← Transportation Engineering · DSSSB JE Civil

Chapter 10 of 12

Traffic Engineering

In the DSSSB JE Civil syllabus under Transportation Engineering · 2 parts

Traffic Characteristics, Studies & Flow Theory · Traffic Control — Signs, Markings, Signals & Intersections

CIVILGYAN TEST SERIES · civilgyantests.com — free study notes for civil engineering exams

📑 Contents (24 sections)

Part 1 of 2

Traffic Characteristics, Studies & Flow Theory

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.

Part 2 of 2

Traffic Control — Signs, Markings, Signals & Intersections

Last reviewed 16 Sept 2026 · 11 min read

Traffic control devices

Traffic control devices regulate, warn and guide road users. Requirements: they must fulfil a need, command attention, convey a clear, simple meaning, command respect and give adequate time for response. Main types: signs, markings, signals and islands, plus speed control devices and barriers.

Traffic signs (IRC:67)

Category	Purpose	Shape and colour (typical)	Examples
Regulatory (mandatory) signs	Inform road users of laws and regulations; violation is an offence	Circular — prohibitory signs: red border, white background, black symbols (with red diagonal bar for some); compulsory signs: blue background with white symbols	No entry, no parking, speed limit, no overtaking, compulsory turn left, compulsory cycle track
Stop sign	Stop before proceeding	Octagonal, red with white "STOP"	Minor road at major road junction
Give way sign	Yield to traffic on the major road	Inverted equilateral triangle with red border	Minor road approaches
Warning (cautionary) signs	Warn of hazards ahead	Equilateral triangle, apex upwards, red border, white background, black symbols	Right/left hand curve, hairpin bend, narrow bridge, school ahead, pedestrian crossing, slippery road, steep ascent
Informatory (guide) signs	Direction, destination, distance and facilities	Rectangular — green background for highways (direction/destination), blue for other roads and facility signs	Direction signs, place identification, parking, hospital, petrol pump, rest area

Placement: on the left side of the road; at a suitable distance ahead of the hazard (greater for high speeds); at a height giving clearance above the carriageway/footpath; retro-reflective sheeting for night visibility; not cluttered or obscured by trees.

Road markings (IRC:35)

Markings on the pavement surface guide and control traffic, supplementing signs.

Type	Examples and meaning
Longitudinal markings	Centre lines — broken (overtaking permitted), continuous (no crossing); double continuous lines (no overtaking, often yellow); lane lines; edge lines; no-overtaking zone markings; barrier lines
Transverse markings	Stop line, give way line, pedestrian crossing (zebra), cycle crossing
Hazard and object markings	Approaches to obstructions (bridge piers, islands), chevron markings, kerb markings (alternate black and white on kerbs of islands)
Arrow and word markings	Turn arrows, "STOP", "BUS", "SCHOOL"
Parking markings, bus stop markings, speed breaker markings	Bays, no-parking zones, hump markings

Colours: white for most markings; yellow for no-parking, no-overtaking/barrier lines and some kerb markings. Materials: road marking paint, hot-applied thermoplastic (durable, common on highways), cold plastic, preformed tapes; retro-reflective glass beads; raised pavement markers (road studs, "cat's eyes").

Traffic signals

Advantages and disadvantages

Advantages	Disadvantages
Orderly movement; assign right of way	Increase rear-end collisions
Reduce right-angle and pedestrian accidents	Can increase delays when poorly timed or at low volumes
Increase capacity of intersections (with proper timing)	Encourage use of alternate routes; cost of installation and maintenance
Allow pedestrians and minor-road traffic to cross	Failures cause confusion
Can be coordinated for progressive movement

Types of signals

Traffic control signals:
- Fixed-time (pre-timed) — fixed cycle and phase durations.
- Traffic-actuated — timing varies with detector inputs: semi-actuated (detectors on minor road) and fully actuated (all approaches).
- Adaptive / area traffic control — timings optimised in real time over a network (e.g. SCOOT, SCATS types).
Pedestrian signals — WALK/DON'T WALK (walking man symbols) with countdown timers.
Special signals — flashing beacons (amber for caution, red flashing for stop), lane-use signals, ramp metering.

Coordination of signals

System	Operation
Simultaneous system	All signals along a road show the same indication at the same time
Alternate system	Adjacent signals show opposite indications
Simple progressive system	Offsets set so that a vehicle at a given speed meets successive green lights (green wave)
Flexible progressive system	Cycle, splits and offsets vary with time of day and traffic

Signal warrants (IRC:93)

Signals are installed only when warranted, e.g. by: minimum vehicular volume, interruption of continuous traffic (minor road traffic cannot cross the major road), minimum pedestrian volume, accident experience, and combination of the above.

