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IRC 86 : 2018Geometric Design Standards for Rural Highways

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CurrentFrequently UsedCode of PracticeTransportation · Roads and Pavement
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OverviewValues34InternationalTablesFAQ14Related

IRC 86:2018 is the Indian Standard (IRC) for geometric design standards for rural highways. IRC 86 covers the geometric design of rural highways — the shape, alignment, and cross-section of roads. Design speed determines every geometric parameter: sight distance, curve radius, superelevation, gradient. Essential for highway planning and alignment design.

Geometric design standards for rural highways including alignment, cross-section, sight distance, horizontal curves, vertical curves, and intersections.

Quick Reference — Top IRC 86:2018 Values

Key design speeds, cross-section widths, alignment parameters, and intersection geometry limits for urban road design in plains.

✓ Verified 2024-05-15
ReferenceValueClause
Design Speed - Arterial Road60-80 km/hCl. 2.4 (Table 2.1)
Right of Way (RoW) - Arterial Road40-60 mCl. 2.5 (Table 2.2)
Carriageway Lane Width— Standard for urban roads.3.5 mCl. 4.2.1
Camber - High Bituminous Surface— For areas with rainfall > 1000 mm/year.2.0 %Cl. 4.3 (Table 4.1)
Camber - Cement Concrete Pavement— For areas with rainfall > 1000 mm/year.1.7 %Cl. 4.3 (Table 4.1)
Median Width (Desirable Min.)— To accommodate U-turns and pedestrian refuge.5.0 mCl. 4.4.1
Footpath Width (Absolute Min.)— Represents clear width available for pedestrians.1.8 mCl. 4.6.1
Cycle Track Width (Min. One-way)2.0 mCl. 4.7.2
Vertical Clearance (Min.)— Across the entire carriageway width.5.5 mCl. 4.11
SSD - Driver Reaction Time (t)2.5 secCl. 5.2.1
SSD - Longitudinal Friction (f)— For design speed of 60 km/h.0.37Cl. 5.2.1 (Table 5.1)
Max. Superelevation (e_max)— In plain and rolling terrain.7.0 %Cl. 6.2.1
Lateral Friction Coefficient (f_lat)— Used for calculating minimum radius of horizontal curve.0.15Cl. 6.2.2
Min. Radius of Horizontal CurveR_min = V² / (127(e_max + f_lat))Cl. 6.2.2
Transition Curve Length— Based on rate of change of centrifugal acceleration.Ls = V³ / (C * R)Cl. 6.4.3
Ruling Gradient (Max.)3.3 % (1 in 30)Cl. 7.2 (Table 7.1)
Limiting Gradient (Max.)5.0 % (1 in 20)Cl. 7.2 (Table 7.1)
Min. Longitudinal Gradient (Drainage)— For kerbed roads to prevent water stagnation.0.5 % (1 in 200)Cl. 7.4
Bus Lay-by Width3.0 mCl. 9.2.1
Min. Inner Radius for U-Turn (Cars)6.5 mCl. 9.4
⚠ Verify against the latest IRC publication and project specifications. Amendments may modify values.

