Traffic engineering

Civil Engineering (B.E./B.Tech) · Transportation · free study notes for exams, placements and interviews

In short

Traffic engineering is the branch of transportation engineering that measures, analyses and controls the movement of vehicles and people on roads so that traffic flows safely, quickly and economically. It covers traffic studies (volume, speed, origin-destination, parking, accidents), flow theory, intersection capacity, signal and rotary design, signs, markings and regulation. In India it is governed by IRC codes (IRC 106, IRC 65, IRC 67, IRC 35, IRC SP 41, IRC SP 43) and the Motor Vehicles Act 1988 as amended in 2019. It carries high weightage in GATE Civil (Transportation Engineering) and in university exams, and is the daily work of road and urban-mobility consultancies, NHAI, state PWDs, municipal corporations and metro/BRT agencies.

Explained

Traffic characteristics and traffic studies

Road user and vehicle characteristics: reaction time (PIEV — Perception, Intellection, Emotion, Volition; taken as 2.5 s for stopping sight distance design), visual acuity, vehicle dimensions, power-to-weight ratio and braking. Wide variation in vehicle types makes Indian traffic heterogeneous (mixed), so PCU conversion is essential.
Traffic volume (flow) study: counts vehicles crossing a section per unit time. Methods — manual count, mechanical/pneumatic tube counters, inductive loops, video/ANPR cameras. Outputs: ADT, AADT, peak-hour volume, directional split, composition.
PCU (Passenger Car Unit): a common unit to add up mixed traffic. IRC 106-1990 equivalency factors (urban roads, for a share of about 5% in the stream) — car/taxi 1.0, two-wheeler 0.5, auto-rickshaw 1.2, LCV 1.4, bus/truck 2.2, cycle 0.4, cycle-rickshaw 1.5, handcart 2.0, tractor-trailer 4.0. Factors rise as the share of that vehicle type in the stream rises (at about 10% share: two-wheeler 0.75, auto-rickshaw and LCV 2.0, bus/truck 3.7, cycle 0.5, handcart 3.0).
Peak Hour Factor (PHF): PHF = hourly volume / (4 × peak 15-min volume). Values near 1.0 mean uniform flow; 0.80–0.95 is typical urban.
Spot speed study: speed at a point; methods — enoscope, radar/laser speed gun, pressure-contact strips, video. Used for speed limits, geometric design checks and accident analysis.
Speed percentiles from the cumulative frequency curve: 85th percentile → speed limit / safe speed; 98th percentile → geometric design speed; 15th percentile → lower speed limit for slow-vehicle restriction; 50th percentile (median) → average speed of the stream.
Speed and delay study: gives running speed, journey speed and delays. Methods — floating car (moving observer) method, licence plate method, elevated observation, GPS logger. Output plotted as speed-distance and delay diagrams; identifies bottlenecks.
Origin–Destination (O-D) study: road-side interview, return postcard, licence-plate matching, tag-on-vehicle, home interview. Results in an O-D matrix used for bypass alignment, corridor planning and desire-line diagrams.
Parking study: measures parking demand and supply. Key terms — parking accumulation (vehicles parked at a time), parking volume (total vehicles parked in a period), parking load (vehicle-hours), average parking duration, parking turnover (vehicles per bay per period), parking index/occupancy (%). Types: on-street (parallel, 30°, 45°, 60°, 90° angle) and off-street (surface lots, multi-storey, mechanised). Parallel parking takes the least width but the most kerb length per car; 90° parking stores the most cars per metre of kerb. Guidance: IRC SP 12 and IRC SP 43.
Accident (road safety) study: records collected in the MoRTH (Ministry of Road Transport and Highways) format; analysis uses the collision diagram (sketch of movements at a spot) and the condition diagram (scaled site plan of physical features). Causes grouped as road user, vehicle, road and environment. Remedies follow the 5 E's — Education, Enforcement, Engineering, Environment, Emergency care. Blackspot identification and rectification is a standing NHAI/MoRTH programme; India records roughly 1.7 lakh road deaths a year (MoRTH annual report figures change each year).

