Guidelines for Planning and Design of Roundabouts (First Revision)
2017 Edition

IRC 65 outlines recommended procedures for the geometric design and planning of traffic rotaries. It includes definitions related to geometry, traffic flow, driver behavior, and operational performance. Design considerations for single-lane and multilane roundabouts are covered. Planning topics such as intersection hierarchy, Passenger Car Unit (PCU) calculations, and site selection criteria are discussed. Geometric details include central island and circulatory carriageway dimensions, entry and exit layouts, splitter islands, entry flaring and angles, design speeds, and sight distances. The code also references related standards for markings, signage, drainage, and landscaping to ensure comprehensive guidance for safe and efficient rotary designs.

Key planning aspects involve determining intersection priority, calculating PCU values for roundabouts, and selecting appropriate sites. Entry capacity models based on roundabout diameter use critical gap and follow-up times to estimate maximum entry flows. Formulas express capacity as a function of circulating flow, with parameters varying by roundabout size. Vehicle path radii critical to geometric design are illustrated in the code. References to other IRC manuals cover landscaping, drainage, signage, and marking for integrated planning.

Roundabouts are categorized primarily by size, design speed, and geometric features. Single-lane roundabouts typically have an Inscribed Circle Diameter (ICD) between 28 to 40 meters, while multilane variants range from 40 to 70 meters. Larger traffic circles known as rotaries exceed 70 meters ICD, accommodate higher speeds above 40 km/h, and exhibit different entry angles. Tables provide recommended turning radii and minimum ICD values based on central island size to aid design. Capacity estimation formulas are included to match roundabout dimensions and expected traffic volumes.

Rotaries are large circular junctions with ICDs greater than 70 meters, permitting higher vehicle speeds and weaving maneuvers between entering and circulating traffic. Unlike roundabouts that depend on gap acceptance and priority rules, rotaries involve continuous merging. Key geometric elements include the central island, circulatory carriageway, entry and exit radii and widths, splitter islands, and weaving zones. The code provides detailed definitions and comparisons between roundabouts and rotaries.

Roundabouts are recommended at intersections where minor approach delays are significant under stop or yield controls, where right-turn movements are heavy, and at rural crossroads prone to crossing accidents. They are also suited for multi-leg intersections with complex priority assignment and locations anticipating future traffic increases. However, they require more land and are less suitable where pedestrian crossings on all legs are a priority. Geometric parameters for single-lane roundabouts, including inscribed circle diameter, central island size, and circulatory roadway width, guide layout design. Capacity estimation uses exponential models relating entry capacity to circulating flow, aligned with the Indian Highway Capacity Manual.

IRC 65 details the geometric elements essential for designing roundabouts and rotaries. These include the raised central island that converts conflict points and provides turning radius, the circulatory carriageway surrounding it, and the inscribed circle diameter defining overall size. Entry and exit curve radii and widths control vehicle speeds and paths. Splitter or deflecting islands channelize traffic. Design also addresses weaving areas within rotaries. The code provides extensive figures and tables covering all aspects such as entry angles, design speeds, sight distances, camber, and super-elevation to promote safe and efficient traffic movement.

For single-lane roundabouts, the central island diameter and circulatory carriageway widths correspond to the inscribed circle diameter, which is used for capacity and level of service calculations. Table 6.2 specifies these relationships, showing decreasing carriageway widths as central island size increases. A truck apron around smaller central islands is recommended to facilitate vehicle deflection. Positioning ensures approach centerlines intersect the inscribed circle center, guaranteeing all vehicles, including those from outer lanes, are deflected appropriately.

The central island must be located such that the center lines of all approach roads pass through the center of the inscribed circle. This arrangement ensures deflection of all entering vehicles, including those on the extreme left lane. The size and location of the central island enforce this deflection. Table 6.2 provides dimensions linking inscribed circle diameter, central island diameter, and circulatory carriageway width for single-lane roundabouts. Provision of a truck apron is advisable for smaller central islands to aid light vehicle maneuvering.

The geometry of entry and exit curves depends on the number of approach lanes and design speed. Table 6.3.7 specifies radii for these curves and carriageway widths at entry and exit for different road types. Entry and exit curves are designed as tangents to the central island and outer circulatory edge, ensuring smooth vehicle transitions. Proper curve geometry reduces speed and enhances safety at roundabout approaches and departures.

Splitter islands, raised or kerbed, separate entering and exiting traffic streams and aid in traffic channelization. Minimum offsets range from 0.25 to 0.5 meters for standard islands and 0.5 to 1.0 meters for raised islands with a curvature radius of about 1 meter. These dimensions ensure smooth vehicle flow and effective separation. Illustrations in the code demonstrate appropriate shapes and sizes for splitter islands, supporting safety and operational efficiency.

The entry angle is the geometric angle formed between entering and circulating traffic streams, ranging between 20° and 60°. It must be greater than the exit angle to control vehicle speeds effectively. For large roundabouts, the entry angle is determined by geometric constructions involving midpoints between kerbs and median lines. This precise measurement ensures safe conflict angle management and aids in design verification.

Design speed influences geometric parameters such as turning radii and sight distances. Excessively low design speeds can lead to increased delays and reduced level of service. Minimum approach sight distances are specified for various speeds (e.g., 105 m for 80 km/h). Super-elevation is incorporated, typically around 2%, to promote safe turning. These parameters balance safety with operational efficiency in roundabout designs.

Camber is the transverse slope to facilitate surface drainage, generally ranging from 2.5% to 3.5% on straight sections. Super-elevation counteracts centrifugal forces on curves and varies with speed and radius, reaching up to 7% on sharp curves. These slopes maintain vehicle stability and pavement durability. Detailed graphical guidance is provided for single and multilane roundabouts.

Placement of road signs and pavement markings must ensure clear guidance and uninterrupted traffic flow. Designs should comply with IRC 35 (road markings) and IRC 67 (road signs). Typical layouts include lane markings, stop lines, directional arrows, and standard signage symbols and colors. Proper positioning and visibility are essential for safety and driver information.

Roundabout design must accommodate non-motorized users such as bicycles, cycle rickshaws, hand carts, and animal carts with appropriate PCU values. Entry capacity models consider the mix of motorized and non-motorized traffic. Roundabouts maintain speeds below 40 km/h to enhance safety for vulnerable users. Dedicated signage and markings should clearly define NMT paths to ensure safe coexistence with motor vehicles.