--- title: "Highway capacity; factors affecting capacity and LOS" subject: "Transportation Engineering II" chapter: 1 key: "semester_7/transportation_engineering_ii:highway-capacity-factors-level-of-service" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/highway-capacity-factors-level-of-service site: BCT Engineering --- # Highway capacity; factors affecting capacity and LOS Transportation Engineering II · Chapter 1: Traffic Engineering - Asked in: 2074 Asoj, 8 marks - Video: [Roadway Capacity and Level of Service](https://www.youtube.com/watch?v=yIBEXU_XScc) — WIT Solapur - Professional Learning Community - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/highway-capacity-factors-level-of-service - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-1 ## Question Describe highway capacity. Explain the factors which affect capacity and level of service. ## 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. ## Answer #### 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: 1. **Basic capacity:** the maximum number of passenger cars that can pass in a lane under ideal (best possible) roadway and traffic conditions. 2. **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. 3. **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 1. **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. 2. **Traffic factors:** composition (share of buses, trucks and slow vehicles), directional distribution of traffic, lane distribution and the presence of pedestrians and cyclists. 3. **Control factors:** signals (green time, cycle length, progression), signs, speed limits and parking restrictions. 4. **Environmental factors:** weather (rain, fog), lighting at night, surface condition and side friction (pedestrians, bus stops, parking and roadside activities). 5. **Driver factors:** familiarity with the road, behaviour (urban commuters versus tourists) and lane discipline. 6. **Operating (service) measures used to judge LOS:** speed and travel time, freedom to manoeuvre, comfort, interruptions and safety, v/c ratio and density. 7. **Peaking:** the peak hour factor (PHF) and the variation of flow over time. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does possible capacity differ from basic capacity? A. Possible capacity is under prevailing conditions, basic under ideal conditions B. Possible capacity is under ideal conditions, basic under prevailing conditions C. Possible capacity includes a safety margin for delay, basic does not D. They are the same, only the units differ Answer: A. Basic capacity assumes ideal roadway and traffic; possible capacity adjusts it for actual lane width, shoulders, gradient, heavy vehicles, etc. (Key point: Basic vs possible capacity) 2. What is practical (design) capacity? A. The maximum flow at LOS F B. The flow at jam density C. The ideal-condition maximum flow in a lane D. The flow without unreasonable delay at the chosen LOS, about 75-80 percent of possible capacity Answer: D. Practical capacity is the design value, kept below possible capacity so drivers are not unreasonably delayed. (Key point: Practical capacity) 3. Which statement about LOS E and F is correct? A. E is unstable flow at or near capacity; F is forced breakdown flow with flow below capacity B. E is free flow; F is stable flow C. E is forced flow; F is flow at capacity with uniform speed D. Both E and F are stable flow with v/c below 0.5 Answer: A. At LOS E flow is near capacity and unstable; at F the flow breaks down into stop-and-go with throughput below capacity. (Key point: Level of service E and F) 4. Which of these is a traffic (not roadway) factor affecting capacity? A. Lane width B. Share of buses and trucks in the stream C. Gradient D. Lateral clearance Answer: B. Traffic composition is a traffic factor; lane width, gradient and clearance are geometric factors. 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