Traffic flow theory

Three fundamental parameters: flow or volume q (veh/h), space-mean speed v (km/h), density or concentration k (veh/km). Fundamental relation q = k × v.
Time mean speed (Vt): arithmetic mean of spot speeds at a point. Space mean speed (Vs): harmonic mean, i.e. length divided by mean travel time. Vt ≥ Vs always; Vt = Vs + σ²/Vs approximately. Flow calculations must use space mean speed.
Headway and spacing: time headway h (s) between successive vehicles at a point → q = 3600/h̄; space headway (spacing) s (m) → k = 1000/s̄.
Greenshields linear model: v = vf (1 − k/kj), where vf = free-flow speed, kj = jam density. Substituting in q = kv gives a parabolic q–k curve with capacity qmax = vf·kj/4 at k = kj/2 and v = vf/2. The q–v curve is also parabolic; the v–k relation is linear.
Other models: Greenberg logarithmic v = vo ln(kj/k); Underwood exponential v = vf e^−k/ko. Greenshields is the one examined.
Fundamental diagram: the q–k curve has a stable (uncongested) left branch and an unstable (congested) right branch; the same flow occurs at two densities, so flow alone cannot describe a stream.
Shock wave speed (between states 1 and 2): ω = (q2 − q1)/(k2 − k1); a negative value means the wave moves upstream — the queue grows backwards, the mechanism of a traffic jam.
Traffic stream distributions: arrivals at a point in light traffic follow a Poisson distribution; the gaps between them are negative exponential. Used for gap acceptance, pedestrian crossing and unsignalised intersection capacity.
Critical gap and gap acceptance: the minimum time gap in the major stream that a minor-road driver accepts. Larger critical gaps lower the minor-stream capacity.

Capacity and level of service

Capacity: the maximum hourly rate at which vehicles can reasonably be expected to pass a point under prevailing road and traffic conditions. Basic capacity = ideal conditions; possible capacity = prevailing conditions; practical (design) capacity = a lower value used for design so operation stays acceptable.
Theoretical capacity (single lane): C = 1000·V/S, where V = speed (km/h) and S = average spacing (m).
IRC 64-1990 design service volumes (rural highways, PCU/day, plain terrain, indicative): single lane ≈ 2,000; intermediate lane ≈ 6,000; two-lane with hard shoulder ≈ 15,000; four-lane divided ≈ 35,000 (both directions). Use current code tables; values differ with terrain and curvature.
Urban capacity (IRC 106-1990, PCU/h, total for the road): roughly 1,500 for a two-lane two-way urban road, 2,400 for a two-lane one-way, 3,000 for a four-lane undivided two-way and 3,600 for a four-lane divided arterial — again indicative.
Level of Service (LOS): a qualitative measure A to F based on density, speed and delay. A = free flow; C = stable but restricted; D = approaching unstable; E = at capacity, v/c ≈ 1; F = forced/breakdown flow with queueing. Signalised intersections use control delay per vehicle (HCM: LOS A ≤ 10 s, F > 80 s). Indian practice adapts HCM thresholds; Indo-HCM (CSIR-CRRI, 2017) gives Indian LOS criteria for mixed traffic.
Factors reducing capacity: lane width below 3.5 m, inadequate shoulders, lateral clearance, heavy-vehicle share, gradients, pedestrians and parking, poor pavement, side friction and encroachment.

