- 2074 Asoj · 8 marks
Highway capacity; factors affecting capacity and LOS
Short answer
Highway capacity is the maximum vehicles per hour a road can carry under existing conditions; basic, possible, and practical types. Level of service grades A (free flow) to F (forced flow). Affecting factors: lane width, alignment, traffic composition, controls, weather, driver behavior, and peak hour factor.
Describe highway capacity. Explain the factors which affect capacity and level of service.
Highway 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 in 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 of 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).
Level of service
| LOS | Operating condition | v/c (typical) |
|---|---|---|
| A | Free flow, very low volume, high speed, driver free to choose speed | 0.0-0.3 |
| B | Stable flow, some restriction of speed and manoeuvre | 0.3-0.5 |
| C | Stable flow, speed and manoeuvre controlled by traffic | 0.5-0.7 |
| D | Approaching unstable flow, little freedom, small increase in flow causes large drop in speed | 0.7-0.85 |
| E | Unstable flow at or near capacity, speeds low and uniform | 0.85-1.0 |
| F | Forced, breakdown flow; stop-and-go; flow below capacity | variable |
IRC:106-1990 (urban roads) and IRC:64-1990 (rural roads) give design service volumes at LOS C and LOS B respectively; Nepal Road Standard 2070 also designs for LOS B-C.
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 and the 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 and side friction (pedestrians, bus stops, parking and roadside activities).
- Driver factors: familiarity with the road, behaviour (urban commuters versus tourists) and lane discipline.
- Operating (service) measures used to judge LOS: speed and travel time, freedom to manoeuvre, comfort, interruptions and safety, v/c ratio and density.
- Peaking: the peak hour factor (PHF) and the variation of flow over time.
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- Traffic capacity; factors affecting capacity and LOS · Transportation Engineering II, chapter 1
- PIEV theory and level of service · Transportation Engineering II, chapter 1
- Factors affecting pavement design · Transportation Engineering II, chapter 2 · asked 3×
- Factors affecting bearing capacity; water table · Foundation Engineering, chapter 6 · asked 5×
- Factors affecting foundation design, improving bearing capacity · Building Technology, chapter 2
- Factors controlling highway alignment · Transportation Engineering I, chapter 2 · asked 12×
- Service reservoir capacity, 10,000 m3/day city · Water Supply Engineering, chapter 7