Sight Distance in Highway: Stopping, Overtaking and Intermediate Sight Distance with IRC Values

What is sight distance in highway engineering?

It is the distance along the road up to which a driver can keep seeing an object of a set height. IRC:73-2023 (clause 5.2.1) says it depends on three things: how high the driver’s eye is, how high the object is, and how high and how far to the side any obstruction stands.

A road can be smooth, wide and well drained and still be dangerous if drivers cannot see far enough. A crest that hides a broken-down tractor, a wall on the inside of a bend, or tall sugarcane at a village junction all cut sight distance. So the designer checks it at every section of the road, in plan and in profile.

The 2023 code is the one to quote now. Its introduction says the revision merged IRC:66-1976, the old recommended practice for sight distance on rural highways, and IRC:SP:23-1983 on vertical curves into one document. Trucks get no separate values: they need longer to stop, but their drivers sit higher and see further over obstructions, and the code treats the two effects as cancelling out.

What are the types of sight distance?

There are three main types, plus two special cases for night driving and junctions. Each one is measured with a fixed eye height and a fixed object height.

Table 1: Types of sight distance and how each is measured (IRC:73-2023, clauses 5.2 to 5.8)

Why 0.15 m for stopping? It is the height of an object below which, the code says, there is no crash risk: think of a stone or a fallen sack. For overtaking, the object is an oncoming vehicle, so it is given the same 1.2 m height as the eye. Both drivers must be able to see each other.

How do you calculate stopping sight distance?

Stopping sight distance has two parts. The first is the distance the vehicle covers while the driver reacts. The second is the distance it covers after the brakes go on, until it stops.

SSD = 0.278 V t + V² / (254 f)

Here V is the speed in km/h, t is the perception and brake reaction time in seconds, and f is the coefficient of longitudinal friction between tyre and road (how much grip the tyres get when braking in a straight line). The 0.278 converts km/h into metres per second.

IRC takes t as 2.5 seconds. This is the PIEV time: Perception (seeing the danger), Intellection (understanding it), Emotion (deciding) and Volition (acting). Most drivers react faster, but the code chooses a value that covers nearly all drivers in nearly all conditions. The friction value falls from 0.40 at 20 km/h to 0.35 at 100 km/h, because tyres grip less at higher speed.

Worked example (our arithmetic): a 60 km/h road on level ground

• Reaction distance: 0.278 × 60 × 2.5 = 41.7 m
• Braking distance: 60² ÷ (254 × 0.36) = 3,600 ÷ 91.44 = 39.4 m
• Stopping sight distance: 41.7 + 39.4 = 81.1 m, which IRC rounds to a design value of 80 m

The code’s own example, at 70 km/h, comes to 48.65 m + 53.59 m = 102.24 m, which it rounds up to 105 m.

On a slope, the braking part changes. The braking distance becomes V² / 254 (f + 0.01G), where G is the gradient in per cent, positive uphill and negative downhill. A vehicle going downhill needs longer to stop. At 60 km/h on a 4% downgrade, the braking distance grows to 44.3 m and SSD to about 86 m. On a 4% upgrade it falls to 35.4 m and SSD to about 77 m. In IRC’s example, the 70 km/h road on a 1 in 40 grade (2.5%) needs about 100 m uphill and 110 m downhill. The code says this grade correction must always be applied on one-way roads and on divided highways whose two carriageways have separate profiles.

At 80 km/h the driver covers 56 m before the brakes even bite. That is why a bend or crest that hides only 100 m of road is too short for this speed, however good the brakes are.

What are the IRC values of stopping sight distance?

Table 2 gives the design values from IRC:73-2023 Table 5.2, worked out with the 2.5 second reaction time. We recomputed every row from the formula and the figures agree.

Table 2: Stopping and intermediate sight distance by design speed (IRC:73-2023 Table 5.2)

To use the table you need the road’s design speed, and that depends on the class of road and the terrain. IRC:73-2023 Table 3.2 sets a “ruling” speed, which is the normal target, and a “minimum” speed for stretches where land or cost will not allow the ruling one. The road classes themselves are explained in our guide to types of roads in India.

