---
title: "Intersection sight distance numerical"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:n-81c-intersection-sd"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/n-81c-intersection-sd
site: BCT Engineering
---

# Intersection sight distance numerical

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2081 Chaitra, 3 marks
- Video: [Sight Distance at Intersection part :-2 #HighwayEngineering](https://www.youtube.com/watch?v=ZtB961jRnS0) — Advance Civil Tech
- 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-81c-intersection-sd
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

A 4 lane major highway passing through rolling terrain in heavy rain fall area with a 2% grade intersects a 2 lane minor highway with 3% grade at an uncontrolled right angled intersection. Determine the minimum intersection sight distance required given the design speeds of major and minor approaches are 80 kmph and 60 kmph respectively.

## Short answer

For an uncontrolled right-angle intersection with major road at 80 km/h (2% grade) and minor road at 60 km/h (3% grade), minimum intersection sight distance equals stopping sight distance on each approach. Using t=2.5 s, f=0.35, and downgrade case: major road SSD = 132 m, minor road SSD = 86 m. A sight triangle with legs 132 m and 86 m must be kept clear of obstructions.

## Answer

**Given:** Major road: $V_1 = 80$ km/h, grade 2%. Minor road: $V_2 = 60$ km/h, grade 3%. Uncontrolled right-angle intersection.

**Principle:** At an uncontrolled intersection each driver must be able to see the other vehicle in time to stop. The **approach sight triangle** is therefore formed with legs equal to the **stopping sight distance** of each approach (IRC/AASHTO). The grade is included in the braking term.

**Assumptions:** reaction time $t = 2.5$ s; $f = 0.35$ (NRS/IRC design value; no further reduction is made for the rain); the approach is taken on the downgrade, which is the worse case:
$$
SSD = 0.278\,V\,t + \frac{V^2}{254\,(f - G)}
$$

#### Major road (80 km/h, G = 2%)
$$
SSD_1 = 0.278(80)(2.5) + \frac{80^2}{254(0.35-0.02)} = 55.6 + 76.35 = 132.0\ \text{m}
$$

#### Minor road (60 km/h, G = 3%)
$$
SSD_2 = 0.278(60)(2.5) + \frac{60^2}{254(0.35-0.03)} = 41.7 + 44.3 = 86.0\ \text{m}
$$

**Answer:** the minimum intersection sight distance is **132 m along the major road and 86 m along the minor road** (a sight triangle with legs 132 m and 86 m must be kept clear of obstructions). If the approaches are upgrades the values would be 123.7 m and 79.0 m; on level ground, 127.6 m and 82.2 m.

## Check yourself

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

1. At an uncontrolled intersection, what are the legs of the approach sight triangle?
   A. The stopping sight distances of each approach
   B. The overtaking sight distances of each approach
   C. The widths of each road
   D. The lane widths
   Answer: A. Each driver must see the other vehicle in time to stop, so the legs equal the SSD of each approach. (Key point: Principle of approach sight triangle)

2. Which grade condition is taken as the worst case for SSD?
   A. Level ground
   B. Upgrade
   C. Downgrade, as braking distance is longest
   D. Grade has no effect
   Answer: C. SSD = 0.278Vt + V^2/(254(f - G)) on the downgrade, giving the largest distance. (Key point: Worst case for grade)

3. What is SSD for the major road (80 km/h, G = 2% down, f = 0.35, t = 2.5 s)?
   A. About 86 m
   B. About 132 m
   C. About 56 m
   D. About 190 m
   Answer: B. 55.6 + 80^2/(254 x 0.33) = 55.6 + 76.35 = 132 m. (Key point: Major road SSD)

4. Minor road (60 km/h, G = 3% down) SSD is
   A. About 132 m
   B. About 41.7 m
   C. About 86 m
   D. About 123 m
   Answer: C. 41.7 + 3600/(254 x 0.32) = 41.7 + 44.3 = 86 m. (Key point: Minor road SSD)

## Related in Transportation Engineering I

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