---
title: "30th hour volume and design controls"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:design-volume-controls"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/design-volume-controls
site: BCT Engineering
---

# 30th hour volume and design controls

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2065 Kartik (old course), 8 marks
- Video: [30th Highest Hourly Volume | Highway Engineering #2 | Civil Engineering Concepts | By Kamalakar Sir](https://www.youtube.com/watch?v=tORVnyFqiiE) — Utkarsh Engineers Classes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/design-volume-controls
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

A 30th hourly volume is generally accepted as the design volume of traffic, why? What are the other design controls for the geometrics of roads? Discuss in brief.

## Short answer

The 30th highest hourly volume is used as design volume because it represents the knee of the volume-frequency curve, balancing cost and congestion. Other geometric design controls include design speed, topography, traffic volume and composition, design vehicle, human factors, environmental factors, economic factors, and road classification.

## Answer

#### Why the 30th hour volume is used as the design volume
If the hourly traffic volumes of a year (8760 hours) are arranged in descending order and plotted as a percentage of the AADT, the curve falls steeply at first and then flattens, forming a **knee** at about the 30th hour (the 30th highest hourly volume is about 12-18% of AADT on rural roads and 8-12% on urban roads).

```text
 Hourly volume
 (% AADT) |\
          | \
          |  \_______ knee at ~30th hour
          |          ------------
          +---------------------- hours (descending)
```
- Designing for the **highest hourly volume** would make the road very wide and costly, yet that capacity would be used for only a few hours in a year.
- Designing for a much **lower** volume (e.g. the 100th hour) would cause congestion during many more hours.
- To the left of the knee, the volume rises steeply, so designing for an earlier hour greatly increases the cost while serving only a few extra hours. To the right of the knee, the curve is flat, so a later hour saves little. Hence the 30th highest hourly volume is a good **economic balance:** it is exceeded in only 29 hours of the year.

#### Other design controls (briefly)
1. **Design speed:** fixes the radius, sight distance, superelevation and gradients.
2. **Topography:** plain, rolling, hilly, mountainous.
3. **Traffic volume and composition:** number of lanes and capacity (PCU), proportion of trucks.
4. **Design vehicle:** its dimensions and turning path decide the lane width, radius and widening.
5. **Human factors:** reaction time, eye height, driver behaviour.
6. **Environmental factors:** noise, air pollution, aesthetics, land use.
7. **Economic factors:** available funds, benefit-cost.
8. **Road class and safety:** NH, feeder, district roads (NRS 2070).

## Check yourself

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

1. Why is the 30th hourly volume taken as design volume?
   A. It is the highest hourly volume of the year
   B. It is the average hourly volume
   C. It lies at the knee of the curve and is an economic balance, exceeded only 29 hours a year
   D. It is the lowest hourly volume
   Answer: C. At the knee, higher design hours add little volume reduction while lower hours cause sharply more congested hours. (Key point: 30th hour)

2. Why not design for the highest hourly volume?
   A. It would be too narrow
   B. It makes the road very wide and costly but is used for only a few hours
   C. It is illegal
   D. It causes lower speeds
   Answer: B. A road for the peak hour would be uneconomic as it is used only a few hours a year. (Key point: Maximum hour)

3. Design speed fixes which of the following?
   A. Only the lane colour
   B. Only the camber
   C. Radius, sight distance, superelevation and gradients
   D. Only the right of way
   Answer: C. Many geometric elements are derived from design speed. (Key point: Design speed)

4. The design vehicle decides
   A. lane width, radius and widening based on its dimensions and turning path
   B. the 30th hour
   C. the camber only
   D. the AADT
   Answer: A. Dimensions and turning path of the design vehicle govern lane width, curve radius and widening. (Key point: Design vehicle)

## Related in Transportation Engineering I

- [Design controls for valley curves](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Avalley-design-controls) — Transportation Engineering I, chapter 3
- [Design and construction problems of hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-problems) — Transportation Engineering I, chapter 5, asked 11×
- [Factors controlling geometric design](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ageometric-design-factors) — Transportation Engineering I, chapter 3, asked 7×
- [Special considerations in hill road design](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-special-considerations) — Transportation Engineering I, chapter 5, asked 5×

## Related in other subjects

- [Short note: thirtieth highest hourly volume](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Ashort-note-thirtieth-highest-hourly-volume) — Transportation Engineering II, chapter 1
- [Peak hour volume and PHF from counts](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Apeak-hour-volume-phf-urban-approach) — Transportation Engineering II, chapter 1
- [Signal design: roads X 15 m and Y 10.5 m](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Asignal-design-roads-x-15m-y) — Transportation Engineering II, chapter 1
- [Signal timing: roads A 15 m and B 12 m](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Asignal-pedestrian-road-a-15m-b) — Transportation Engineering II, chapter 1, asked 2×