--- title: "Traffic capacity; factors affecting capacity and LOS" subject: "Transportation Engineering II" chapter: 1 key: "semester_7/transportation_engineering_ii:traffic-capacity-factors-los-timing" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/traffic-capacity-factors-los-timing site: BCT Engineering --- # Traffic capacity; factors affecting capacity and LOS Transportation Engineering II · Chapter 1: Traffic Engineering - Asked in: 2068 Baisakh, 6 marks - Video: [Topic 4 Highway Capacity and LOS (M Maharjan)](https://www.youtube.com/watch?v=kw0-xDZMipc) — Michael “Doctor S” Schrader - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/traffic-capacity-factors-los-timing - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-1 ## Question Define traffic capacity. Describe the factors affecting capacity and level of service. ## 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. ## Answer #### 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: 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 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 1. **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. 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, side friction (pedestrians, bus stops, parking and roadside activities). 5. **Driver factors:** familiarity, behaviour (urban commuters versus tourists) and lane discipline. 6. **Operating (service) measures:** speed and travel time, freedom to manoeuvre, comfort, interruptions and safety, v/c ratio, density. 7. **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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which defines capacity? A. Average number of vehicles per day B. Maximum vehicles passing a point in a given time (normally one hour) under prevailing conditions C. Number of vehicles at jam density D. Minimum flow at LOS A Answer: B. Capacity is the maximum hourly flow under prevailing roadway, traffic and control conditions. (Key point: Definition of capacity) 2. Which capacity is the maximum under ideal conditions? A. Possible capacity B. Practical capacity C. Design capacity D. Basic capacity Answer: D. Basic capacity is for ideal roadway and traffic; possible capacity adjusts for actual conditions. (Key point: Types of capacity) 3. Practical (design) capacity is roughly what share of possible capacity? A. 75-80 percent B. 100 percent C. 30 percent D. 120 percent Answer: A. It is the flow without unreasonable delay, about 75-80 percent of possible capacity. (Key point: Practical capacity) 4. At which level of service is capacity reached, and what LOS are roads designed for? A. LOS A; design for LOS F B. LOS E; design for LOS B to C C. LOS F; design for LOS E D. LOS C; design for LOS A Answer: B. Capacity is reached at LOS E, and roads are designed for LOS B-C for flow without undue delay. 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