---
title: "Short note: spacing and headway"
subject: "Transportation Engineering II"
chapter: 1
key: "semester_7/transportation_engineering_ii:short-note-spacing-headway"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/short-note-spacing-headway
site: BCT Engineering
---

# Short note: spacing and headway

Transportation Engineering II · Chapter 1: Traffic Engineering

- Asked in: 2068 Baisakh, 4 marks
- Video: [Traffic Flow, Density, Headway, and Speed | NCEES Civil Engineering PE Exam [Section 5.1.1.1]](https://www.youtube.com/watch?v=AN85spukBoc) — Daniel Findley
- 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-spacing-headway
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-1

## Question

Write a short note on spacing and head way.

## Short answer

Spacing is distance between successive vehicle fronts (s=1000/k m); headway is time between fronts at a point (h=3600/q s). They are related by speed: s = v·h. Gap is time between rear and next front.

## Answer

**Spacing** is the distance between the front of one vehicle and the front of the next vehicle in the same lane, measured in metres. Average spacing $s$ is related to density $k$ (veh/km) by

$$
s = \frac{1000}{k}\ \text{m}
$$

**Headway** is the time between the passage of the front of one vehicle and the front of the next vehicle at a fixed point, in seconds. Average headway $h$ is related to flow $q$ (veh/h) by

$$
h = \frac{3600}{q}\ \text{s}
$$

Spacing and headway are linked by speed: $s = v\,h$ ($v$ in m/s). 

```text
 |<------- spacing (m) ------->|
 [==== car 1 ====]     <-gap->  [==== car 2 ====]
 front              rear        front
 |<------ headway (s) at a point ------>|
```

- The **gap** is the time between the rear of one vehicle and the front of the next, and the **clearance** is the corresponding distance. So $h = $ gap + (vehicle length / speed) and $s = $ clearance + vehicle length.
- Headways are used in capacity (saturation headway about 2 s per vehicle at a signal gives 1800 veh/h), safety and queue analysis; short headways (below about 1 s) are unsafe. Spacing is used to compute density and to find the storage space needed for queues.
- For random arrivals the headways follow a negative exponential distribution.

## Check yourself

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

1. Average spacing s (m) from density k (veh/km)?
   A. s = 3600/k
   B. s = k/1000
   C. s = 1000/k
   D. s = 1000 k
   Answer: C. s = 1000/k metres. (Key point: Spacing)

2. Average headway h (s) from flow q (veh/h)?
   A. h = 3600/q
   B. h = q/3600
   C. h = 1000/q
   D. h = 60 q
   Answer: A. h = 3600/q seconds. (Key point: Headway)

3. How are spacing and headway linked?
   A. s = h / v
   B. s = v x h (v in m/s)
   C. s = h + v
   D. They are unrelated
   Answer: B. Distance = speed x time gives s = v h. (Key point: Relation)

4. A saturation headway of 2 s per vehicle corresponds to what flow?
   A. 900 veh/h
   B. 3600 veh/h
   C. 1800 veh/h
   D. 2 veh/h
   Answer: C. 3600/2 = 1800 veh/h. (Key point: Saturation headway)

## Related in other subjects

- [Canal head regulator design, Q 150](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-head-regulator-design-q150) — Irrigation and Drainage Engineering, chapter 7