Overview

Status
Current
Usage level
Frequently Used
Domain
Transportation — Roads and Pavement
Type
Code of Practice
Typically used with
IS 73IRC 38IRC 67IRC 35
Also on InfraLens for IRC 86
34Key values11Tables14FAQs
Practical Notes
! NOTE: Correct edition is IRC 86:2018 (First Revision)
! Design speed is the KEY parameter — everything else (curves, gradients, sight distance) follows from it.
! NH design speed: 100 km/h in plains — don't confuse with speed limit (which may be lower).
! Sight distance must be checked at every horizontal and vertical curve — inadequate sight distance causes accidents.
! Superelevation max 7% — higher values cause slow vehicles to slide inward.
! When designing rural highways, prioritize the safety aspect by ensuring adequate sight distances (SSD and OSD) even if it means slightly increasing construction costs. This is crucial for avoiding accidents, especially on two-lane roads where overtaking is frequent.
! For horizontal curves, always use the design speed for calculating the ruling minimum radius. If the available space restricts this, consider lowering the design speed for that section or providing additional super-elevation, but never compromise on the absolute minimum radius.
! In undulating terrain, the gradient requirements must be carefully balanced with the need for good sight distances on vertical curves. Ensure the sight line offset is maintained, especially at crest curves.
! Shoulder width is often underestimated. For rural highways, especially those with future potential for higher traffic volumes or use by slow-moving vehicles, providing wider shoulders (even exceeding the minimum) is a sound investment for traffic management and emergency stops.
! The selection of design speed is paramount and dictates many subsequent geometric parameters. It should be based on the likely future traffic and the functional classification of the road (NH, SH, MDR, ODR).
! When transitioning from a curve to a tangent or vice-versa, ensure a smooth transition using vertical curves. The length of these curves should be adequate to avoid abrupt changes in gradient.
! For intersections on rural roads, simple at-grade intersections are common. However, if traffic volumes warrant, consider channelization and appropriate sight distances within the intersection area to improve safety and traffic flow.
! Earthen shoulders should have adequate camber to facilitate drainage. This prevents waterlogging and erosion, which can compromise the structural integrity of the road edges.
! The concept of 'valley curves' is as important as 'summit curves'. Ensure adequate comfort and safety for vehicles negotiating sag curves, especially at night or in adverse weather conditions.
! When dealing with horizontal curves, always check the sight line clearance for obstructions. This includes vegetation, buildings, and other structures that might impede visibility, especially for approaching vehicles.
! The permissible gradient in Sahyadri region is a critical consideration. Stick strictly to the specified limits to prevent vehicles from stalling or losing control on steep inclines.
! Super-elevation should be introduced gradually using transition curves where possible. Abrupt changes in super-elevation can cause discomfort and instability for vehicles.
! The geometric design should consider the environmental impact. Minimize earthwork and preserve natural drainage patterns as much as possible.
! Always refer to the latest version of IRC codes. For example, IRC 86:2018 supersedes earlier versions and incorporates updated design practices.
! For PMGSY roads, specific geometric standards apply, often adapted from IRC codes but with considerations for lower traffic volumes and simpler construction methods.
Frequently referenced clauses
Cl. 3 — Road classification and design speedCl. 4 — Cross-section elements (carriageway, shoulder, median)Cl. 5 — Sight distance (SSD, OSD, ISD)Cl. 6 — Horizontal curve design (radius, superelevation, widening)Cl. 7 — Vertical curve design (summit, valley)Cl. 8 — Intersection designCl. 2.1.1 — ObjectivesCl. 3.1 — Design SpeedCl. 4.1 — Sight DistanceCl. 4.2 — Stopping Sight Distance (SSD)Cl. 4.3 — Overtaking Sight Distance (OSD)Cl. 5.1 — Horizontal AlignmentCl. 6.1 — Vertical AlignmentCl. 7.1 — Cross-Section Elements
Pulled from IRC 86:2018. Browse the full clause & table index below in Tables & Referenced Sections.
geometric designrural highwayroad alignmentsight distancehorizontal curvevertical curveIRC

