Highway Engineering

Transportation and the role of roads

Modes of transportation: roads (highways), railways, waterways (inland and ocean), airways and pipelines.

Characteristics of road transport:

• Maximum flexibility — door-to-door service, routes and schedules changeable.
• The only mode that can serve every area (feeder to all other modes).
• Suitable for short and medium distances, perishable goods and passenger travel.
• Lower initial investment than railways; but higher energy use per tonne-km, accidents and pollution.

Roads promote economic development, agriculture and markets, social services (health, education), national integration and defence.

History of road construction

Development of roads in India

Twenty-year road development plans

Classification of roads

Rural roads (Nagpur classification, modified)

ODR and VR together are rural roads.

Urban roads (IRC)

Arterial roads (through traffic, limited access), sub-arterial roads, collector streets (collect traffic from local streets to arterials) and local streets (access to properties).

Road patterns

Highway planning

Objectives

To plan an overall road network for maximum utility, to fix priorities for phased development, to plan future development accounting for traffic growth, and to work out financing.

Planning surveys

1. Economic studies — population, agricultural and industrial products, existing facilities, per capita income.
2. Financial studies — sources of funds, revenue from vehicle taxes, tolls, betterment levies.
3. Traffic studies — volume, origin–destination, traffic flow, accidents, future growth.
4. Engineering studies — topography, soil, materials, drainage, road inventory, location surveys.

Results lead to a master plan — a long-term plan of the road network, and phasing of works.

Saturation system (maximum utility per unit length)

Used to choose between alternative road networks:

1. Assign utility units to villages/towns by population ranges (and to agricultural and industrial products per tonne).
2. For each alternative, total the utility units served.
3. Compute utility per unit length = total utility units ÷ road length.
4. The network with the highest utility per unit length is given priority.

Major programmes

• National Highways Development Project (NHDP, 1998) — phased upgrading of NHs; Golden Quadrilateral (about 5846 km linking Delhi, Mumbai, Chennai and Kolkata) and the North–South (Srinagar to Kanyakumari) and East–West (Silchar to Porbandar) corridors, port connectivity and further phases.
• Pradhan Mantri Gram Sadak Yojana (PMGSY, 2000) — all-weather road connectivity to eligible unconnected habitations (population 500+ in plains; 250+ in hilly, tribal and desert areas), later phases for upgrading and consolidation.
• Bharatmala Pariyojana (2017) — economic corridors, inter-corridor and feeder roads, border and international connectivity, coastal and port connectivity roads, expressways.
• Sagarmala (port-led development) and PM Gati Shakti National Master Plan (2021) for integrated multimodal infrastructure planning.

India has one of the largest road networks in the world (second largest by length).

Highway alignment

The alignment is the position or layout of the centre line of a highway on the ground. It includes:

• Horizontal alignment — straight paths and horizontal curves in plan.
• Vertical alignment — gradients and vertical curves in the longitudinal section.

Improper alignment leads to higher construction, maintenance and vehicle operating costs and more accidents; once built, an alignment is very expensive to change — so it must be fixed carefully.

Requirements of an ideal alignment

1. Short — the most direct route between terminals, as far as possible.
2. Easy — easy to construct and maintain, with easy gradients and curves for vehicle operation.
3. Safe — adequate sight distances, stable slopes and embankments, safe geometric features.
4. Economical — minimum total cost including construction, maintenance and vehicle operation (life-cycle cost).

These requirements often conflict (e.g. the shortest route may cross hills or swamps), so a balanced choice is needed.

Factors controlling alignment

Obligatory points

Traffic

The alignment should suit present and future traffic — desire lines from origin–destination studies, traffic flow patterns and connections to towns and ports.

Geometric design

Gradients, curve radii and sight distances govern alignment. The alignment may be deviated to avoid steep gradients and sharp curves; uniform design standards throughout are desirable (avoid sudden changes).

Economics

Total cost — construction (earthwork, cross-drainage structures, pavement, land), maintenance and vehicle operating costs (fuel, tyres, time) — should be minimum. Balance of cut and fill reduces earthwork cost.

Other considerations

• Drainage — avoid areas of poor natural drainage; minimise the number of cross-drainage structures.
• Hydrological factors — high flood levels, water table, seepage, stream crossings.
• Geological and soil conditions — stable, strong foundations; avoid weak and expansive soils.
• Environmental — minimum damage to forests, wildlife and communities; noise and air impacts.
• Political and strategic considerations; international boundaries; defence needs.
• Monotony — very long straight stretches cause driver fatigue; gentle curves are introduced.

Special considerations for hill roads

Ineffective rise and fall: when the alignment rises and then falls (or vice versa) unnecessarily between two points, the extra rise and fall increase fuel consumption and time. Also, gradients steeper than the ruling gradient make part of the rise effectively "ineffective" (braking on descent, low gear on ascent).

Engineering surveys for highway location

1. Map study

Using topographic maps (e.g. Survey of India toposheets with contours):

• Possible routes are marked on the map avoiding obstacles, and ruling gradients are checked using contour spacing.
• Obligatory points, river crossings and passes are identified.
• A few alternative routes are selected for field reconnaissance.

2. Reconnaissance

A field inspection along the alternative routes, often a walk-over (or drive) with simple instruments — Abney level, tangent clinometer, compass, barometer, pedometer, GPS — to examine:

• Valleys, ponds, marshes, rivers, hills, permanent structures.
• Approximate gradients, length of gradients and radii of curves.
• Number and type of cross-drainage structures, HFLs.
• Soil type, geological features, sources of construction materials, water.
• Nature of the terrain (plain, rolling, mountainous, steep).

A few alignments are then selected for detailed study.

3. Preliminary survey

Collection of topographical and other data along the selected alternative routes to compare them and choose the best.

Conventional method: a traverse (chain/tape and compass or theodolite) along each alternative; levelling along the centre line and cross-sections; hydrological and soil surveys; traffic surveys; then plans and longitudinal sections are prepared, earthwork quantities and costs estimated, and alternatives compared.

Modern methods: aerial photographic surveys, satellite imagery and GIS; LiDAR (laser scanning) and drone (UAV) surveys; total stations and DGPS; digital terrain models for rapid evaluation of alternatives and computation of quantities.

4. Final location and detailed survey

The centre line of the chosen alignment is pegged on the ground:

• Centre line pegs at regular intervals (closer on curves); curves set out; bench marks established along the route.
• Detailed levelling — longitudinal section and cross-sections at regular intervals (closer in hilly terrain).
• Soil investigations — subgrade CBR, borrow areas, foundations for bridges.
• Hydrological and drainage studies — catchment areas, HFL, waterway for cross-drainage structures.
• Material surveys — quarries and borrow pits.
• Land acquisition details (right of way, property boundaries).

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

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.

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

• 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.

• 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.

• 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 $P=\frac{W{v}^2}{gR}$ acts outward. It is resisted by superelevation (raising the outer edge) and lateral friction.

Overturning vs skidding on curves:

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

Attainment of superelevation:

1. Elimination of the crown (outer half rotated about the crown to a flat, then to the camber slope).
2. 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.
3. 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

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 ($LR$ = constant), recommended by IRC; lemniscate; cubic parabola.

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 $S$.