Table 3: Ruling and minimum design speed, km/h (IRC:73-2023 Table 3.2)

So a village road across plain land, at its ruling speed of 50 km/h, needs 60 m of stopping sight distance. A two-lane state highway on the same land, at 100 km/h, needs 180 m. Expressways are designed at 120 km/h and must have 500 m, the desirable minimum for that speed, throughout. For rural roads, the Rural Roads Manual (IRC:SP:20) gives the same stopping values at 20 to 40 km/h and adds 40 m for 35 km/h.

How is overtaking sight distance calculated?

Overtaking sight distance is the sum of three lengths: d1, covered by the overtaking car while its driver decides; d2, covered while it actually overtakes; and d3, covered by an oncoming vehicle in the meantime. IRC takes a 2.0 second reaction time here, shorter than for stopping, because the driver is planning the move rather than reacting to a surprise.

The code assumes the slow vehicle runs 16 km/h below the design speed, that the overtaking car follows it for a moment, then speeds up to the design speed, and that the oncoming vehicle also travels at the design speed. An overtaking move takes roughly 8 to 14 seconds. During the first third of it the overtaking car can still drop back, so the oncoming vehicle’s share is taken as its travel during about two-thirds of the move.

Table 4: Overtaking sight distance (IRC:73-2023 Table 5.3)

Each value is the speed in metres per second multiplied by the total time, then rounded. At 80 km/h that is 22.2 m/s × 21 s = 467 m, shown as 470 m. The values are for a car overtaking on level ground; the code gives no separate figures for grades.

Overtaking zones and “No Overtaking” stretches. Where a road gives full OSD, the overtaking zone should ideally be five times the OSD long and never less than three times, with signs at its start and end. Where OSD cannot be given, “No Overtaking” signs go up on both sides at the start of the stretch, with repeaters at intervals of no more than 400 m (IRC:67 covers the signs). At curves that do not give even intermediate sight distance, the code calls for restrictive centre-line markings under IRC:35; the paint used for those lines is covered in our post on thermoplastic road marking paint. Overtaking distance is not considered on single-lane roads and is mostly not feasible on hill roads, says IRC:SP:20.

Where is intermediate sight distance used?

Intermediate sight distance is twice the stopping sight distance. It does not give enough room for a normal overtake, but it lets a careful driver overtake and it makes the road far safer than the bare minimum. IRC:73-2023 clause 5.6 uses it in five places:

• On two-lane roads where full OSD is not practical, such as undulating land, ISD should be available throughout.
• On all new 4, 6 and 8-lane divided highways, ISD must be provided at every point. Bare SSD is allowed only on existing divided roads that cannot be improved.
• Undivided four-lane roads are designed like divided ones, because vehicles can overtake within their own half.
• At grade-change locations, such as vehicular underpasses, bridges, rail over-bridges and under-bridges, loops and ramps, the design uses ISD.
• On expressways, 500 m is required throughout.

Under every one of these sits the same floor: no road, anywhere, may give less than the safe stopping sight distance.

How much clear width is needed on the inside of a curve?

On a horizontal curve the danger is beside the road, not on it. A wall, a house, a cut hillside or a row of trees on the inside of the bend can block the view even when the road surface is wide open. The clear width needed is called the set-back distance, and IRC:73-2023 clause 6.6 gives it as:

m = R − (R − n) cos θ, where θ = S ÷ 2(R − n), in radians

Here m is the set-back from the road centre line to the obstruction at the middle of the curve, R is the curve radius at the centre line, n is the distance from the centre line to the middle of the inner lane, and S is the sight distance needed. This form applies when the curve is longer than the sight distance. On a narrow single-lane road, n is taken as zero.

Worked example (our arithmetic): a two-lane road with a 7 m carriageway (so n = 1.75 m), a 200 m radius curve, and a 60 km/h design speed (S = 80 m). θ = 80 ÷ (2 × 198.25) = 0.2018 radians, so m = 200 − 198.25 × cos 0.2018 = about 5.8 m. Everything tall within 5.8 m of the centre line at the middle of that bend has to go, or the curve has to be redesigned. The code’s first answer is to remove the obstruction; only if that is not possible should the alignment change.

A tighter curve needs more set-back for the same speed, which is one reason sharp bends on hill roads feel so blind. How the same curve is banked for speed is in our guide to superelevation in road design.

What about summit curves and valley curves?

On a summit curve, the crest of a rise, the road itself hides what lies beyond. The curve is made long enough that a line from a 1.2 m eye height clears the crest and reaches a 0.15 m object at the stopping distance, or a 1.2 m object at the overtaking distance. Where a bend and a crest overlap, the sight distance must be there both across the inside of the bend and along the road surface over the crest.

