Geometric Design of Highways Notes

Last reviewed 16 Sept 2026 · 14 min read

Design controls and criteria

Design speed — depends on road class and terrain. For National and State Highways in plain terrain, IRC gives a ruling design speed of 100 km/h and a minimum of 80 km/h; values reduce for rolling, mountainous and steep terrain and lower road classes.
Terrain classification by cross slope of the country: plain (0–10%), rolling (10–25%), mountainous (25–60%), steep (greater than 60%).
Traffic — volume, composition (PCU), design hour volume, directional distribution.
Design vehicle dimensions (IRC limits): maximum width 2.5 m; maximum height about 3.8 m (4.2 m for double-deckers and container vehicles); maximum length about 11 m for a single-unit truck, 16 m for a semi-trailer and 18 m for a truck–trailer combination.
Human factors — perception–reaction time, driver behaviour.
Environment and economy.

Cross-section elements

Pavement surface characteristics

Characteristic	Remarks
Friction (skid resistance)	IRC recommends a longitudinal coefficient of friction of 0.35 to 0.40 (depending on speed) for stopping sight distance and a lateral coefficient of friction of 0.15 for horizontal curve design
Unevenness (roughness)	Measured by bump integrator (mm/km) or International Roughness Index; affects comfort, safety and vehicle operating cost
Light reflection	Light-coloured (concrete) surfaces give better night visibility but more glare
Drainage	Surface should shed water quickly — camber, cross slopes

Skidding — wheels lock and slide (longitudinal skid) or the vehicle slides sideways on curves (lateral skid). Slipping — wheels revolve more than the distance travelled (on starting or sudden acceleration on slippery surfaces).

Camber (cross slope)

Transverse slope given to the carriageway to drain surface water quickly.

Surface type	Camber (heavy rainfall – light rainfall)
Cement concrete and high-type bituminous	2.0% – 1.7% (1 in 50 – 1 in 60)
Thin bituminous surface	2.5% – 2.0%
Water bound macadam, gravel	3.0% – 2.5%
Earth	4.0% – 3.0%

Shapes: parabolic (elliptic), straight-line (two straight slopes), and a combination (straight slopes with a rounded crown). Height of crown above edges = camber × half the width (straight line).

Too steep a camber causes discomfort, erosion of edges and a tendency of vehicles to drift to the edges; too flat a camber causes ponding.

Widths

Element	IRC value (typical)
Single-lane carriageway	3.75 m
Two-lane carriageway without raised kerbs	7.0 m
Two-lane carriageway with raised kerbs	7.5 m
Intermediate carriageway	5.5 m
Multi-lane pavements	3.5 m per lane
Roadway (formation) width — NH/SH, plain and rolling terrain, two-lane	12 m (carriageway + shoulders)
Right of way (land width) — NH/SH, open areas, plain terrain	Normal 45 m (range 30–60 m)

Shoulders — provided along the edges for stopped vehicles, lateral support to the pavement, emergency lane; commonly 2.5 m wide on each side in rural two-lane roads; cross slope about 0.5% steeper than the camber.
Median (traffic separator) — separates opposing traffic on divided highways; prevents head-on collisions; width varies with land availability (wider medians in rural areas).
Kerbs: low/mountable kerb (about 10 cm) — vehicles can climb; semi-barrier kerb (about 15 cm) — along medians and parking lanes; barrier kerb (about 20 cm) — prevents vehicles leaving the pavement, near footpaths and bridges; submerged kerbs at edges of rural roads to confine pavement layers.
Building line and control line — setback distances regulating construction along highways.
Other elements: footpaths, cycle tracks, guard rails, crash barriers, parking lanes, bus bays, frontage roads, service roads.

Sight distance

Sight distance is the length of road visible ahead to a driver at any instant.

Stopping sight distance (SSD)

The minimum distance required to stop a vehicle moving at design speed safely, without collision, on seeing an obstruction.

FormulaStopping sight distance

S S D = lag distance 0.278 V t + braking distance \frac{V ^{2}}{254 f}

$V$ in km/h; $t$ = total perception–reaction time (IRC: 2.5 s); $f$ = longitudinal coefficient of friction (0.35–0.40).

On a gradient of $n$ %: braking distance $= \frac{V ^{2}}{254 ( f \pm 0.01 n )}$ (+ ascending, − descending)

In m/s units: $S S D = v t + \frac{v ^{2}}{2 g f}$

PIEV theory — reaction time consists of Perception, Intellection, Emotion and Volition.
On two-way single-lane roads: required sight distance = 2 × SSD.
Height of driver's eye 1.2 m and height of object 0.15 m are used for SSD (IRC).

Intermediate sight distance (ISD)

ISD = 2 × SSD — provided where overtaking sight distance cannot be provided, to give limited overtaking opportunities with caution. Object height = eye height = 1.2 m.

Overtaking sight distance (OSD)

The minimum distance required for a vehicle to safely overtake a slower vehicle on a two-lane two-way road, considering an oncoming vehicle.

FormulaOvertaking sight distance

O S D = d_{1} + d_{2} + d_{3}

$d_{1} = 0.28 v_{b} t$ — distance travelled by the overtaking vehicle while following the slow vehicle (reaction time $t$ = 2 s)
$d_{2} = 0.28 v_{b} T + 2 s$ — during the overtaking manoeuvre
- Spacing $s = 0.2 v_{b} + 6$ (m)
- Time $T = \frac{4 s \times 3.6}{A} = \frac{14.4 s}{A}$ (s), $A$ = acceleration in km/h/s
$d_{3} = 0.28 v T$ — distance travelled by the oncoming vehicle during the manoeuvre

$v_{b}$ = speed of the overtaken vehicle (km/h) — if not given, take $v_{b} = v - 16$ km/h; $v$ = design speed.

