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

# Short note: traffic flow parameters

Transportation Engineering II · Chapter 1: Traffic Engineering

- Asked in: 2066 Bhadra, 4 marks
- Video: [Relation between fundamental traffic flow parameters, Flow, Speed and Density, A simple explanation](https://www.youtube.com/watch?v=MOxya1UVxgc) — Transportation Engg Talks | Satish IITR
- 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-traffic-flow-parameters
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-1

## Question

Write a short note on traffic flow parameters.

## Short answer

Traffic flow parameters are volume (q, veh/h), speed (time mean, space mean), and density (k, veh/km), linked by q = k·v_s. Microscopic parameters include headway (h=3600/q s) and spacing (s=1000/k m); capacity is q_max = v_f·k_j/4.

## Answer

Traffic flow is described by three basic macroscopic parameters, which are related to each other, and several microscopic ones.

**1. Flow or volume ($q$):** the number of vehicles passing a section per unit time, in veh/h or PCU/h. Related measures: AADT, peak hour volume, design hourly volume, PHF and rate of flow.

**2. Speed ($v$):** the distance covered per unit time. 
- *Spot speed:* at a point.
- *Time mean speed:* average of spot speeds, $\bar v_t = \frac{1}{n}\sum v_i$.
- *Space mean speed:* average over a length, $\bar v_s = \frac{n}{\sum 1/v_i} = L/\bar t$.
- *Running speed* (excluding stops) and *journey speed* (including stops).

**3. Density ($k$):** the number of vehicles per unit length of road, in veh/km. It is related to occupancy.

**Fundamental relation**

$$
q = k\,\bar v_s
$$

With a linear model $v = v_f(1 - k/k_j)$ the capacity is $q_{max} = v_f k_j/4$ at $k = k_j/2$.

**4. Time headway ($h$) and space headway or spacing ($s$):** $h = 3600/q$ and $s = 1000/k$; they describe how vehicles are spaced in the stream.

**5. Other parameters:** capacity, level of service, v/c ratio, gap, clearance, platoon size and composition.

These parameters are measured by traffic surveys (volume count, spot speed, floating car) and used for design of roads and signals.

## Check yourself

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

1. How is space mean speed computed from n spot speeds v_i?
   A. Arithmetic mean of v_i
   B. n divided by the sum of (1/v_i)
   C. Maximum of v_i
   D. Sum of v_i divided by distance
   Answer: B. Space mean speed is the harmonic mean, n / sum(1/v_i) = L / mean travel time; the arithmetic mean is the time mean speed. (Key point: Space mean speed)

2. With v = vf(1 - k/kj), where does the maximum flow occur and what is its value?
   A. At k = kj, q = vf kj
   B. At k = kj/2, q = vf kj/4
   C. At k = 0, q = vf
   D. At k = kj/4, q = vf kj/2
   Answer: B. q = vf k(1 - k/kj) is maximum at k = kj/2, giving qmax = vf kj/4. (Key point: Capacity in linear model)

3. If flow q is in veh/h and density k in veh/km, the time headway and spacing are:
   A. h = q/3600 and s = k/1000
   B. h = 3600/q and s = 1000/k
   C. h = 1000/q and s = 3600/k
   D. h = q x 3600 and s = k x 1000
   Answer: B. Average time headway is h = 3600/q seconds and space headway s = 1000/k metres. (Key point: Headway and spacing)

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