Guidelines for Capacity of Urban Roads in Plain Areas

IRC 106: Background - Key Points & Definitions

• Urban Road Classification:

  • Arterials: Principal through traffic; spaced <1.5 km in CBD, >8 km in fringes; limited parking; pedestrian crossing at intersections only.
  • Sub-arterials: Similar to arterials but lower mobility; spacing 0.5 km (CBD) to 3-5 km (fringes).
  • Collector Streets: Collect traffic from locals to arterials; full access; limited parking restrictions.
  • Local Streets: Access to properties; low volume; unrestricted parking and pedestrian movement.

• Key Capacity-Related Definitions:

  • Speed: Space mean speed used (km/h).
  • Density: Vehicles per km per lane; max at jam density.
  • Capacity: Max hourly volume under prevailing conditions (vehicles/hour).
  • Design Service Volume: Max hourly volume maintaining a designated Level of Service (LOS).

• Level of Service (LOS):

  • Ranges from A (free flow) to F (breakdown).
  • LOS impacts design capacity; factors include speed, comfort, freedom to maneuver.
  • Example speeds relative to free flow speed:
    • LOS A: ~90%
    • LOS B: ~70%
    • LOS C: ~50%
    • LOS D: ~40%
    • LOS E: ~33%

Summary Table: Urban Road Classification & LOS Speeds

IRC 106: Scope — Key Points & Definitions

The scope covers urban road classification, capacity, and design criteria for traffic flow and service levels.

Urban Road Classification (Clause 1.5)

Capacity-Related Definitions

• Speed: Space mean speed (km/h) used for analysis.
• Density: Vehicles per km per lane; max = jamming density.
• Capacity: Max hourly volume (veh/hr) under prevailing conditions.
• Design Service Volume: Max hourly volume maintaining a chosen Level of Service (LOS).

Level of Service (LOS) Summary (Clause 5)

Formula: Capacity (Simplified)

[ \text{Capacity} = \frac{\text{Number of lanes} \times \text{Base capacity per lane (veh/hr)}}{\text{Adjustment factors}} ]

Adjustment factors include lane width, heavy vehicles, gradient, and access control.

Visual: Urban Road Hierarchy & Flow

IRC 106: Classification of Urban Roads

Key Points from IRC 106 (Clause 3)

Urban roads are classified based on their function and traffic characteristics into the following:

• Arterial Roads: Major roads for through traffic with high speed and volume.
• Sub-Arterial Roads: Connect arterial roads with collector roads.
• Collector Roads: Distribute traffic from arterials to local roads.
• Local Roads: Provide access to individual properties and low traffic volumes.

Typical Design Parameters (from IRC 106)

Design Criteria Highlights (Clause 6)

• Design Service Volume: Based on Passenger Car Units (PCU).
• Level of Service (LOS): Used to assess operational conditions (A to F).
• Capacity: Depends on lane width, lateral clearance, and traffic mix.

Passenger Car Unit (PCU) Factors (Clause 7)

flowchart LR
    A[Arterial Roads] --> B[Sub-Arterial Roads]
    B --> C[Collector Roads]
    C --> D[Local Roads]

Summary: IRC 106 classifies urban roads into Arterial, Sub-Arterial, Collector, and Local, each with specific right-of-way, carriageway widths, and design speeds. Use PCU factors to convert mixed traffic into equivalent passenger cars for capacity and LOS assessments.

IRC 106: Capacity-Related Definitions & Key Specifications

Key Definitions:

• Speed (Space Mean Speed): Average speed over a road section (km/h).
• Time Mean Speed: Average speed of vehicles at a point over time.
• Density: Number of vehicles per km per lane; max at jammed condition (jamming density).
• Capacity: Max hourly volume (vehicles/hr) under prevailing conditions.
• Design Service Volume: Max hourly volume maintaining a specified Level of Service (LOS).

Level of Service (LOS):

Capacity Concept:

• Capacity = Maximum hourly vehicles passing a point under prevailing conditions.
• Design service volume ≤ Capacity, ensuring target LOS.

Summary Diagram: Capacity & LOS Relationship

graph LR
A[Free Flow (LOS A)] --> B[Stable Flow (LOS B, C)]
B --> C[Near Capacity (LOS D)]
C --> D[At Capacity (LOS E)]
D --> E[Breakdown (LOS F)]

This framework guides urban road planning, lane provision, and traffic management per IRC 106.

Level of Service (LOS) - IRC 106 Key Points

Definition:

• LOS is a qualitative measure of traffic operational conditions, perceived by drivers/passengers.
• Ranges from A (best) to F (worst), describing freedom to maneuver, speed, comfort, and convenience.

LOS Descriptions & Speed Relationships (Urban Roads)

Important Formulas & Concepts

• Capacity (C): Maximum hourly volume (vehicles/hour) under prevailing conditions.
• Design Service Volume: Max hourly volume maintaining the chosen LOS.
• Density (k): Vehicles per km per lane.
• Speed (v): Space mean speed, km/h.
• Flow (q): Vehicles per hour per lane.

Fundamental relation:

[ q = k \times v ]

Conceptual Diagram: LOS vs. Volume-Flow

graph LR
A[LOS A] --> B[LOS B]
B --> C[LOS C]
C --> D[LOS D]
D --> E[LOS E]
E --> F[LOS F]
F --> G[Breakdown]

Summary

• LOS guides design capacity and service volume.
• Urban LOS affected by traffic heterogeneity, intersections, parking, pedestrians.
• Use LOS to balance traffic flow quality and infrastructure cost.