Intersections and traffic control devices

Intersection types: at-grade (uncontrolled, priority/stop-yield controlled, channelised, rotary, signalised) and grade-separated (underpass, flyover, interchanges — trumpet, diamond, cloverleaf, directional).
Conflict points: a plain four-arm crossroad has 32 conflict points (16 crossing, 8 merging, 8 diverging); channelisation, rotaries and signals reduce or time-separate them.
Rotary (roundabout): movements are converted into weaving manoeuvres with one-way circulation. Design elements — entry/exit radius, central island radius, weaving length, weaving width, entry width. Rotaries suit moderate flows of roughly 500–3,000 PCU/h total and sites with four or more arms; they fail when any one approach dominates or flow is high (then signals or grade separation). Reference: IRC 65-2017.
Wardrop's weaving formula for practical rotary capacity (see Formulas section).
Traffic signs — IRC 67-2022: three groups — mandatory/regulatory (circular; red border — STOP and GIVE WAY are the exceptions, octagonal and inverted triangle), cautionary/warning (equilateral triangle, apex up, red border) and informatory (rectangular, usually blue/green). Overhead and gantry signs on expressways carry route and destination information.
Road markings — IRC 35-2015: white for most longitudinal and transverse markings, yellow for centre lines on undivided roads, no-parking kerb marking and no-overtaking zones. Broken centre line = overtaking permitted; continuous line = do not cross; double line (one continuous, one broken) = crossing allowed only from the broken side. Also stop line, give-way triangles, zebra crossing, chevrons, hatching, rumble strips.
Other devices: traffic islands (divisional, channelising, refuge, rotary), delineators, studs, hazard markers, variable message signs, speed humps and tables (IRC 99 for traffic-calming measures).
Traffic regulation and law: Motor Vehicles Act 1988 and the Motor Vehicles (Amendment) Act 2019 (sharply higher penalties, National Road Safety Board, Good Samaritan protection, e-governance of licensing); Central Motor Vehicles Rules 1989; one-way systems, turn bans, speed limits notified by state governments, lane discipline and no-entry hours for goods vehicles.

Traffic signal design

Why signals: they separate conflicting movements in time, reduce right-angle collisions, allow pedestrian crossing and give orderly control where flows are heavy.
Signal terms: cycle (one full sequence), cycle time C, phase (part of the cycle allotted to one set of movements), interval, green, amber (3 s typical) and all-red times, lost time, effective green g = G + a − l, saturation flow s (PCU/h of green), critical lane volume, split, offset (time difference between adjacent signals' green starts in a coordinated system).
Types of signals: fixed-time, vehicle-actuated (semi- or fully-actuated via loop/radar detectors), adaptive/area traffic control (ATCS — SCATS, SCOOT, CoSiCoSt by C-DAC used in several Indian cities), pedestrian signals, flashing signals, special signals.
Design methods:
1. Trial-cycle method — assume a cycle, allot green in proportion to critical volumes, check adequacy.
2. Approximate method (IRC) — green times in proportion to approach volumes with pedestrian and clearance checks.
3. Webster's method — optimum cycle minimising delay (standard examination method).
Webster's steps: (i) find design-hour flow q and saturation flow s for each approach; (ii) ratio y = q/s for the critical lane of each phase; (iii) Y = Σy (must be < 1, else the intersection is over-saturated); (iv) total lost time L = Σ(l per phase) + all-red; (v) optimum cycle Co = (1.5L + 5)/(1 − Y); (vi) distribute effective green in proportion to y; (vii) add amber/clearance to get displayed green; (viii) check pedestrian crossing time and minimum green (usually ≥ 10–12 s).
Pedestrian crossing time = crossing width / walking speed (1.2 m/s assumed) + start-up time (about 7 s).
Signal coordination: linked or progressive systems with a common cycle; offset = distance / progression speed, giving a "green wave". Reference: IRC 93-1985 for signal installation and IRC SP 41 for at-grade intersection design.
Advantages: orderly flow, fewer severe conflicts, economical compared with grade separation, allows pedestrian phases. Limitations: adds delay and stops to all traffic, rear-end collisions may rise, poor design or over-saturation causes queue spillback and gridlock; unwarranted signals invite violation.

