---
title: "Short note: thirtieth highest hourly volume"
subject: "Transportation Engineering II"
chapter: 1
key: "semester_7/transportation_engineering_ii:short-note-thirtieth-highest-hourly-volume"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/short-note-thirtieth-highest-hourly-volume
site: BCT Engineering
---

# Short note: thirtieth highest hourly volume

Transportation Engineering II · Chapter 1: Traffic Engineering

- Asked in: 2070 Chaitra (old course), 4 marks
- Video: [30th highest hourly volume, Traffic Engineering](https://www.youtube.com/watch?v=e-nW9DI-uB0) — Civil Engineering & Stuff
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/short-note-thirtieth-highest-hourly-volume
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-1

## Question

Write a short note on thirtieth highest hourly traffic volume.

## Short answer

The 30th highest hourly volume is the hourly volume exceeded only 29 hours per year, found by ranking all 8760 hourly counts. It represents the knee of the curve where further design increase saves little, balancing economy and congestion, and is used as the design hourly volume for rural roads, typically 8–15% of AADT.

## Answer

The **thirtieth highest hourly volume** (30 HV) is the hourly traffic volume that is exceeded in only 29 hours of the year. It is obtained by counting the traffic in every hour of the year (8,760 hours), arranging the hourly volumes in descending order, and taking the 30th value.

```text
 Hourly volume
 (% of AADT)
 |*
 | *
 |   *
 |      *___        <- knee at about the 30th hour
 |          `--.___
 +--------------------> hours (ranked)
   1   30    100   ...  8760
```

**Why it is used**
- On a graph of ranked hourly volume (as a percentage of AADT) against rank, the curve is steep up to about the 30th hour and then flattens (the "knee"). Designing for an hour beyond the knee saves very little, because the volumes there are nearly the same, while designing for one of the highest hours (for example the 1st or 5th) is uneconomical because the road would be under-used almost all the time.
- Designing for the 30th hour means the road is congested for fewer than 30 hours in a year, a reasonable economic balance.
- For rural roads the 30th highest hourly volume is taken as the **design hourly volume (DHV)**. It is commonly about 8-15 percent of the AADT (the factor $K$), and the capacity (design service volume) of the road is fixed to carry it:

$$
DHV = K \times AADT
$$

- It is also used in highway planning in Nepal (Nepal Road Standard 2070) and in IRC practice (IRC:SP:19) to fix the number of lanes, while for urban roads the peak hour volume and PHF are used instead.

## Check yourself

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

1. What is the 30th highest hourly volume?
   A. Volume exceeded in only 29 hours of the year
   B. Volume exceeded in 30 days
   C. Average of the top 30 hours
   D. 30 percent of AADT
   Answer: A. Hourly volumes of the year are ranked in descending order and the 30th value is taken. (Key point: Definition)

2. Why is the 30th hour chosen for design?
   A. It is the peak of the year
   B. It is at the knee of the curve, balancing economy and congestion
   C. It is the lowest hourly volume
   D. It is easiest to count
   Answer: B. The curve flattens near the 30th hour: higher design is uneconomical and lower gives little benefit. (Key point: Why the 30th hour)

3. How is design hourly volume obtained?
   A. DHV = AADT / K
   B. DHV = K x AADT, with K about 8-15 percent for rural roads
   C. DHV = AADT x 365
   D. DHV = PHF x 24
   Answer: B. DHV = K x AADT. (Key point: DHV formula)

## Related in Transportation Engineering II

- [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
- [Flexible pavement life with 140 cvpd traffic](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-life-years-140-cvpd) — Transportation Engineering II, chapter 2
- [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

## Related in other subjects

- [30th hour volume and design controls](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Adesign-volume-controls) — Transportation Engineering I, chapter 3