What is Traffic Engineering?
Traffic engineering is a branch of civil engineering that applies scientific principles to the planning, design, and operation of roads, streets, and highways — as well as to the traffic using them — to achieve safe, efficient, and convenient movement of people and goods. Its basic objective is to achieve free and rapid flow of traffic with the least number of accidents.
To achieve this objective, traffic engineers study three fundamental areas: traffic characteristics, traffic studies and analysis, and traffic controls and regulations. Each of these feeds into the design and management decisions that make roads safer and more efficient.
Three Main Traffic Characteristics
1. Road User Characteristics
Roads are ultimately designed for human users, and human beings are highly variable in their physical, mental, and psychological capabilities. Road user characteristics include everything that affects a driver’s or pedestrian’s ability to use a road safely: vision (field of view, night vision, colour blindness), reaction time (PIEV theory), hearing ability, physical strength, emotional state, and psychological responses like risk perception and attention span.
Understanding these characteristics directly influences the design of traffic signals (cycle lengths must match reaction times), road signs (must be visible and comprehensible), speed limits (must be achievable by most users), and sight distance requirements (based on driver eye height and reaction time).
2. Vehicular Characteristics
Vehicular characteristics encompass all measurable physical and performance attributes of vehicles that use the road network. These include: vehicle dimensions (width, length, height), weight of loaded vehicles, engine power and pulling capacity, maximum speed, braking ability, and turning radius. These parameters directly determine geometric design elements like lane width (must accommodate vehicle width plus clearance), turning radius of intersections, gradient limits (based on engine power), and vertical clearances.
3. Braking Characteristics
Braking characteristics determine how quickly a vehicle can be stopped after the driver decides to brake. They affect the safe stopping distance between vehicles, the design of sight distance, and the spacing of traffic signals. A braking test is conducted to find the skid resistance of a pavement surface. At least two of the following three parameters are needed to calculate skid resistance: initial velocity (v), braking distance (L), and duration of brake application (t).
Passenger Car Unit (PCU)
Since roads carry a mixture of vehicle types with very different performance characteristics, traffic volumes are expressed in a common unit — the Passenger Car Unit (PCU). The PCU value of a vehicle class is the ratio of the road’s capacity with passenger cars only to its capacity with that vehicle class only.
| Vehicle Class | IRC PCU Factor (Rural Plain Terrain) |
|---|---|
| Passenger car, Tempo, Auto-rickshaw, Agri. factor | 1.0 |
| Bus, Truck, Agricultural tractor-trailer | 3.0 |
| Motor cycle, Scooter, Pedal cycle | 0.5 |
| Cycle rickshaw | 1.5 |
| Horse drawn vehicle | 4.0 |
| Small bullock cart / Hand cart | 6.0 |
| Large bullock cart | 8.0 |
Seven Types of Traffic Studies
| Study Type | Purpose |
|---|---|
| Traffic Volume Study | Count vehicles (AADT, ADT, 30th HHV) for road planning |
| Traffic Speed Study | Measure spot, mean, journey speeds for design and regulation |
| Origin & Destination (O&D) Study | Determine trip patterns for highway and transit planning |
| Traffic Flow Characteristics | Understand speed-density-volume relationships |
| Traffic Capacity Study | Determine maximum flow and level of service |
| Parking Study | Assess parking demand, accumulation, and turnover |
| Accident Analysis Study | Identify causes and locations of accidents for prevention |
The traffic survey (also called traffic census) is the structured process of collecting traffic data for any of these studies. A notation like “1950 M65” in traffic survey data means a survey over 1950 vehicles of mixed traffic with a design speed of 65 km/h.
Key Summary
- Traffic engineering objective: free, rapid flow with minimum accidents
- Three characteristics: road user (human), vehicular (vehicle), braking (stopping)
- PCU = standard unit to compare different vehicle types (passenger car = 1.0)
- Seven traffic studies: volume, speed, O&D, flow, capacity, parking, accident
- Traffic survey = traffic census = data collection for analysis