Traffic management and demand measures

Transport System Management (TSM) / low-cost measures: one-way pairs, turn restrictions, channelisation, signal retiming, kerb-side parking control, bus-bay and lay-by provision, pedestrian footpaths and foot-over-bridges, segregated cycle tracks.
Travel Demand Management (TDM): congestion/parking pricing, staggered office hours, carpooling, park-and-ride, priority lanes for buses, restraint on private-vehicle use in core areas.
Road safety audit: systematic independent safety check at design, construction and operation stages — mandated on National Highways projects; IRC SP 88-2019 is the Indian manual.
Traffic forecasting for design: growth-rate (compound) method, or elasticity with respect to vehicle registration/income; design volume projected over the design life (20 years for geometric design and 15–20 years for pavement).

Key points

Traffic engineering — measurement, analysis and control of traffic for safe, efficient and economical movement.
q = k·v — flow equals density times space-mean speed; the single most used relation in flow theory.
PCU — common unit for mixed traffic; IRC 106 gives factors (small share: car 1.0, two-wheeler 0.5, bus/truck 2.2), rising with that type's share.
85th percentile speed — basis for the posted speed limit; 98th percentile for geometric design speed.
Space mean speed — harmonic mean, always ≤ time mean speed; only it is valid in q = kv.
Greenshields — v = vf(1 − k/kj); capacity = vf·kj/4 at v = vf/2 and k = kj/2.
Shock wave speed — ω = Δq/Δk; negative value means the queue propagates upstream.
Capacity vs design service volume — capacity is the maximum possible; design service volume is the lower value used for planning at a chosen LOS.
Level of Service — A (free flow) to F (breakdown); judged by density on highways and control delay at signals.
PHF — hourly volume ÷ (4 × peak 15-minute count); measures how peaked the flow is.
Webster's optimum cycle — Co = (1.5L + 5)/(1 − Y); requires Y = Σ(q/s) < 1.
Saturation flow — discharge rate during fully saturated green; estimated from approach width per IRC 106.
Conflict points — 32 at an uncontrolled four-arm intersection; reduced by channelisation, rotary or signals.
Rotary — weaving-based control, good for about 500–3,000 PCU/h and multi-arm junctions; IRC 65 governs.
Sign shapes — circular mandatory, triangular cautionary, rectangular informatory (IRC 67); markings per IRC 35.
Parking terms — accumulation, turnover, index, duration; parallel parking needs least width, 90° stores most cars per kerb metre.
Accident analysis tools — collision diagram (movements) and condition diagram (site features); remedies via the 5 E's.
Legal frame — Motor Vehicles Act 1988 with the 2019 Amendment; CMVR 1989; enforcement by state transport and traffic police.

Formulas and examples

q = k × v — q = flow (veh/h), k = density (veh/km), v = space mean speed (km/h)

Vt = (Σvi)/n (time mean speed, km/h); Vs = n / Σ(1/vi) (space mean speed, harmonic mean); Vt ≈ Vs + σ²/Vs

q = 3600 / h̄ (h̄ = average time headway, s); k = 1000 / s̄ (s̄ = average space headway, m)

Greenshields: v = vf (1 − k/kj); q = vf (k − k²/kj); qmax = vf·kj / 4 at k = kj/2, v = vf/2

Shock wave speed ω = (q2 − q1)/(k2 − k1), m/s or km/h

PHF = V(hour) / (4 × V(peak 15 min))

Theoretical lane capacity C = 1000 V / S — V in km/h, S = average centre-to-centre spacing in m

Webster: Co = (1.5 L + 5) / (1 − Y); L = Σ lost time per phase + all-red (s); Y = Σ yi; yi = qi / si

Effective green for phase i: gi = (Co − L) × yi / Y; displayed green Gi = gi + li − ai

Webster delay per vehicle d ≈ 0.9 [ C(1 − λ)² / (2(1 − λx)) + x² / (2q(1 − x)) ] — λ = g/C, x = q/(λs) degree of saturation

Wardrop rotary practical capacity Qp = 280 w [1 + e/w] [1 − p/3] / [1 + w/L] — w = weaving width (m), e = average entry width (m), L = weaving length (m), p = proportion of weaving traffic; Qp in PCU/h. Valid for w = 6–18 m, e/w = 0.4–1, w/L = 0.12–0.4, p = 0.4–1.

