- 2068 Baisakh · 6 marks
Traffic capacity; factors affecting capacity and LOS
Short answer
Traffic capacity is the maximum vehicles that can pass a point per hour under prevailing conditions. Factors include roadway geometry (lane width, alignment), traffic composition, control devices, weather, driver behavior, and peaking. Level of service grades flow quality from A (free flow) to F (breakdown); design is typically for LOS B or C.
Define traffic capacity. Describe the factors affecting capacity and level of service.
Traffic capacity
Capacity of a road is the maximum number of vehicles (or PCUs) that can pass a given point or section of a lane or road in one direction (or both directions on a two-lane road) during a given time period (normally one hour) under the prevailing roadway, traffic and control conditions (HCM definition; IRC:106 for urban roads).
Types of capacity:
- Basic capacity: the maximum number of passenger cars that can pass in a lane under ideal (best possible) roadway and traffic conditions.
- Possible capacity: the maximum number that can pass under the actual (prevailing) roadway and traffic conditions, i.e. after adjusting the basic capacity for lane width, shoulders, gradient, heavy vehicles, etc.
- Practical capacity (design capacity): the maximum number that can pass without unreasonable delay or restriction to the drivers' freedom to manoeuvre, at the chosen level of service. It is about 75-80 percent of the possible capacity (the design service volume at LOS C for urban roads in IRC:106, and at LOS B for rural roads in IRC:64).
Factors affecting capacity and level of service
- Roadway (geometric) factors: lane width, lateral clearance (shoulder, kerb), number of lanes, horizontal and vertical alignment, gradient, sight distance, presence of a median. Narrow lanes (below 3.65 m) reduce capacity.
- Traffic factors: composition (share of buses, trucks and slow vehicles), directional distribution of traffic, lane distribution and the presence of pedestrians and cyclists.
- Control factors: signals (green time, cycle length, progression), signs, speed limits and parking restrictions.
- Environmental factors: weather (rain, fog), lighting at night, surface condition, side friction (pedestrians, bus stops, parking and roadside activities).
- Driver factors: familiarity, behaviour (urban commuters versus tourists) and lane discipline.
- Operating (service) measures: speed and travel time, freedom to manoeuvre, comfort, interruptions and safety, v/c ratio, density.
- Peaking: the peak hour factor (PHF) and the variation of flow over time.
Level of service
LOS is the quality of traffic flow, graded from A (free flow) to F (breakdown) by speed, travel time, freedom to move, comfort and safety (HCM; IRC:106 and IRC:64). Capacity is reached at LOS E; roads are designed for LOS B to C so that traffic flows without undue delay.
✓ Answer checked by Claude Opus 5.5 (high effort)
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