A valley curve, a dip, gives no trouble in daylight. At night the headlights decide what the driver can see. IRC:73-2023 clause 5.8 takes the headlight as 0.75 m above the road with a useful beam 1 degree upward, and requires the lit length to be at least the safe stopping sight distance. The cross slope that drains a straight stretch is a separate check, explained in the guide to camber in road construction.

How is sight distance provided at intersections?

At a junction, each driver must be able to see vehicles coming on the other road. The area that must stay clear is a sight triangle at each corner, formed by a length (a “leg”) along each road. Buildings, parked vehicles, hedges, trees, tall crops, walls and fences inside that triangle are all obstructions.

• Uncontrolled junctions (roads of equal importance, no priority): each leg should equal the desirable minimum sight distance for that road’s design speed, and never less than the SSD. For major and other district roads and village roads, legs equal to the SSD are allowed.
• STOP-controlled junctions (minor road meets a major road): the leg along the major road is the distance a major-road vehicle covers while a stopped car crosses. On a two-lane road IRC takes 8 seconds for a car and 10 seconds for a truck, adding 0.5 seconds for a car and 0.7 for a truck for each extra lane.

The 8 seconds is worked out in the code from Indian conditions: a car crossing from the stop line travels 3 m to the road edge, plus its own 4.7 m length, plus 10 m of road (7 m of lanes and 1.5 m paved shoulder each side), which makes 17.7 m. With 2.5 seconds to react, 3.71 seconds to reach 20 km/h at 1.5 m/s² and 1.33 seconds for the rest, the crossing takes 7.54 seconds, rounded up to 8.

Table 5: Leg length along the major road for a car crossing from a STOP-controlled minor road (IRC:73-2023 Table 5.5, design values)

The code asks for the larger of this value and the desirable minimum sight distance where possible, and never less than the table value. Six and eight-lane highways are access controlled, so they should have no at-grade STOP-controlled junctions at all.

How is sight distance kept clear on site?

Sight distance starts on the drawing. IRC:73-2023 clause 5.9 asks for it to be checked on plans and profiles early, while the alignment can still move, and again in design software once superelevation, crash barriers and bridge parapets are all modelled. The surveyor who sets out the alignment on the ground is part of this check.

On the ground, the code (clause 5.2.4) lists the usual culprits: median plantation, kerbs, crash barriers, the fill slope at a bridge approach, trees, and the back slope of a cutting. Sight lines should fall within the toe of the earthwork or within the earthen shoulder, so the shape of the bank matters too; the guide to embankment construction covers slopes and shoulders. Planting along the road and in the median is checked against IRC:SP:21 and IRC:SP:103.

Opening a sight line is ordinary earthwork, and the machine depends on the job:

• Cutting back the hillside on the inside of a blind hill-road bend is excavator work, for example with a 20-tonne class Tata Hitachi EX 210 Infra.
• Clearing bushes, rubble and dumped soil from village junction corners and verges is a typical job for a backhoe loader such as the JCB 3DX.
• Trimming a crest and dressing shoulders to the profile is motor grader work, for example with the Mahindra RoadMaster G9595.

Cutting back a slope or widening a curve? Compare excavator models and talk to a dealer near your site.

During construction the risk is self-made. A heap of aggregate, a parked roller or a diversion board on the inside of a curve or at a junction corner cuts sight distance as surely as a wall. Keep stacks and parked machines out of those zones, and check the sight lines again after any widening job. The full list of machines a road package uses is in the guide to machinery required for road construction.

Where the figures in this guide come from. Definitions, eye and object heights, the stopping and overtaking formulas, Tables 2, 4 and 5, the grade correction, the use of intermediate sight distance, the set-back equation, headlight criteria and the intersection rules are from IRC:73-2023, Geometric Design Standards for Non-Urban (Rural) Roads, read from the printed pages. Design speeds are from its Table 3.2. Village-road values and the note on overtaking on hill and single-lane roads are from the Rural Roads Manual, IRC:SP:20-2002. The 60 km/h examples and the set-back example are our own arithmetic.

More on how roads are designed and built is in the road construction guides, and the road construction layers guide shows what sits under the surface. When the job is ready, compare motor graders and speak to a dealer near your site.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.