Signal design terms

Cycle — one complete sequence of indications; phase — part of the cycle allotted to one set of movements.
Green, amber (change/clearance, typically about 2–4 s) and red intervals; all-red clearance.
Lost time — start-up lost time and clearance lost time per phase.
Saturation flow $s$ — maximum discharge rate from a queue during green; Webster's approximation for approach width $w$ (5.5–18 m): $s \approx 525 w$ PCU/h.
Effective green = actual green + amber − lost time.

FormulaWebster's method of optimum signal cycle

Optimum cycle length:

C_{0} = \frac{1.5 L + 5}{1 - Y}

$L$ = total lost time per cycle (s); $Y = \sum y_{i}$ , $y_{i} = \frac{q _{i}}{s _{i}}$ = ratio of design flow to saturation flow for the critical approach of phase $i$ .

Effective green time for phase $i$ :

g_{i} = \frac{y _{i}}{Y} (C_{0} - L)

Pedestrian green time — enough to cross the carriageway at walking speed (IRC uses about 1.2 m/s) plus an initial interval (about 7 s).

Other methods (IRC:93): trial cycle method, approximate method based on pedestrian crossing requirements, HCM method.

Traffic islands

Raised areas that guide and protect traffic and pedestrians:

Divisional islands — separate opposing streams (medians).
Channelising islands — guide traffic into proper paths at intersections.
Pedestrian loading islands (refuge islands) — safe waiting space for pedestrians/bus passengers.
Rotary (central) islands — at roundabouts.

Intersections

Conflict points

Intersection (two-way roads)	Crossing conflicts	Merging	Diverging	Total
Four-legged	16	8	8	32
Three-legged (T)	3	3	3	9

Crossing conflicts are the most dangerous; intersection design aims to reduce and separate conflicts.

Types of intersections

At-grade intersections:

Unchannelised — simple, for low volumes.
Channelised — islands and markings define paths for each movement (turning lanes, flared approaches).
Rotary (roundabout) — vehicles circulate one way around a central island, converting crossing conflicts into weaving manoeuvres.
Signalised intersections.

Grade-separated intersections (interchanges): flyovers and underpasses separate crossing movements.

Interchange	Features
Trumpet	For three-legged (T/Y) junctions
Diamond	Major road grade-separated; minor road has at-grade junctions with ramps — for major–minor crossings; economical
Cloverleaf (full)	Four loops and four outer ramps — eliminates all crossing conflicts; needs large area; weaving between loops
Partial cloverleaf	Some loops omitted — suits site constraints
Directional interchange	Direct ramps for heavy turning volumes — costly, multi-level
Rotary interchange	A roundabout above or below the through road

Rotary intersections (IRC:65)

Merits and demerits

Merits	Demerits
Orderly, continuous flow; no stopping required	Needs large area — difficult in built-up areas
Crossing conflicts eliminated (replaced by weaving) — fewer severe accidents	Not suitable for very high volumes (weaving capacity limits) or very low volumes
No signal control cost	Longer travel distance for all; not convenient for heavy pedestrian traffic
Suitable for five or more approaching roads	Right-turning traffic has to travel around

Rotaries are suitable for total intersection volumes of about 500 to 3000 vehicles per hour with a significant share of turning traffic.

Design elements

Design speed: about 40 km/h (rural) and 30 km/h (urban).
Shape of central island: circular, elliptical, turbine or tangent shapes.
Radius of central island: commonly about 1.33 times the radius of the entry curve.
Weaving length: about 45–90 m (rural) and 30–60 m (urban).
Width of carriageway at entry and exit (depends on traffic and lanes).

FormulaWidth and capacity of a rotary weaving section

Width of weaving section:

w = \frac{e _{1} + e _{2}}{2} + 3.5 m

$e_{1}$ = entry width; $e_{2}$ = width of the non-weaving section/exit.

Practical capacity (Wardrop's formula, IRC:65):

Q_{p} = \frac{280 w ( 1 + \frac{e}{w} ) ( 1 - \frac{p}{3} )}{1 + \frac{w}{l}}

$e$ = average entry width $= (e_{1} + e_{2}) /2$ ; $l$ = weaving length; $p$ = proportion of weaving traffic $= \frac{b + c}{a + b + c + d}$ , where $a$ , $d$ = non-weaving traffic and $b$ , $c$ = weaving traffic in the section.

Downloaded from civilgyantests.com · Not for resale. Verify every code clause and date against the official source.

Finished reading? Test yourself.

A timed chapter test from the DSSSB JE Civil series, on exactly this chapter.

Practice this chapter →