Traffic Characteristics, Studies & Flow Theory Notes

Last reviewed 16 Sept 2026 · 10 min read

Traffic engineering

Traffic engineering deals with the planning, geometric design and traffic operations of roads, streets and highways — their networks, terminals and abutting lands — for safe, convenient and economic movement of people and goods.

Traffic safety is traditionally addressed by the three Es — Engineering, Enforcement and Education (often extended with Environment and Emergency care).

Road user characteristics

Physical: vision, hearing, strength, reaction time.
Mental: knowledge, skill, experience, intelligence.
Psychological: attentiveness, fear, anger, maturity, impatience.
Environmental: traffic stream, atmosphere, facilities.

PIEV theory

The total perception–reaction time of a driver is made up of:

Perception — time to see or perceive an object.
Intellection — time to understand the situation.
Emotion — time for emotional reactions (fear, anger, superstition, etc.).
Volition — time to decide and initiate action.

It varies from about 0.5 s to 4 s or more with conditions; IRC uses a total reaction time of 2.5 s for stopping sight distance.

Vision: the cone of clear vision is narrow (a few degrees), fairly clear vision extends to about 10–12°, and peripheral vision is wider but less clear — signs must be placed within the driver's clear field of view.

Vehicular characteristics

Static: dimensions (width, length, height), weight, axle configuration, turning radius — affect lane width, clearances, pavement design and curve widening.
Dynamic: speed, acceleration and deceleration, braking performance, power — affect sight distances, gradients, intersection design.
Resistances: rolling, air, grade, curve and inertia resistances determine power requirements.

Traffic volume studies

Traffic volume — number of vehicles crossing a section per unit time.

Term	Meaning
ADT	Average daily traffic over a period
AADT	Annual average daily traffic (365-day average)
Design hourly volume (DHV)	Hourly volume used for design — commonly the 30th highest hourly volume of the year
Peak hour factor (PHF)	$P H F = \frac{peak hourly volume}{4 \times peak 15-minute volume}$ (≤ 1)
Directional distribution	Split of traffic by direction in the peak hour
Passenger car unit (PCU)	Converts mixed traffic into equivalent passenger cars

Methods: manual counts (tally sheets), mechanical/automatic counters (pneumatic tubes, inductive loops), video and camera-based counting, radar and infrared sensors, moving observer method.

Uses: planning, geometric and pavement design, signal design, capacity analysis, traffic trends, economic evaluation.

PCU values (IRC)

Typical PCU equivalency factors for rural roads (IRC:64) include: passenger car/jeep/van 1.0; motorcycle/scooter 0.5; light commercial vehicle 1.5; truck or bus 3.0; truck–trailer/tractor–trailer 4.5; bicycle 0.5; cycle rickshaw 2.0; bullock cart 8.0. For urban roads (IRC:106), PCU values vary with the percentage of each vehicle type in the traffic stream (ranges are given; e.g. two-wheelers about 0.5–0.75, buses and trucks about 2.2–3.7). Indo-HCM (2017) gives dynamic PCU values from Indian field studies.

Speed studies

Spot speed study

Speed of vehicles at a particular location — measured by radar guns, enoscope (mirror box over a known base length), pressure contact tubes, video.

Cumulative speed distribution is used to find percentile speeds:

Percentile speed	Use
85th percentile	Setting the maximum speed limit
98th percentile	Geometric design speed
15th percentile	Minimum speed limit (on expressways)
50th percentile	Median speed

Modal speed (most frequent) and pace (the 10 km/h range containing the most vehicles) are also reported.

Time mean speed and space mean speed

FormulaMean speeds

Time mean speed — arithmetic mean of spot speeds at a point:

v_{t} = \frac{\sum v _{i}}{n}

Space mean speed — harmonic mean of spot speeds (average speed over a length of road):

v_{s} = \frac{n}{\sum \frac{1}{v _{i}}}

Relation: $v_{t} = v_{s} + \frac{σ _{s}^{2}}{v _{s}}$ → $v_{t} \geq v_{s}$

Space mean speed is used in traffic flow relations ( $q = k v_{s}$ ).

Speed and delay studies

Give running speed, journey (overall) speed and the location, cause and duration of delays along a route.

Journey speed = distance ÷ total journey time (including stops).
Running speed = distance ÷ running time (excluding stopped delays).

Methods: floating car (moving observer) method, license plate method, interview technique, elevated observations, GPS probe vehicles.

FormulaFloating car (moving observer) method

The test car travels with the stream in direction N and against it in direction S.

q_{n} = \frac{x _{s} + y _{n}}{t _{s} + t _{n}}, \overset{ˉ}{t}_{n} = t_{n} - \frac{y _{n}}{q _{n}}

$q_{n}$ = flow in direction N
$x_{s}$ = number of vehicles met while the test car travels in direction S (opposite)
$y_{n}$ = vehicles overtaking the test car minus vehicles overtaken by it, while travelling in direction N
$t_{s}$ , $t_{n}$ = journey times of the test car in directions S and N
$\overset{ˉ}{t}_{n}$ = mean journey time of the stream in direction N; mean speed = length ÷ $\overset{ˉ}{t}_{n}$

Origin–destination (O–D) studies

Determine where trips start and end, their purpose, mode and route — used for planning new roads, bypasses, public transport and parking.

Methods: roadside interview, license plate method, return postcard method, tag-on-car method, home interview surveys, workplace surveys, mobile phone and GPS data.

Presentation: O–D matrix (trip table) and desire lines (straight lines between origins and destinations with thickness proportional to trips).

Parking studies

In–out survey, fixed period sampling, license plate method (records duration).
Parameters: parking accumulation (vehicles parked at a time), parking volume (total vehicles parked in a period), parking load (area under accumulation curve, vehicle-hours), parking duration, parking turnover = parking volume ÷ number of bays, parking index (occupancy) = accumulation ÷ capacity × 100.

Accident studies

Objectives: identify black spots, causes, and remedial measures; evaluate safety measures.
Records: first information reports, accident report forms; collision diagrams (show type and path of vehicles) and condition diagrams (show roadside features).
Causes: road user (speeding, drunk driving, fatigue, distraction), vehicle (brake or tyre failure, lighting), road (poor geometry, surface, signs, drainage), environment (fog, rain).
Accident rates:

Rate per 100 million vehicle-km = \frac{A \times 1 0 ^{8}}{365 \times T \times A D T \times L}

( $A$ = accidents in $T$ years on a section of length $L$ km.)

Road safety audit (IRC:SP:88) at design, construction and operation stages.

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Traffic Characteristics, Studies & Flow Theory