Guidelines for Capacity of Urban Roads in Plain Areas
1990 Edition

IRC 106 classifies urban roads into a hierarchy based on their role and traffic characteristics: 1) Arterial roads serve as primary routes for large-volume, high-speed through traffic connecting major urban centers; 2) Sub-arterial roads link arterial roads with collector roads, handling moderate traffic at reduced speeds; 3) Collector roads channel traffic from local streets to higher-class roads and typically serve neighborhoods and commercial areas with moderate to low speeds; 4) Local roads provide direct access to properties, support low traffic volumes, and are not intended for through movement. This classification facilitates organized traffic distribution and access management within urban environments.

IRC 106 describes Level of Service (LOS) as a qualitative indicator reflecting traffic operational conditions and user experience on urban roads. LOS is rated from A to F, where A represents free-flowing traffic with approximately 90% of free-flow speed and excellent comfort, and F denotes breakdown conditions with heavy congestion. Intermediate levels (B through E) indicate varying degrees of stability, speed reduction, and maneuvering difficulty. These LOS categories help in setting capacity standards and design service volumes to ensure urban roads meet desired performance and comfort levels amid diverse and mixed traffic conditions.

Design Service Volumes (DSV) in IRC 106 are assigned based on the functional classification and fringe conditions of urban roads. Arterial roads, spaced about 1.5 km in central business districts to over 8 km on city outskirts, accommodate the highest traffic volumes and have the largest DSVs. Sub-arterial roads, with spacing from 0.5 km (CBD) to 3-5 km (fringe), have moderate DSVs, connecting arterials and collectors. Collector streets serve local neighborhoods with lower DSVs, and local roads primarily provide access with minimal DSVs. Adjustments to DSV are made if fringe conditions differ from the typical road classification, and LOS C or D is commonly targeted for design to balance flow and comfort.

IRC 106 suggests several measures to enhance the capacity of urban roads: prohibiting on-street parking and encouraging off-street parking to reduce obstructions; segregating opposing traffic flows via medians to improve safety and flow; providing dedicated lanes for slow-moving vehicles to minimize disruptions; restricting heavy and slow vehicle movements during peak periods; controlling roadside commercial activities to reduce friction; implementing pedestrian and cyclist infrastructure to separate non-motorized traffic; banning conflicting maneuvers at major intersections during busy times; regulating median openings to limit cross traffic; and promoting lane discipline through markings, education, and enforcement. Collectively, these strategies help maintain efficient and stable traffic operations.

The Passenger Car Unit (PCU) concept in IRC 106 converts diverse vehicle types into a common equivalent based on their impact on traffic flow and road space occupancy. Each vehicle class—such as cars, buses, two-wheelers, three-wheelers, bicycles—is assigned a PCU factor reflecting its relative effect compared to a standard passenger car (PCU = 1). By multiplying the number of vehicles of each type by their respective PCU factors and summing the results, mixed traffic volumes are expressed uniformly in PCUs. This standardization facilitates accurate capacity estimation, design service volume calculation, and Level of Service analysis under heterogeneous urban traffic conditions.