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IRC 38 : 1988
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Guidelines for Design of Horizontal Curves for Highways and Design Tables

AASHTO Green Book Ch. 3
CurrentFrequently UsedCode of PracticeTransportation · Roads and Pavement
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OverviewValues27InternationalTablesFAQ13Related

Overview

IRC 38:1988 is the Indian Standard (IRC) for guidelines for design of horizontal curves for highways and design tables. IRC 38 provides design tables for horizontal curves — the most referenced IRC code for road alignment design. Every curve on every highway in India is designed using these tables. Covers radius, superelevation, extra widening, and transition length.

Design of horizontal curves for highways including simple, compound, reverse, and transition curves with design tables for various speeds.

Status
Current
Usage level
Frequently Used
Domain
Transportation — Roads and Pavement
Type
Code of Practice
International equivalents
AASHTO Green Book Ch. 3 · AASHTO (US)
Typically used with
IRC 86IS 73IRC 67
Also on InfraLens for IRC 38
27Key values10Tables13FAQs
Practical Notes
! Every horizontal curve needs: minimum radius check, superelevation design, extra widening, and transition curve.
! Superelevation is developed over the transition length — not abruptly.
! Sharp curves (below absolute minimum radius) are prohibited on NH/SH.
! Always prioritize a higher design speed than the actual traffic speed to ensure safety, especially on rural roads managed by PMGSY.
! When selecting radius, use the table values but also consider sight distance requirements (as per IRC:66) to avoid conflicts.
! For reverse curves, ensure sufficient straight length between them to prevent driver disorientation.
! Transition curves are crucial for rider comfort and safety; do not omit them unless absolutely necessary and traffic speeds are very low.
! The rate of introduction of centrifugal force (C) significantly impacts comfort. Use higher values of C for higher speeds.
! Super-elevation calculations must consider both equilibrium and maximum allowable rates. Ensure smooth transition of super-elevation.
! When designing curves for NHAI projects, always check for any specific circulars or additional requirements beyond IRC 38.
! Site constraints might necessitate a radius smaller than the minimum prescribed. In such cases, a thorough risk assessment and justification are required, and often lower speeds are enforced.
! For urban roads, the minimum radius might be dictated by property lines and ROW. Adjustments to super-elevation and transition curve length may be needed.
! Hill roads present unique challenges; IRC 38 provides general guidance, but specific modifications based on local topography are vital.
! The interaction between horizontal curves and vertical curves (sumps and crests) must be checked for sight distance and drainage.
! Always double-check all calculations from the design tables and formulas, especially for critical parameters like radius and transition length.
! The coefficient of lateral friction (f) is assumed as per the code, but actual conditions (wet/dry) might vary. Design conservatively.
! In areas prone to frost or heavy rainfall, consider the impact on tire-road friction and adjust design parameters where permissible.
! Ensure proper signing and marking of curves as per IRC:65 and other relevant codes to warn drivers of upcoming changes in alignment.
Frequently referenced clauses
Cl. 3 — Minimum radius for different design speedsCl. 4 — Superelevation designCl. 5 — Extra widening on curvesCl. 6 — Transition curve lengthCl. 7 — Design tables for all speedsCl. 2.1.1 - Classification of CurvesCl. 3.1.1 - Design of Simple CurvesCl. 4.1.1 - Design of Transition CurvesCl. 5.1.1 - Design of Compound CurvesCl. 6.1.1 - Design of Reverse CurvesCl. 7.1.1 - Design of Helical CurvesCl. 8.1.1 - Design Tables and their UseCl. 9.1.1 - Special Considerations for Urban Areas and Hill Roads
Pulled from IRC 38:1988. Browse the full clause & table index below in Tables & Referenced Sections.
horizontal curvehighway curvesuperelevationtransition curveroad geometryIRC

International Equivalents

Similar International Standards
AASHTO Green Book Ch. 3AASHTO (US)
HighCurrent
Horizontal Alignment
Both provide horizontal curve design standards with similar physics-based approach.
Key Differences
≠IRC: 7%. AASHTO: 4-12% depending on context.
Key Similarities
≈Both use the same fundamental equation: e+f = V²/gR for curve design.
Parameter Comparison
ParameterIS ValueInternationalSource
Min radius at 100 km/h360m340m (with e=8%)AASHTO
⚠ Verify details from original standards before use