International Equivalents

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Key Values34

Quick Reference Values
NH design speed100 km/h (plain), 80 km/h (rolling), 50 km/h (hilly)
SH design speed80 km/h (plain), 65 km/h (rolling), 40 km/h (hilly)
Carriageway width (2-lane)7.0m
Shoulder width (paved)1.5m each side
Min SSD at 100 km/h180m
Min radius at 100 km/h360m
Max gradient (plain)3.3%
Max superelevation7%
Design Speed (kmph) for NHs100 kmph
Design Speed (kmph) for SHs80 kmph
Design Speed (kmph) for MDRs60 kmph
Design Speed (kmph) for ODRs50 kmph
Minimum Stopping Sight Distance (SSD) for 100 kmph470 m
Minimum Stopping Sight Distance (SSD) for 80 kmph310 m
Minimum Stopping Sight Distance (SSD) for 60 kmph190 m
Minimum Stopping Sight Distance (SSD) for 50 kmph150 m
Minimum Overtaking Sight Distance (OSD) for 100 kmph740 m
Minimum Overtaking Sight Distance (OSD) for 80 kmph530 m
Minimum Overtaking Sight Distance (OSD) for 60 kmph360 m
Minimum Overtaking Sight Distance (OSD) for 50 kmph280 m
Maximum Gradient (Sahyadri Region/Hilly Terrain)1 in 20 (5%)
Maximum Gradient (Plains)1 in 30 (3.3%)
Minimum Radius of Horizontal Curve (Flat Terrain) for 100 kmph475 m
Minimum Radius of Horizontal Curve (Flat Terrain) for 80 kmph290 m
Minimum Radius of Horizontal Curve (Flat Terrain) for 60 kmph145 m
Maximum Super-elevation Rate7%
Absolute Minimum Radius of Horizontal Curve50 m
Minimum Sight Line Offset for Grade Separated Interchanges4.5 m
Minimum Camber on Earthen Shoulders3%
Minimum Lane Width for National Highways3.75 m
Minimum Lane Width for State Highways3.50 m
Minimum Lane Width for Major District Roads3.00 m
Minimum Shoulder Width (paved carriageway) for NHs3.0 m
Minimum Shoulder Width (paved carriageway) for SHs2.5 m
Key Formulas
SSD = 0.278Vt + V²/(254f)
where V=speed(km/h), t=reaction time(2.5s), f=friction(0.35-0.40)
Superelevation e = V²/(127R) - f
SSD = 0.278 * V * T + V^2 / (254 * (f + G))
OSD = 3.6 * (u + ut + 2T) * u
e = V^2 / (g * R)
R = V^2 / (127 * (e + f))

Tables & Referenced Sections

Key Tables
Table 1 — Design speed by road classification
Table 2 — Minimum sight distances
Table 3 — Minimum curve radii
Table 4 — Superelevation values
Table 4.1 — Stopping Sight Distance (SSD) for Different Design Speeds
Table 4.3 — Overtaking Sight Distance (OSD) for Different Design Speeds
Table 5.1 — Ruling Minimum Radius of Horizontal Curves for Different Design Speeds and Maximum Values of Super-elevation
Table 5.5 — Setback Distances for Curves on Plains and Rolling Terrain
Table 6.1 — Ruling and Limiting Gradients
Table 7.1 — Recommended Lane Widths
Table 7.2 — Recommended Shoulder Widths
Key Clauses
Cl. 3 — Road classification and design speed
Cl. 4 — Cross-section elements (carriageway, shoulder, median)
Cl. 5 — Sight distance (SSD, OSD, ISD)
Cl. 6 — Horizontal curve design (radius, superelevation, widening)
Cl. 7 — Vertical curve design (summit, valley)
Cl. 8 — Intersection design
Cl. 2.1.1 — Objectives
Cl. 3.1 — Design Speed
Cl. 4.1 — Sight Distance
Cl. 4.2 — Stopping Sight Distance (SSD)
Cl. 4.3 — Overtaking Sight Distance (OSD)
Cl. 5.1 — Horizontal Alignment
Cl. 6.1 — Vertical Alignment
Cl. 7.1 — Cross-Section Elements

Related Resources on InfraLens

Cross-Referenced Codes
IS 73:2013Paving Bitumen - Specification
→
IRC 38:1988Guidelines for Design of Horizontal Curves fo...
→
IRC 67:2012Code of Practice for Road Signs
→
IRC 35:2015Code of Practice for Road Markings
→

