Traffic engineering is the science of moving people and goods safely and efficiently on roadways. It encompasses traffic flow theory, road capacity, intersection design, and traffic management. For civil engineers in India, IRC 64, SP:41, and SP:87 are key references alongside the HCM (Highway Capacity Manual) for advanced methods.
Traffic Flow Parameters
Fundamental Variables
| Variable | Symbol | Definition | Unit |
|---|---|---|---|
| Volume / Flow | q | Number of vehicles passing a point per unit time | veh/hr |
| Density / Concentration | k | Number of vehicles per unit length of road | veh/km |
| Space Mean Speed | us | Average speed based on travel time (harmonic mean) | km/hr |
| Time Mean Speed | ut | Average speed based on spot measurements (arithmetic mean) | km/hr |
Fundamental Relationship
q = k × us (Greenshields fundamental equation)
This links the three primary traffic flow parameters. At maximum flow (capacity), there is an optimum density and speed.
Greenshields Linear Model
us = uf (1 − k/kj)
- uf = free flow speed (speed when density → 0)
- kj = jam density (speed = 0, maximum packing)
- Capacity: qmax = kj × uf / 4
- Optimum speed: uf/2; Optimum density: kj/2
Passenger Car Unit (PCU)
PCU converts mixed traffic to equivalent passenger cars for capacity analysis:
| Vehicle Type | PCU Value (IRC 64) |
|---|---|
| Passenger car (PC) | 1.0 |
| Auto-rickshaw (3-wheeler) | 0.5 |
| Motorcycle / Scooter | 0.5 |
| Tractor | 4.0 |
| Bus (2-axle) | 3.0 |
| Truck (2-axle) | 3.0 |
| Truck (3-axle) | 4.5 |
| Cycle | 0.5 |
| Cycle rickshaw | 1.5 |
| Animal-drawn cart | 8.0 |
Total PCUs = Σ(number of vehicles of each type × PCU value)
Level of Service (LOS)
LOS is a qualitative measure of traffic operating conditions on an A–F scale:
| LOS | v/c Ratio | Condition | Speed (% of FFS) |
|---|---|---|---|
| A | ≤ 0.20 | Free flow, unimpeded travel | ~100% |
| B | ≤ 0.45 | Reasonably free flow | ~95% |
| C | ≤ 0.70 | Stable flow, restricted freedom | ~85% |
| D | ≤ 0.90 | Approaching unstable flow | ~70% |
| E | ≤ 1.00 | At capacity, unstable | ~60% |
| F | > 1.00 | Forced/breakdown flow, stop-and-go | <60% |
Design target in India: LOS C or better for national/state highways; LOS D acceptable for urban roads.
Road Capacity — IRC 64:1990
| Road Type | Capacity (PCU/day) | Capacity (PCU/hr per lane) |
|---|---|---|
| 2-lane undivided (7.0 m) | 12,000 – 15,000 | ~1500 |
| 4-lane divided (2×7.0 m) | 30,000 – 35,000 | ~1800 per lane |
| 6-lane divided expressway | 60,000+ | ~2000 per lane |
These are practical capacities under Indian mixed-traffic conditions. HCM values are higher (2200 PCU/hr/lane for freeways) but assume homogeneous traffic.
Traffic Volume Studies
Methods of Traffic Count
- Manual count: Observers with tally counters; expensive, short duration (peak hour)
- Automatic Traffic Counter (ATC): Inductive loops in pavement, gives continuous 24×7 data
- Video detection: CCTV + image processing; captures vehicle classification
- Pneumatic tube counters: Axle counters; portable, accurate for counting, misses classification
Peak Hour Factor (PHF)
PHF = Peak Hour Volume / (4 × Peak 15-minute Volume)
PHF close to 1.0 = smooth flow; PHF = 0.75–0.85 = peaky traffic, higher capacity demand.
Design flow = Peak Hour Volume / PHF
Spot Speed Studies
Spot speed = speed of individual vehicles at a specific location. Used to determine:
- Design speed for geometric design
- Speed limits (85th percentile speed)
- Accident analysis
Techniques: Enoscope (stop watch), radar gun, laser (LIDAR), inductive loops.
85th percentile speed = speed below which 85% of vehicles travel; used to set advisory/regulatory speed limits.
Signalized Intersection Design
Key Terms
- Saturation flow (s): Maximum flow that can pass through stop line during green time (~1800–2000 PCU/hr/lane)
- Lost time per phase (L): Start-up delay + clearance time ≈ 4–5 seconds
- Critical lane volume ratio (y): y = q/s for critical lane in each phase
- Green ratio (g/C): Fraction of cycle allocated to a phase
Webster's Optimum Cycle Length
Co = (1.5L + 5) / (1 − ΣYc)
- L = total lost time per cycle = n × li (n = number of phases)
- ΣYc = sum of critical volume ratios
Phase split: gi = (Co − L) × Yi / ΣYc
Worked Example
Two-phase signal, critical volumes: Phase 1 = 800 PCU/hr, Phase 2 = 600 PCU/hr
Saturation flow = 1800 PCU/hr/lane; lost time per phase = 4 sec
Y1 = 800/1800 = 0.444; Y2 = 600/1800 = 0.333; ΣYc = 0.777
Co = (1.5 × 8 + 5) / (1 − 0.777) = 17 / 0.223 = 76 sec ≈ 80 sec
Effective green: Phase 1 = (80 − 8) × 0.444/0.777 = 41 sec; Phase 2 = 31 sec
Traffic Impact Assessment (TIA)
Required for large developments (malls, townships, industrial estates). Steps:
- Determine trip generation (ITE Trip Generation manual or local studies)
- Distribute trips to access points (gravity model or traffic distribution)
- Assign to road network (manual or VISUM/TransCAD software)
- Check LOS with and without development — identify mitigation measures
Frequently Asked Questions
What is the difference between time mean speed and space mean speed?
Time mean speed (ut) is the arithmetic average of speeds observed at a fixed point — it overestimates average speed. Space mean speed (us) is the harmonic mean of travel times over a section — it is the true average for flow calculations. ut ≥ us always; relationship: ut = us + σ²/us (where σ = standard deviation of individual speeds).
Why is v/c ratio important in LOS determination?
The volume-to-capacity ratio (v/c) is the primary indicator of traffic congestion. As v/c approaches 1.0 (LOS E), queuing becomes unstable. Above 1.0 (LOS F), breakdowns occur — a self-reinforcing congestion spiral that's very hard to resolve without physical intervention.