For detailed design volumes and passenger car unit (PCU) factors, refer to Clauses 7 & 8 of IRC 106.

IRC 106: Design Criteria for Urban Roads – Key Points

IRC 106 provides guidelines primarily for urban roads in plain areas. Although the exact clause text is missing, the standard covers:

1. Classification of Urban Roads

• Arterial Roads
• Sub-arterial Roads
• Collector Roads
• Local Roads

2. Design Service Volume (DSV)

• Defines the maximum number of Passenger Car Units (PCU) per hour per lane.
• Typical DSV values (approximate):
  • Arterial: 1800 – 2200 PCU/hr/lane
  • Sub-arterial: 1400 – 1800 PCU/hr/lane
  • Collector: 800 – 1400 PCU/hr/lane
  • Local: < 800 PCU/hr/lane

3. Level of Service (LOS)

• LOS A to F based on traffic density and speed.
• Target LOS for urban roads is generally C or better.

4. Passenger Car Unit (PCU) Conversion Factors

5. Geometric Design Parameters

• Carriageway width: Minimum 7.0 m for two lanes.
• Lane width: 3.0 m per lane.
• Shoulder width: 1.5 m (preferably paved).
• Design speed: 40–60 km/h depending on road type.

Example Formula: Design Service Volume (DSV)

[ \text{DSV} = \text{Number of lanes} \times \text{PCU per lane per hour} ]

flowchart LR
    A[Urban Road Classification] --> B[Arterial]
    A --> C[Sub-Arterial]
    A --> D[Collector]
    A --> E[Local]
    B --> F[Design Speed: 60 km/h]
    C --> G[Design Speed: 50 km/h]
    D --> H[Design Speed: 40 km/h]
    E --> I[Design Speed: 

IRC 106: Passenger Car Units (PCU)

The concept of Passenger Car Units (PCU) is crucial for urban road capacity analysis, converting heterogeneous traffic into equivalent passenger car flow.

Key Points from IRC 106 (Clause 7):

• PCU Definition: One PCU represents the space and time taken by a standard passenger car.
• Purpose: To convert mixed traffic (bicycles, two-wheelers, three-wheelers, buses, trucks) into equivalent passenger car traffic for uniform capacity evaluation.

Typical PCU Values (Approximate):

Usage:

[ \text{Equivalent PCU} = \sum (\text{Number of vehicles of type i} \times \text{PCU}_i) ]

Notes:

• PCU values vary with speed, road width, and traffic conditions.
• IRC 106 provides detailed tables and adjustments for urban road types.

flowchart LR
    A[Mixed Traffic Flow] --> B[Convert each vehicle type using PCU factor]
    B --> C[Calculate Equivalent PCU]
    C --> D[Use for Capacity & LOS Analysis]

This conversion standardizes traffic volume for design and analysis per IRC 106 guidelines.

Recommended Design Service Volumes (IRC 106)

Key Points:

• Design service volumes depend on functional classification: arterial, sub-arterial, collector.
• If fringe conditions differ from classification, use service volumes corresponding to actual fringe conditions.
• Design service volume = max hourly volume maintaining a target Level of Service (LOS).
• LOS ranges from A (free flow) to F (breakdown), with LOS C or D often used for design.

Typical Design Service Volumes (vehicles/hour per lane) - Indicative from IRC 106

Note: Exact values depend on local conditions, lane width, traffic mix.

Measures to Improve Capacity (Clause 9.2)

• Prohibit on-street parking; provide off-street parking.
• Use medians to segregate bi-directional traffic.
• Provide separate lanes for slow-moving vehicles.
• Restrict heavy/slow vehicles during peak hours.
• Control roadside commercial activities.
• Provide pedestrian and cycle facilities.
• Ban conflicting intersection movements during peaks.
• Regulate median openings to control cross traffic.
• Improve lane discipline and overtaking behavior.

Level of Service (LOS) Summary

flowchart LR
    A[Functional Classification]
    A --> B[Arterial]
   

IRC 106: Measures for Improving the Capacity of Urban Roads

Key measures to enhance urban road capacity (Clause 9) include:

• Lane Widening: Increase lane width to standard 3.0–3.5 m for smoother traffic flow.
• Traffic Management: Implement one-way systems, signal optimization, and better lane discipline.
• Intersection Improvements: Use grade separators, channelization, and signal timing to reduce delays.
• Access Control: Limit direct access points to reduce conflict and improve flow.
• Public Transport Priority: Dedicated bus lanes and improved transit facilities reduce private vehicle load.
• Non-Motorized Traffic Facilities: Separate pedestrian and cycle paths to minimize conflicts.
• Parking Management: Restrict on-street parking to free up road space.
• Use of Intelligent Transport Systems (ITS): Adaptive signals and real-time traffic monitoring.

Recommended Design Service Volumes (Excerpt from Clause 8)

Passenger Car Unit (PCU) Factors (Clause 7)

flowchart LR
    A[Increase Lane Width] --> B[Improved Traffic Flow]
    C[Signal Optimization] --> B
    D[Access Control] --> B
    E[Public Transport Priority] --> B
    F[Parking Restrictions] --> B
    B --> G[Increased Road Capacity]

These measures combined help achieve higher Level of Service (LOS) and improved urban mobility.