Frequently Asked Questions14

What is the design speed for national highway?+
100 km/h in plain terrain, 80 km/h in rolling terrain, 50 km/h in hilly/mountainous terrain per IRC 86. Design speed determines minimum curve radii, sight distances, and gradients.
What is stopping sight distance?+
The minimum distance a driver needs to see ahead to stop safely before an obstacle. At 100 km/h: SSD = 180m (includes 2.5s reaction time + braking distance). Every point on the road must provide at least this much visibility.
What is the primary difference between the design speeds for National Highways (NH) and Major District Roads (MDR)?+
The primary difference lies in their intended function and traffic volume. National Highways (NH) are designed for high-speed, high-volume traffic, typically with a higher design speed (e.g., 100 kmph) and stricter geometric standards. Major District Roads (MDR), serving within a district, are designed for moderate traffic volumes and speeds, hence a lower design speed (e.g., 60 kmph) and slightly relaxed geometric parameters compared to NHs.
How is Stopping Sight Distance (SSD) calculated, and why is it crucial?+
SSD is calculated using a formula that considers the design speed, driver's reaction time, and braking efficiency (friction). It's the minimum distance required for a driver to see an object on the road and bring their vehicle to a complete stop before colliding with it. Ensuring adequate SSD is paramount for preventing accidents, especially on two-lane rural roads where visibility is critical.
What is the role of super-elevation in horizontal curves?+
Super-elevation is the banking of the road surface on horizontal curves, where the outer edge is raised higher than the inner edge. Its primary role is to counteract the centrifugal force acting on vehicles, thereby reducing the lateral forces on the tires and enhancing safety and comfort for drivers. It allows vehicles to negotiate curves at higher speeds.
What are the implications of exceeding the maximum permissible gradient on rural roads?+
Exceeding the maximum permissible gradient on rural roads can lead to several safety and operational issues. For ascending gradients, slower vehicles may struggle to maintain speed, potentially causing traffic congestion. For descending gradients, vehicles can gain excessive speed, making braking difficult and increasing the risk of accidents, especially for heavy vehicles.
How does terrain affect the geometric design of rural highways?+
Terrain significantly influences geometric design. In hilly or mountainous terrain (like Sahyadri regions), steeper gradients and sharper curves may be necessary due to topographical constraints, requiring careful consideration to maintain safety. Flat terrain allows for more flexible and often more efficient geometric layouts with gentler curves and gradients.
What is the difference between ruling and limiting gradients?+
The ruling gradient is the maximum gradient provided on a particular section of road for the prevailing terrain. The limiting gradient is a steeper gradient that can be provided on shorter stretches, typically in difficult terrain, where economic considerations or site constraints necessitate it. The ruling gradient ensures smooth and safe traffic flow under normal conditions.
Why are wider shoulders recommended for National Highways compared to other rural roads?+
National Highways are designed for higher traffic volumes and speeds, including potential for breakdown vehicles, slow-moving traffic, and emergency services. Wider shoulders provide a safer space for vehicles to pull over, facilitate maintenance activities, and offer a buffer zone, thereby enhancing overall road safety and operational efficiency.
What is Overtaking Sight Distance (OSD), and why is it important for two-lane roads?+
Overtaking Sight Distance (OSD) is the minimum distance required for a faster vehicle to safely overtake a slower vehicle on a two-lane road. It accounts for the time taken by the overtaking vehicle to accelerate, travel past the slower vehicle, and decelerate to a safe position. It's critical for two-lane roads to prevent head-on collisions during overtaking maneuvers.
Are there any specific considerations for geometric design in the context of the Pradhan Mantri Gram Sadak Yojana (PMGSY)?+
Yes, while PMGSY roads generally adhere to IRC standards, they often have specific adaptations. These adaptations typically involve design speeds suitable for rural connectivity, potentially simpler intersection designs, and considerations for construction feasibility and cost-effectiveness in remote areas, while still prioritizing basic safety parameters.
What is the minimum sight line offset for grade-separated interchanges on rural highways?+
The minimum sight line offset for grade-separated interchanges is a critical safety parameter to ensure clear visibility of merging and diverging traffic. IRC 86:2018 specifies this offset to prevent collisions in these complex intersection areas. The value typically relates to the required sight distance within the interchange ramps.
How does the transition curve length impact the design of horizontal curves?+
Transition curves, also known as spiral curves, are introduced between a straight tangent and a circular curve (or between two curves of different radii). They gradually introduce or remove the radial acceleration and super-elevation. Adequate transition curve length ensures a smooth change in forces on the vehicle and driver, preventing discomfort and potential instability, thus improving safety.
What are the key elements of a rural highway cross-section?+
The cross-section of a rural highway typically includes the carriageway (lanes), shoulders, drainage ditches or side slopes, and potentially median elements or guardrails depending on the road type and location. Each element is designed to ensure safe passage of traffic, effective drainage, and structural integrity of the roadway.

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