Key Values27

Quick Reference Values
Min radius at 100 km/h360m
Min radius at 80 km/h230m
Max superelevation7%
Transition curve typeSpiral (clothoid)
Design Speed (kmph)30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160
Minimum Radius of Curvature (m) for Design Speed 30 kmph45
Minimum Radius of Curvature (m) for Design Speed 50 kmph125
Minimum Radius of Curvature (m) for Design Speed 80 kmph375
Minimum Radius of Curvature (m) for Design Speed 100 kmph600
Minimum Radius of Curvature (m) for Design Speed 120 kmph900
Maximum degree of curve (degrees) for Design Speed 30 kmph12.73
Maximum degree of curve (degrees) for Design Speed 50 kmph4.58
Maximum degree of curve (degrees) for Design Speed 80 kmph1.53
Maximum degree of curve (degrees) for Design Speed 100 kmph0.96
Maximum degree of curve (degrees) for Design Speed 120 kmph0.64
Minimum length of transition curve (m) for 50 kmph60
Minimum length of transition curve (m) for 80 kmph120
Minimum length of transition curve (m) for 100 kmph180
Minimum length of transition curve (m) for 120 kmph250
Rate of introduction of centrifugal force (m/s^3) for 50 kmph0.75
Rate of introduction of centrifugal force (m/s^3) for 80 kmph0.80
Rate of introduction of centrifugal force (m/s^3) for 100 kmph0.85
Rate of introduction of centrifugal force (m/s^3) for 120 kmph0.90
Super-elevation rate (e) for 50 kmph (no grooving)0.07
Super-elevation rate (e) for 80 kmph (no grooving)0.07
Super-elevation rate (e) for 100 kmph (no grooving)0.05
Super-elevation rate (e) for 120 kmph (no grooving)0.05
Key Formulas
e + f = V²/(127R)
where e=superelevation, f=side friction(0.15), V=speed(km/h), R=radius(m)
Transition length Ls = V³/(46.7CR) where C=rate of change of centripetal acceleration
R = (V^2 / 127 * f) for minimum radius (where R is radius in meters, V is design speed in kmph, f is coefficient of lateral friction)
L_T = (V^3 / (C * R)) for length of transition curve (where L_T is length in meters, V is design speed in m/s, C is rate of introduction of centrifugal force in m/s^3, R is radius in meters)
e = (V^2 / (224 * R)) for equilibrium super-elevation (where e is super-elevation rate, V is design speed in kmph, R is radius in meters)
tan(Δ/2) = (T / R) for deflection angle in simple curves (where Δ is total deflection angle, T is tangent length, R is radius)
e_max = 0.07 (for roads without grooved surfaces) or 0.10 (for roads with grooved surfaces) - Maximum permissible super-elevation

Tables & Referenced Sections

Key Tables
Table 1 — Minimum radius and superelevation by speed
Table 2 — Extra widening by radius and vehicle length
Table 3 — Transition curve lengths
Table 1 - Minimum Radius of Curvature and Maximum Degree of Curve for Different Design Speeds
Table 2 - Length of Transition Curve for Different Design Speeds and Rates of Introduction of Centrifugal Force
Table 3 - Recommended Values of Super-elevation for Different Design Speeds and Radius of Curvature
Table 4 - Geometric Design Elements for Different Types of Curves
Table 5 - Stationing Details for Simple Curves
Table 6 - Stationing Details for Compound Curves
Table 7 - Stationing Details for Reverse Curves
Key Clauses
Cl. 3 — Minimum radius for different design speeds
Cl. 4 — Superelevation design
Cl. 5 — Extra widening on curves
Cl. 6 — Transition curve length
Cl. 7 — Design tables for all speeds
Cl. 2.1.1 - Classification of Curves
Cl. 3.1.1 - Design of Simple Curves
Cl. 4.1.1 - Design of Transition Curves
Cl. 5.1.1 - Design of Compound Curves
Cl. 6.1.1 - Design of Reverse Curves
Cl. 7.1.1 - Design of Helical Curves
Cl. 8.1.1 - Design Tables and their Use
Cl. 9.1.1 - Special Considerations for Urban Areas and Hill Roads

Related Resources on InfraLens

Cross-Referenced Codes
IRC 86:2018Geometric Design Standards for Rural Highways
→
IS 73:2013Paving Bitumen - Specification
→
IRC 67:2012Code of Practice for Road Signs
→