On divided roads (one-way), $d_{3}$ is not needed: OSD = $d_{1} + d_{2}$ .

Overtaking zones are marked where OSD is available; minimum length about 3 × OSD, desirable 5 × OSD.
For OSD, eye height and object height are both taken as 1.2 m.

Sight distance at intersections

A sight triangle must be kept clear so that drivers approaching from intersecting roads can see each other in time to stop (based on SSD of each approach).

Horizontal alignment

Superelevation

When a vehicle moves on a curve, centrifugal force $P = \frac{W v ^{2}}{g R}$ acts outward. It is resisted by superelevation (raising the outer edge) and lateral friction.

FormulaSuperelevation and radius

Equilibrium:

e + f = \frac{V ^{2}}{127 R}

$e$ = rate of superelevation (tan θ); $f$ = lateral friction (0.15); $V$ in km/h; $R$ in m.

IRC design steps (mixed traffic):

Superelevation to counteract centrifugal force of 75% of design speed with no friction:

e = \frac{( 0.75 V ) ^{2}}{127 R}

If $e \leq e_{ma x}$ → adopt. If $e > e_{ma x}$ , adopt $e = e_{ma x}$ and check friction: $f = \frac{V ^{2}}{127 R} - e_{ma x} \leq 0.15$ .
If $f > 0.15$ → restrict the speed: $V_{a} = 127 R (e_{ma x} + 0.15)$ (or increase the radius).

Maximum superelevation (IRC): 7% in plain and rolling terrain and snow-bound areas; 10% in hilly areas not bound by snow; 4% on urban roads with frequent intersections. Minimum superelevation = camber (for drainage).

Ruling minimum radius: $R = \frac{V ^{2}}{127 ( e + f )}$ (with ruling design speed); absolute minimum radius with minimum design speed.

Overturning vs skidding on curves:

Overturning (no superelevation) if $\frac{v ^{2}}{g R} > \frac{b}{2 h}$ ; skidding if $\frac{v ^{2}}{g R} > f$ .
Since usually $f < b /2 h$ , a vehicle skids before it overturns.

Attainment of superelevation:

Elimination of the crown (outer half rotated about the crown to a flat, then to the camber slope).
Rotation to full superelevation — about the centre line (most common; balances cut and fill), about the inner edge (drainage advantages), or about the outer edge.
The rate of change of cross slope along the transition is limited — commonly 1 in 150 in plain and rolling terrain and 1 in 60 in mountainous terrain.

Extra widening on curves

FormulaWidening on horizontal curves

W_{e} = W_{m} + W_{p s} = \frac{n l ^{2}}{2 R} + \frac{V}{9.5 R}

$W_{m}$ = mechanical widening (off-tracking of rear wheels); $n$ = number of lanes; $l$ = wheel base (commonly 6.1 m).
$W_{p s}$ = psychological widening (drivers' tendency to keep more clearance on curves).

IRC recommends widening for radii up to about 300 m on two-lane roads. The widening is provided on the inner side (on hill roads) or equally on both sides, attained gradually along the transition.

Transition curves

A transition curve has a radius decreasing from infinity (at the tangent) to $R$ (at the circular curve).

Objectives: gradual introduction of centrifugal force; gradual introduction of superelevation and extra widening; pleasing appearance; comfort and safety.

Types: spiral (clothoid) — ideal, radius inversely proportional to length ( $L R$ = constant), recommended by IRC; lemniscate; cubic parabola.

FormulaLength of transition curve (IRC) — adopt the highest

1. Rate of change of centrifugal acceleration:

L_{s} = \frac{0.0215 V ^{3}}{C R}, C = \frac{80}{75 + V} (0.5 \leq C \leq 0.8 m/s^{3})

2. Rate of introduction of superelevation (rate 1 in $N$ ; $N$ = 150 plain/rolling, 60 hilly):

Rotation about the centre line: $L_{s} = \frac{E N}{2}$
Rotation about the inner edge: $L_{s} = E N$
where $E = e (W + W_{e})$ = total raising of the outer edge

3. IRC empirical minimum:

Plain and rolling terrain: $L_{s} = \frac{2.7 V ^{2}}{R}$
Mountainous and steep terrain: $L_{s} = \frac{V ^{2}}{R}$

Shift of the circular curve: $S = \frac{L _{s}^{2}}{24 R}$

Set-back distance

The clearance required from the centre line of the inner lane to an obstruction (buildings, cut slopes) on the inner side of a horizontal curve to provide the required sight distance $S$ .

FormulaSet-back distance

When the sight distance $S$ is less than the curve length $L_{c}$ :

Single-lane road: $m = R - R cos \frac{α}{2}$ , where $\frac{α}{2} = \frac{S}{2 R}$ radians

Multi-lane road (sight line along the centre of the inner lane, $d$ = distance from the centre line of the road to the centre of the inner lane):

m = R - (R - d) cos \frac{θ}{2}, \frac{θ}{2} = \frac{S}{2 ( R - d )} radians

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Geometric Design of Highways