Parking: parking load (vehicle-hours) = Σ (accumulation × time interval); turnover = vehicles parked / (bays × period); parking index = (parking load / parking capacity in vehicle-hours) × 100

Accident rate R = (A × 10⁸) / (365 × T × V × L) — accidents per 100 million vehicle-km; A = accidents, T = years, V = AADT, L = length (km)

Traffic growth: Vn = Vo (1 + r)ⁿ — r = annual growth rate, n = years

Example 1 — PCU conversion and capacity check. An urban arterial with two lanes under one-way operation carries, in the peak hour: 900 cars, 1,400 two-wheelers, 150 auto-rickshaws, 80 buses, 60 LCVs.

Cars: 900 × 1.0 = 900; two-wheelers: 1,400 × 0.5 = 700; autos: 150 × 1.2 = 180; buses: 80 × 2.2 = 176; LCVs: 60 × 1.4 = 84.
Total = 900 + 700 + 180 + 176 + 84 = 2,040 PCU/h.
Against the indicative 2,400 PCU/h for two lanes one-way, v/c = 2,040/2,400 = 0.85 → operating around LOS D/E, so the corridor needs demand or capacity action.
If the peak 15-minute count is 620 vehicles (2,590 total vehicles in the hour): PHF = 2,590/(4 × 620) = 2,590/2,480 = 1.04 — greater than 1 is impossible, so the counts are inconsistent and must be rechecked (a useful exam trap: PHF ≤ 1 always).

Example 2 — Time mean vs space mean speed, flow and density. Four vehicles cross a 500 m test section in 30, 36, 40 and 45 s.

Speeds: 500/30 = 16.67 m/s; 500/36 = 13.89; 500/40 = 12.50; 500/45 = 11.11 m/s.
Vt = (16.67 + 13.89 + 12.50 + 11.11)/4 = 54.17/4 = 13.54 m/s = 48.8 km/h.
Vs = 4 / (1/16.67 + 1/13.89 + 1/12.50 + 1/11.11) = 4 / (0.0600 + 0.0720 + 0.0800 + 0.0900) = 4/0.3020 = 13.25 m/s = 47.7 km/h. Vt > Vs, as expected.
If the average time headway at a point is 2.4 s, q = 3600/2.4 = 1,500 veh/h; then k = q/Vs = 1,500/47.7 = 31.4 veh/km, and average spacing = 1000/31.4 ≈ 31.8 m.

Example 3 — Greenshields model and shock wave. A highway lane has free-flow speed vf = 80 km/h and jam density kj = 120 veh/km.

Capacity qmax = vf·kj/4 = 80 × 120/4 = 2,400 veh/h, occurring at k = 60 veh/km and v = 40 km/h.
At k = 30 veh/km: v = 80(1 − 30/120) = 80 × 0.75 = 60 km/h; q = 30 × 60 = 1,800 veh/h.
A signal stops traffic downstream, creating a jam state (k2 = 120 veh/km, q2 = 0) behind the upstream state (k1 = 30, q1 = 1,800).
ω = (0 − 1800)/(120 − 30) = −1800/90 = −20 km/h — the stopping wave moves backwards at 20 km/h, so in 3 minutes the queue extends 20 × 3/60 = 1.0 km upstream.

