---
title: "Need of crash studies; highway lighting design"
subject: "Transportation Engineering II"
chapter: 1
key: "semester_7/transportation_engineering_ii:crash-studies-need-highway-lighting-design"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/crash-studies-need-highway-lighting-design
site: BCT Engineering
---

# Need of crash studies; highway lighting design

Transportation Engineering II · Chapter 1: Traffic Engineering

- Asked in: 2080 Baisakh, 2+6 marks
- Video: [STREET LIGHTING ~ Traffic Engineering (2170613) GTU](https://www.youtube.com/watch?v=nmIu1sU0iSc) — vatsal patel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/crash-studies-need-highway-lighting-design
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-1

## Question

Mention the need of crash studies. Discuss the steps to design highway lighting system.

## Short answer

Crash studies identify crash extent, black spots, causes and cost, guide remedial measures, and evaluate safety improvements. Lighting design: classify road and fix illuminance, choose arrangement, mounting height and luminaire, compute spacing S = F U_f M/(E W), check uniformity and glare, locate poles, and do electrical design.

## Answer

#### Need of crash studies
- To find the extent, trend, severity and types of crashes and their cost to the nation.
- To identify **black spots** and the contributing causes (human, vehicle, road) so that money is spent where it saves the most lives.
- To design suitable remedial measures (engineering, education, enforcement) and rank them by cost-benefit.
- To judge the effect of measures through before-and-after studies, and for road safety audit and policy (for example Nepal's Road Safety Action Plan and the DoR black-spot programme).
- For legal, insurance and research purposes.

#### Steps to design a highway lighting system
1. **Classify the road and fix the need:** arterial, collector or local; traffic volume, speed, crash record, intersections, bridges, curves and pedestrian crossings. Fix the required average illuminance and uniformity (IS 1944: about 30 lux on major roads, 15 lux on other main roads, 4-8 lux on secondary and residential roads; uniformity min/avg $\ge$ 0.4 on main roads and 0.3 on minor roads).
2. **Choose the arrangement:** single side, staggered, opposite or central (twin-arm on the median) according to the road width.
3. **Select the mounting height $H$ and luminaire type:** cut-off or semi-cut-off to limit glare; $H$ about 8-12 m on highways, 6-8 m on local roads.
4. **Select the lamp:** LED (now preferred), high-pressure sodium or metal halide; get its lumen output $F$.
5. **Find the utilisation factor $U_f$ and maintenance factor $M$** (0.7-0.8).
6. **Compute the spacing** by the lumen method:

$$
S = \frac{F \, U_f \, M}{E\, W}
$$

7. **Check** $S/H$ (not more than about 3 for cut-off and 3.5 for semi-cut-off luminaires), the uniformity ratio and glare; adjust the lamp or height if needed.
8. **Locate the poles:** lateral offset at least 0.6 m behind the kerb (1.5 m or more on high-speed roads), use breakaway or frangible poles, and place additional lights at junctions and curves.
9. **Electrical design and economics:** cable size, voltage drop, switching (timer or photocell), power and maintenance cost.

## Check yourself

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

1. What is a key purpose of crash studies regarding black spots?
   A. To collect fines
   B. To identify black spots and causes so money is spent where it saves the most lives
   C. To remove all signs
   D. To count vehicles
   Answer: B. Crash studies locate hazardous spots and contributing human, vehicle and road causes. (Key point: Black spot identification)

2. Before-and-after studies of crashes are used to:
   A. Judge the effect of remedial measures
   B. Set vehicle speed limits only
   C. Count pedestrians
   D. Calculate lamp spacing
   Answer: A. They test whether a remedial measure actually reduced crashes. (Key point: Before-and-after studies)

3. In the lumen method, lamp spacing is S = ?
   A. F x Uf x M / (E x W)
   B. E x W / (F x Uf x M)
   C. F x E / (W x M)
   D. H x E / Uf
   Answer: A. S = F Uf M / (E W): lamp lumens, utilisation and maintenance factors over required illuminance and road width. (Key point: Lumen method spacing)

4. Which check is made after computing the spacing?
   A. S/H should be 3 to 5, with uniformity and glare checked
   B. S/H should be 0.5
   C. S must equal road width
   D. Mounting height must be 1 m
   Answer: A. The spacing-to-height ratio of 3-5, uniformity ratio (min/avg at least 0.3) and glare are checked, then lamp or height adjusted. (Key point: Spacing check)

5. Why are breakaway or frangible poles and a lateral offset used?
   A. To reduce lamp cost
   B. To lessen severity of run-off-road crashes into poles
   C. To increase glare
   D. To raise illuminance
   Answer: B. Poles are set back from the kerb (at least 0.6 m, 1.5 m or more on high-speed roads) and made frangible to reduce crash severity. (Key point: Pole location safety)

## Related in Transportation Engineering II

- [Street lighting importance and design factors](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Astreet-lighting-importance-design-factors) — Transportation Engineering II, chapter 1
- [Pavement design factors; IRC rigid design steps](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Apavement-factors-irc-rigid-steps) — Transportation Engineering II, chapter 2, asked 2×
- [At-grade intersection requirements; rotary design steps](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aintersection-requirements-rotary-design-steps) — Transportation Engineering II, chapter 1
- [At-grade intersection requirements; street lighting importance](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aintersection-requirements-street-lighting-importance) — Transportation Engineering II, chapter 1
- [Plain CC pavement thickness by IRC, 7 m highway](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aplain-cc-pavement-thickness-irc-7m) — Transportation Engineering II, chapter 2
- [Ideal bridge site; tunnel ventilation need and methods](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aideal-bridge-site-tunnel-ventilation) — Transportation Engineering II, chapter 5

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