Frequently Asked Questions13

What is the minimum curve radius for 100 km/h?+
360m per IRC 38 (with 7% superelevation and 0.15 side friction). Below this radius, the design speed must be reduced or the road reclassified.
What is the primary purpose of IRC 38:1988?+
IRC 38:1988 provides comprehensive guidelines for the design of horizontal curves on highways in India. Its main purpose is to ensure the safety and comfort of road users by specifying appropriate geometric elements like radius of curvature, length of transition curves, and super-elevation for various design speeds.
How is the minimum radius of curvature determined according to IRC 38?+
The minimum radius of curvature is determined based on the design speed of the highway and the assumed coefficient of lateral friction. The code provides specific minimum radii in Table 1 for different design speeds. The formula R = (V^2 / 127 * f) is used for calculation, where V is the design speed in kmph and f is the coefficient of lateral friction.
What is a transition curve and why is it important?+
A transition curve is a spiral curve used to gradually introduce super-elevation and change the radius from infinity (on a straight) to the radius of the circular curve. It is important for providing a smooth ride, gradually introducing centrifugal force, and preventing abrupt changes in steering, thereby enhancing driver comfort and safety.
How is super-elevation calculated, and what are the limiting factors?+
Super-elevation is calculated using the formula e = (V^2 / (224 * R)) for equilibrium, aiming to counteract the centrifugal force. IRC 38 specifies maximum permissible super-elevation rates (e_max) of 0.07 for roads without grooved surfaces and 0.10 for roads with grooved surfaces. This ensures that the vehicle does not slide outwards.
What are the differences in design for compound and reverse curves?+
Compound curves consist of two or more circular curves of different radii joined together, usually in the same direction. Reverse curves are two successive curves of same or different radii which curve in opposite directions. Both require careful consideration of tangent lengths and the separation between curves to avoid abrupt directional changes and ensure sight distances.
Are there specific considerations for urban roads or hill roads in IRC 38?+
Yes, IRC 38:1988 includes a clause (Cl. 9.1.1) addressing special considerations for urban areas and hill roads. Urban roads might have space constraints leading to smaller radii, while hill roads demand extra caution due to steeper gradients, limited sight distances, and challenging topography, often requiring specialized design approaches.
How do I use the design tables provided in IRC 38?+
The design tables in IRC 38, such as Table 1 and Table 2, provide direct values for parameters like minimum radius, maximum degree of curve, and length of transition curves for various design speeds. Engineers can use these tables for quick reference and to ensure compliance with the code's requirements, though manual calculation is also presented.
What is the rate of introduction of centrifugal force (C), and how does it affect design?+
The rate of introduction of centrifugal force (C) is a measure of how quickly the centrifugal force increases along the transition curve, expressed in m/s³. A higher value of C means a more rapid increase in force, leading to a shorter transition curve but potentially less comfort. IRC 38 provides recommended values for C in Table 2 for different design speeds.
What is the difference between 'degree of curve' and 'radius of curve'?+
The degree of curve is the angle subtended at the center by an arc of 30.5 meters (100 feet) of the curve. The radius of curve is the radial distance from the center of the circle to any point on the curve. A smaller radius means a sharper curve, and a larger degree of curve also indicates a sharper curve.
Are there any provisions for grooved surfaces in super-elevation design?+
Yes, IRC 38:1988 mentions grooved surfaces. For roads with grooved surfaces, a higher maximum super-elevation rate of 0.10 is permissible, compared to 0.07 for roads without grooved surfaces. Grooving helps to improve tire-road friction and reduce the risk of skidding.
What is a spiral transition curve, and when is it mandatory?+
A spiral transition curve is a curve whose radius varies linearly from infinity to the radius of the circular curve. It is generally mandatory for all curves where the design speed is above 60 kmph, and highly recommended for speeds below that, especially where there are significant changes in alignment or speed.
How does IRC 38 interact with other IRC codes for horizontal curve design?+
IRC 38:1988 is primarily focused on the geometric design of horizontal curves. It works in conjunction with other IRC codes such as IRC:66 (Guidelines for Sight Distance for Identifying Horizontal Curves) and IRC:73 (Road Geometry – Urban Roads) to provide a complete design solution. Engineers must consult these related codes for a holistic design.

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