Example 4 — Webster signal design, two-phase. A four-arm intersection is to run on two phases. Critical design flows and saturation flows:

Phase 1 (N–S): q = 1,000 PCU/h, s = 2,800 PCU/h → y1 = 1000/2800 = 0.357.
Phase 2 (E–W): q = 800 PCU/h, s = 2,500 PCU/h → y2 = 800/2500 = 0.320.
Y = 0.357 + 0.320 = 0.677 (< 1, so a signal is feasible).
Lost time: start-up/clearance 2 s per phase plus amber 3 s per phase → L = 2(2 + 3) = 10 s.
Co = (1.5 × 10 + 5)/(1 − 0.677) = 20/0.323 = 61.9 s → adopt 60 s (cycles are rounded to 5 s).
Total effective green = Co − L = 60 − 10 = 50 s.
g1 = 50 × 0.357/0.677 = 26.4 s → say 26 s; g2 = 50 × 0.320/0.677 = 23.6 s → say 24 s.
Displayed green G = g + lost time − amber = g + 2 − 3 = g − 1 → G1 = 25 s, G2 = 23 s; with amber 3 s each, cycle = 25 + 3 + 23 + 3 = 54 s, plus 6 s all-red/intergreen adjustment to return to the adopted 60 s. Check pedestrian need: crossing a 14 m carriageway at 1.2 m/s needs 14/1.2 ≈ 11.7 s + 7 s start-up ≈ 19 s — available cross green of 23 s is adequate.
Degree of saturation, phase 1: λ = 26/60 = 0.433; x = 1000/(0.433 × 2800) = 1000/1212 = 0.825 — acceptable but close to the practical limit of 0.85–0.90.

Example 5 — Parking and accident rates. A 200 m kerb length is to be used for parking cars 5 m long and 2.5 m wide.

Parallel parking: bays = 200/5.9 = 33.9 → 33 full bays (5.9 m per bay including manoeuvring allowance).
90° parking: bays = 200/2.5 = 80 cars (but needs about 5 m of carriageway width).
45° parking: approximate bays = 200/(2.5/sin45°) = 200/3.54 ≈ 56 cars.
Accident rate: a 20 km stretch with AADT 25,000 records 54 accidents in 3 years. R = (54 × 10⁸)/(365 × 3 × 25,000 × 20) = 5.4 × 10⁹ / 5.475 × 10⁸ = 9.86 accidents per 100 million vehicle-km.

Intersection control	Suitable flow (total, PCU/h)	Main merit	Main limitation
Uncontrolled / priority	Up to ~500	No delay to major road	Unsafe as flows rise
Channelised at-grade	500–1,500	Guides movements, cheap	Needs land, still conflicts
Rotary	500–3,000	No stopping, multi-arm	Fails with one dominant arm or high flow
Signalised	1,500–8,000	Time separation, pedestrian phase	Stop-and-go delay, spillback if over-saturated
Grade separated	Above ~8,000	No at-grade conflict	Very costly, land and structure

Speed type	Definition	Use
Spot speed	Instantaneous speed at a point	Speed limits, accident study
Running speed	Distance ÷ time in motion	Capacity, operating cost
Journey (overall) speed	Distance ÷ total time including stops	Congestion and delay measure
Time mean speed	Arithmetic mean of spot speeds	Spot-speed reporting
Space mean speed	Harmonic mean over a length	q = kv, flow analysis
Design speed	98th percentile speed	Geometric design (IRC 73/86)

Where it is used

Highway and urban-road consultancies (L&T Infrastructure Engineering, AECOM India, WSP, LEA Associates, RITES, Egis, SYSTRA): traffic surveys, forecast studies, feasibility and DPR traffic chapters, road safety audits.
NHAI, MoRTH, NHIDCL, state PWDs: toll-plaza lane sizing from PCU counts, blackspot rectification, wayside amenity spacing, access control and traffic management plans during construction.
Municipal corporations and urban mobility agencies (BMC, MCD, BBMP, PMC; DIMTS and UMTC as consultants): signal design and retiming, ATCS control rooms, parking policy and pricing, pedestrianisation, BRT and bus-priority corridors.
Traffic police and enforcement technology: e-challan with ANPR and red-light violation detection cameras, speed enforcement, one-way and no-entry schemes — expanded under the Motor Vehicles (Amendment) Act 2019.
Toll, concession and finance roles: traffic and revenue (T&R) studies for BOT/HAM/TOT concessions, where forecast AADT and PCU composition drive toll revenue projections.
Research and standards: CSIR-CRRI (Indo-HCM), IIT transportation groups, Indian Roads Congress committees; also ADB/World Bank-funded state road projects requiring traffic and safety deliverables.

