Guidelines for Planning and Design of Roundabouts (First Revision)

The scope of IRC 65 covers the recommended practices for traffic rotaries, including geometric design, planning considerations, and performance parameters. Key sections include:

• Definitions and Terminologies (Clause 2) covering geometric, flow, driver behavior, and performance parameters.
• Roundabouts design: single lane (3.1) and multilane (3.2).
• Planning considerations such as intersection hierarchy (5.1), Passenger Car Unit (PCU) for roundabouts (5.2), and site selection (5.3).
• Detailed geometric design aspects like central island, circulatory carriageway (6.1), entry and exit design (6.3), splitter islands (6.4), entry flaring (6.5), entry angle (6.6), design speed (6.8), and sight distance (6.11).

For example, the width of certain elements is specified as 1.0 to 1.2 times the maximum entry width (Clause 1.2). The code also references related standards for road markings, signs, drainage, and landscaping.

This comprehensive scope ensures proper planning and design of rotaries for safe and efficient traffic flow.

Sources: Clause 1.2, Clause 2.1, Clause 3.1, Clause 3.2, Clause 5.1, Clause 5.2, Clause 5.3, Clause 6.1, Clause 6.3, Clause 6.4, Clause 6.5, Clause 6.6, Clause 6.8, Clause 6.11

Key planning considerations in IRC 65 include intersection hierarchy, passenger car unit (PCU) calculations for roundabouts, and site selection criteria as outlined in Clause 5. These guide the geometric and operational design of roundabouts. For capacity estimation, Table 9.1 provides entry capacity models based on roundabout diameter (D) with critical gap (T), follow-up time (T1), and formulas for entry capacity (C) in PCUs/hr as functions of circulating flow (Q). The table is reproduced below:

These formulas allow calculation of entry capacity based on circulating flow, critical for planning roundabout performance. Additionally, Fig. 14.1 (not shown here) illustrates vehicle path radii important for geometric design. Planning also references multiple IRC and external manuals for comprehensive guidelines on landscaping, drainage, signage, and markings.

Sources: Clause 5, Clause 9.1, Table 9.1, Clause 14.1

Types of roundabouts per IRC 65 are mainly distinguished by size, speed, and design features as summarized in Table 4.1. Key points include:

• Single lane roundabouts: Inscribed Circle Diameter (ICD) between 28-40 m.
• Double lane roundabouts: ICD between 40-70 m.
• Rotaries: Larger than 70 m ICD, allow higher speeds (>40 kmph), and have smaller entry angles.

Table 6.4 provides turning radii and minimum ICD for normal roundabouts based on central island diameter (CID):

For capacity estimation, Table 9.1 gives entry capacity models based on roundabout diameter (D):

These formulas and tables help design roundabouts with appropriate size, speed control, and capacity per IRC 65 clauses 4.1, 6.4, and 9.1.

Sources: Clause 4.1, Clause 6.4, Clause 9.1

Rotary intersections are larger traffic circles with an Inscribed Circle Diameter (ICD) greater than 70 m, allowing higher speeds (>40 kmph) and involving weaving behavior between entering and circulating traffic, unlike roundabouts which rely on gap acceptance and priority rules (Clause 4.1). Key geometric parameters include:

• Central Island: Raised area converting direct conflicts to angular ones, providing turning radius.
• Circulatory Carriageway: Clockwise path around the central island.
• Entry Radius & Width: Curvature and width at entry to control speed.
• Exit Radius & Width: Curvature and width at exit to facilitate acceleration.
• Non-Weaving Width: Width used by circulating traffic.
• Splitter Island: Kerbed island separating entry and exit traffic.
• Weaving Section & Length: Area where merging/diverging occurs within the rotary.

Refer to Fig. 2.1 for geometric elements and Table 4.1 for comparison with roundabouts (Clause 2.1, 3.2, 4.1).

Sources: Clause 2.1, Clause 3.2, Clause 4.1

As per IRC 65 Clause 5.3, roundabouts are suitable at intersections where minor road delays under 'Stop' or 'Give Way' signs are high, where right-turning traffic is significant, at rural cross intersections prone to crossing accidents, and where future traffic growth is expected. They are also effective at multi-leg intersections where priority is hard to define. However, roundabouts require more land and are not ideal where pedestrian crossing priority is needed.

