Traffic Control — Signs, Markings, Signals & Intersections — Transportation Engineering Notes

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.

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Traffic Control — Signs, Markings, Signals & Intersections