Interview questions

Q: Why is PCU used in Indian traffic analysis? A: Indian traffic is highly heterogeneous, so vehicles of very different size, speed and manoeuvrability cannot be added directly; PCU factors (IRC 106) convert them into equivalent cars so one number can be compared with capacity.
Q: Which percentile speed is used for setting a speed limit and why? A: The 85th percentile from the spot-speed cumulative curve, because the majority of drivers choose a safe speed and restricting the fastest 15% gives a limit that most will accept and obey.
Q: Why must space mean speed, not time mean speed, be used in q = kv? A: Density is a spatial measure, so it must pair with a speed averaged over space; the harmonic mean (space mean speed) preserves travel time and keeps the relation dimensionally and physically consistent.
Q: What does a negative shock wave speed mean on site? A: The boundary between two traffic states is moving upstream against the flow — the queue is growing backwards, which is how a stoppage spreads into a jam.
Q: Why does capacity in Greenshields' model occur at half the free-flow speed? A: Flow q = vf(k − k²/kj) is a parabola in k; setting dq/dk = 0 gives k = kj/2, and the linear speed-density relation then gives v = vf/2, so qmax = vf·kj/4.
Q: When would you choose a rotary over a signal? A: When total flow is moderate (roughly up to 3,000 PCU/h), the approach volumes are reasonably balanced, there are four or more arms or awkward angles, and enough land is available; a dominant approach or high flow calls for signals or grade separation.
Q: What is saturation flow and how does it affect signal timing? A: It is the maximum discharge rate during fully utilised green (PCU/h). Lower saturation flow raises y = q/s, increases Y and therefore the optimum cycle and delay; it is reduced by narrow approaches, parked vehicles, pedestrians and turning conflicts.
Q: What happens if Y = Σ(q/s) approaches or exceeds 1 in Webster's method? A: The optimum cycle tends to infinity — the intersection is over-saturated and no signal timing can clear the demand; the answer is extra capacity, demand management, turn bans or grade separation.
Q: What is the difference between a collision diagram and a condition diagram? A: The collision diagram shows vehicle and pedestrian movements, types and severity of past accidents using arrows and symbols, not to scale; the condition diagram is a scaled plan of the physical site features — geometry, signs, markings, obstructions.
Q: Difference between capacity and design service volume? A: Capacity is the maximum flow the facility can carry; design service volume is the lower flow adopted for design so that the chosen Level of Service (usually B or C) is maintained through the design life.
Q: Why is an all-red (intergreen) interval provided and not just amber? A: Amber warns drivers to stop, but vehicles already past the stop line need time to clear the conflict area; an all-red interval ensures the intersection is empty before the next phase gets green.
Q: How are traffic signs classified and what shapes identify them? A: Per IRC 67 — mandatory/regulatory signs are circular with a red border (STOP octagonal, GIVE WAY inverted triangle), cautionary signs are upward triangles with a red border, informatory signs are rectangular.

Common mistakes

Using time mean speed in q = kv, or averaging speeds arithmetically when a harmonic mean is required.
Mixing units in flow problems — veh/h with veh/s, m with km, or forgetting to convert m/s to km/h before computing density.
Forgetting that total effective green = C − L in Webster's method, and distributing the full cycle time as green.
Reporting PHF greater than 1, or computing it from a 15-minute count without multiplying by four.
Adding mixed vehicles directly without PCU conversion, or applying rural PCU factors to urban roads (and vice versa).
Treating capacity and design service volume as the same number, and quoting IRC table values without stating terrain and road type.
In accident-rate problems, omitting the 365 (days) or the length term, so answers are off by orders of magnitude.