Key geometric design parameters from Clause 6.2 and Table 6.2 for single lane roundabouts are:

**Note: A truck apron is desirable at 4 m central island diameter to aid vehicle deflection.

For capacity estimation (Clause 9.1, Table 9.1), entry capacity C (PCUs/hr) depends on roundabout diameter and circulating flow Q (PCUs/hr) as:

C = A * exp(-B * Q)

Where A and B vary with diameter:

This model is from the Indian Highway Capacity Manual (INDO-HCM).

These guidelines help select and design roundabouts for efficient traffic management with safety and capacity considerations.

Sources: Clause 5.3, Clause 6.2, Table 6.2, Clause 9.1, Table 9.1

IRC 65 provides comprehensive geometric design parameters for roundabouts and rotaries, detailed in Clause 2.1 and Section 6. Key geometric elements include:

• Central Island: Raised portion around which vehicles maneuver, converting direct conflict points into angular ones and providing turning radius (Clause 2.1).
• Circulatory Carriageway: Clockwise curved path around the central island; width measured between outer kerb edge and central island kerb face.
• Inscribed Circle Diameter: Defines roundabout size, measured between outer edges of the circulatory path.
• Entry and Exit Radius: Curvature radii at entry and exit to control vehicle speed and movement.
• Splitter/Deflecting Island: Kerbed island separating opposing vehicles on the same arm.
• Weaving Section and Width: Road section and width where merging/diverging occurs within the rotary.

These parameters govern safe vehicle paths, speeds, and conflict point reduction. Detailed figures and definitions are in Clause 2.1 and Section 6 (6.1 to 6.13) covering central island, entry/exit design, entry angle, design speed, sight distance, camber, and super-elevation.

For example, Fig. 2.1 illustrates these geometric elements clearly.

This structured approach ensures safe, efficient roundabout design per IRC 65.

Sources: Clause 2.1, Section 6 (6.1 to 6.13)

For IRC 65, the key specifications for Central Island and Circulatory Carriageway in single lane roundabouts are as follows:

• The diameter of the roundabout is defined as the diameter of the central island used for capacity and Level of Service (LoS) estimation (Clause 6.1).

• Table 6.2 provides the relationship between Inscribed Circle Diameter, Central Island Diameter, and Width of Circulatory Carriageway:

Note: It is desirable to provide a Truck Apron around the central island to ensure deflection of light vehicles (Clause 6.2).

• The central island should be positioned so that the center lines of approach roads pass through the center of the inscribed circle, ensuring all vehicles are deflected, including those from the extreme left lane (Clause 6.2.1).

Sources: Clause 6.1, Table 6.2, Clause 6.2, Clause 6.2.1

As per Clause 6.2.1 of IRC 65, the central island should be positioned so that the center lines of the approach roads pass through the center of the inscribed circle. This ensures that all vehicles, including those from the extreme left lane, are deflected when approaching the roundabout. The size and position must enforce this deflection.

The key specifications for single lane roundabouts are given in Table 6.2, which relates the Inscribed Circle Diameter, Central Island Diameter, and Width of Circulatory Carriageway as follows:

Note: For a 4 m central island diameter, it is desirable to provide a truck apron to ensure deflection of light vehicles.

This guidance is sourced from the Design Manual for Roads & Bridges, UK, TD 16/07, 2007, as referenced in Clause 6.2.

Sources: Clause 6.2.1, Table 6.2

As per IRC 65 Clause 6.3, the entry and exit design of roundabouts depends on variables such as entry width, entry flaring, and entry angle. The key specifications for radius of entry and exit curves and carriageway widths at entry and exit are given in Table 6.3.7. This table relates the number of approach lanes and roundabout design speed to the radius of curves and carriageway widths as follows:

The entry and exit curves are designed as continuations of the inside curve tangential to the central island and outside curve tangential to the outside edge of the circulatory roadway, ensuring smooth vehicle movement. Refer to Fig. 6.1 in Clause 6.1 for geometric illustration of these curves.

This design ensures safe and efficient vehicle maneuvering at roundabout entries and exits.

Sources: Clause 6.3, Table 6.3.7, Clause 6.1 Fig. 6.1

As per IRC 65 Clause 6.4 and 6.4.6, splitter or channelizing islands are designed with specific geometric details to guide traffic flow safely. The minimum offset for splitter islands ranges from 0.25 m to 0.5 m, and for raised splitter islands, the offset is between 0.5 m to 1.0 m with a recommended radius of curvature R = 1.0 m. Figures 6.4 and 6.5 in the code illustrate these dimensions and shapes, ensuring smooth vehicle movement and effective channelization. These geometric parameters are critical for the safe and efficient design of traffic islands.

