---
title: "Stopping distance and stopping sight distance"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:stopping-distance-relation"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/stopping-distance-relation
site: BCT Engineering
---

# Stopping distance and stopping sight distance

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2063 Kartik (old course), 1+4+3 marks
- Video: [Stopping Sight Distance | Highways | Transportation](https://www.youtube.com/watch?v=fQTSASSMU5U) — APSEd
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/stopping-distance-relation
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

Define stopping distance. Enumerate the various factors affecting stopping distance. Explain the relationship between the stopping distance and stopping sight distance.

## Short answer

Stopping distance equals lag distance (speed × reaction time) plus braking distance (v²/[2g(f±G)]). Factors: speed, reaction time, friction, gradient, brake efficiency, and weather. Stopping sight distance is set equal to stopping distance for design speed with t=2.5 s; available sight distance on curves must meet or exceed it.

## Answer

**Stopping distance** is the total distance travelled by a vehicle from the instant the driver sights an object or hazard ahead until the vehicle comes to a complete stop. It is the sum of the **lag distance** (travelled during the reaction time) and the **braking distance:**

$$
SD = v\,t + \frac{v^2}{2g\,(f \pm G)}
$$

#### Factors affecting stopping distance
1. Speed of the vehicle.
2. Reaction time (PIEV) of the driver.
3. Coefficient of longitudinal friction (type and condition of road surface and tyres, wet or dry).
4. Gradient of the road.
5. Brake efficiency and the weight/loading of the vehicle.
6. Weather and visibility, driver's alertness (also fatigue, alcohol).

#### Relation between stopping distance and stopping sight distance
- The **stopping sight distance (SSD)** is the clear length of road ahead that the road must give so that the driver can see an object and stop in time.
- It is **equal to the stopping distance** computed for the design speed with the design values of $t = 2.5$ s and $f$: so the design SSD is simply the stopping distance:
  $$
  SSD = 0.278\,V\,t + \frac{V^2}{254\,(f \pm 0.01n)}
  $$
- The **available** sight distance at any point of a horizontal or vertical curve (measured with a driver eye height 1.2 m and an object height 0.15 m) must be **not less than** the stopping distance.
- If the available sight distance is less than the stopping distance, the vehicle cannot stop before the object, so the road geometry (curve length, set-back, speed limit) is revised.

## Check yourself

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

1. Stopping distance is the sum of
   A. braking distance and overtaking distance
   B. lag distance during reaction time and braking distance
   C. lag distance and sight distance
   D. braking distance and the length of the vehicle
   Answer: B. SD = v t + v^2/(2g(f +/- G)): distance travelled during reaction plus the braking distance. (Key point: Definition of stopping distance)

2. Which factor does NOT directly affect stopping distance?
   A. Speed of the vehicle
   B. Driver's reaction (PIEV) time
   C. Coefficient of longitudinal friction
   D. Width of the carriageway
   Answer: D. Speed, reaction time, friction, gradient, brake efficiency and weather affect it; carriageway width does not. (Key point: Factors affecting stopping distance)

3. What is the relation between stopping sight distance (SSD) and stopping distance?
   A. SSD is twice the stopping distance
   B. SSD is the clear road length that must be at least the stopping distance computed for the design speed
   C. SSD is the braking distance only
   D. SSD is unrelated to design speed
   Answer: B. The design SSD equals the stopping distance for the design speed with t = 2.5 s and the design f. (Key point: Relation of SSD to stopping distance)

4. What happens if the available sight distance on a curve is less than the stopping distance?
   A. The vehicle can still stop in time
   B. Only the speed of heavy vehicles matters
   C. The superelevation is increased
   D. The vehicle cannot stop before the object, so the geometry (curve length, set-back, speed limit) is revised
   Answer: D. The available sight distance must be not less than the stopping distance; otherwise the design is changed. (Key point: Available sight distance check)

## Close variants set in other exams

- [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 Transportation Engineering I

- [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×
- [Stopping sight distance at 50 kmph](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-66m-ssd-50) — Transportation Engineering I, chapter 3
- [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×
- [Percent grade from stopping distance difference](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-71bh-grade-ssd) — Transportation Engineering I, chapter 3

## Related in other subjects

- [Rainfall-runoff relationship, factors affecting runoff](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arainfall-runoff-relation-factors) — Engineering Hydrology, chapter 4