/(254f) = 61.35 m. For two-way traffic on a two-lane road use 61.4 m, adopt about 65 m. For a single-lane road use twice the SSD, 122.7 m, adopt about 125 m.

## Answer

#### Assumed data (IRC 66 / NRS 2070)
Design speed $V=50$ km/h, total reaction time $t=2.5$ s, coefficient of longitudinal friction $f=0.37$ (IRC value for 50 km/h), level road.

#### Basic stopping sight distance
Lag distance $=0.278Vt=0.278(50)(2.5)=34.75$ m. Braking distance $=\dfrac{V^2}{254f}=\dfrac{50^2}{254(0.37)}=26.60$ m.

$$
SSD=0.278Vt+\frac{V^2}{254f}=61.35\ \text{m}
$$

#### (i) Two-way traffic on a two-lane road
Each vehicle keeps to its own lane and only has to stop for an obstruction in its path, so
**SSD ≈ 61.4 m ≈ 65 m** (IRC gives 60 m for 50 km/h).

#### (ii) Two-way traffic on a single-lane road
Vehicles from opposite directions share the same lane and may approach head-on, so each must be able to stop within half of the sight distance. The sight distance is twice the SSD:

$$
SD=2\times SSD=2(61.35)=122.70\ \text{m}
$$

**≈ 125 m** (IRC gives 120 m).

## Check yourself

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

1. Which two parts make up SSD?
   A. Lag distance 0.278Vt plus braking distance V^2/(254f)
   B. Lag distance plus overtaking distance
   C. Braking distance only
   D. V^2/(254f) plus 2s spacing
   Answer: A. During reaction the vehicle travels 0.278Vt, then brakes over V^2/254f: 34.75 + 26.60 = 61.35 m. (Key point: SSD components)

2. Why is SSD (not twice) enough for two-way traffic on a two-lane road?
   A. Vehicles brake twice as hard
   B. Each vehicle keeps its own lane and only stops for an obstruction in its path
   C. The road is wider so friction is higher
   D. Opposite traffic cannot be present
   Answer: B. Opposing vehicles are in separate lanes, so a single SSD applies. (Key point: Two-lane road)

3. What sight distance is needed for two-way traffic on a single-lane road?
   A. SSD, about 61 m
   B. SSD / 2, about 31 m
   C. 2 x SSD, about 125 m
   D. 3 x SSD, about 184 m
   Answer: C. Vehicles can approach head-on in the same lane, so both must stop within the available distance. (Key point: Single-lane road)

## Related in Transportation Engineering I

- [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
- [Safe speed from stopping criterion on curve](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-78c-speed-stopping) — Transportation Engineering I, chapter 3
- [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
- [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×
- [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
- [Sight distance with obstruction on curve](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-78b-sd-obstruction) — Transportation Engineering I, chapter 3

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