Sources: Clause 6.4, Clause 6.4.6, Fig. 6.4, Fig. 6.5

As per IRC 65 Clause 6.6 and 6.6.3, the entry angle is a geometric measure representing the conflict angle between entering and circulating traffic streams at a roundabout. It should be between 20° and 60°, and importantly, the entry angle must be larger than the exit angle. For large roundabouts (Clause 6.6.1), the entry angle is determined by constructing the curve EF as the locus of midpoints between the nearside kerb and median line (or splitter island edge), then drawing BC as the tangent to EF at the give way line. The curve AD represents the locus of midpoints of the circulatory carriageway. The entry angle is the angle ACB formed by these constructions. This method provides a precise geometric approach to measure the entry angle for design and safety evaluation.

Sources: Clause 6.6, Clause 6.6.1, Clause 6.6.3

As per Clause 6.8 of IRC 65, the design speed is a key parameter for geometric design and is influenced by sight distance and turning radius. Excessive reduction in design speed can increase delays and reduce service levels at roundabouts. The relationship between operating speed and central island radius is illustrated in Fig. 6.10 (not provided here). Additionally, Table 6.5 under Clause 6.5 specifies the minimum approach sight distance required for various speeds, which directly impacts design speed selection. For example, at 80 km/h, the approach sight distance should be 105 m. Super-elevation (e) is considered in design, typically around +0.02, to aid in safe turning at the design speed. These parameters ensure safe and efficient vehicle movement on roundabouts and other geometric elements.

Sources: Clause 6.8, Clause 6.5, Table 6.5, Clause 6.8.2

As per IRC 65 Clause 6.13, camber is the transverse slope provided on the carriageway to facilitate drainage. Super-elevation is the transverse slope provided on curves to counteract centrifugal force. Clause 6.13.2 specifies camber for rotaries and bigger roundabouts, typically higher than straight sections to aid drainage and vehicle stability. Figures 6.17 and 6.18 illustrate camber and super-elevation in single lane and two lane roundabouts respectively. Key points include:

• Camber: Usually ranges from 2.5% to 3.5% on straight sections.
• Super-elevation: Varies with design speed and curve radius; typically up to 7% on sharp curves.

Unfortunately, exact formulas and tabulated values are not provided in the retrieved context. For detailed design, refer to the figures and clauses mentioned for graphical and tabular guidance on camber and super-elevation in roundabouts.

Sources: Clause 6.13, Clause 6.13.2, Clause 6.17, Clause 6.18

As per IRC 65, Clause 6.16 and 6.19, road signs and pavement markings must be placed at convenient and suitable locations to ensure safe and uninterrupted traffic flow. The lane marking and signing should follow the guidelines of IRC 35 "Code of Practice for Road Markings" and IRC 67 "Code of Practice for Road Signs" respectively. Typical sign and marking plans for roundabouts are illustrated in Fig. 6.19 and Fig. 6.20 of IRC 65. Key specifications include:

• Placement of signs and markings for clear guidance and safety.
• Use of standard symbols and colors as per IRC 67.
• Pavement markings such as lane lines, stop lines, and directional arrows as per IRC 35.

Unfortunately, specific formulas or tables for signs and markings are not detailed in IRC 65 but are referenced to IRC 35 and IRC 67 codes.

For detailed design, refer to:

• IRC 35 for pavement marking dimensions, colors, and patterns.
• IRC 67 for sign types, sizes, and placement criteria.

This ensures uniformity and compliance with Indian road safety standards.

Sources: Clause 6.16, Clause 6.19

For Non-Motorized Transportation (NMT) at roundabouts per IRC 65, key considerations include accommodating bicycles, cycles, cycle rickshaws, hand carts, and animal carts with appropriate Passenger Car Unit (PCU) values and ensuring safe interaction with motorized traffic. Table 5.2 provides PCU values for various NMT modes based on roundabout diameter, for example, for 20<D≤30 m diameter:

Entry capacity of roundabouts considering circulating flow Q (PCUs/hr) is given by the formula (Clause 9.1):

C = A * exp(-B * Q)

where A and B depend on diameter D:

These values help estimate entry capacity considering NMT and motorized traffic mix. Roundabouts maintain low speeds (<40 kmph) to enhance safety for NMT users (Clause 4.1). Design should ensure clear signage and markings (Clause 6.20) for NMT paths.

This summary highlights key formulas, tables, and specifications for NMT at roundabouts from IRC 65.

Sources: Clause 4.1, Clause 5.2, Clause 6.20, Clause 9.1, Table 5.2, Table 9.1