Width of Pavement (Carriageway) — IRC Specifications, Lane Width & Standards

What is Width of Pavement (Carriageway)?

The width of pavement, also called carriageway width, is the paved portion of the road cross-section actually available for moving vehicular traffic. It is determined by two key factors: the width of individual traffic lanes and the total number of lanes required based on expected traffic volume.

Getting this right is critical — a carriageway that is too narrow forces vehicles dangerously close to each other and to the road edge, while one that is too wide wastes expensive construction resources. IRC (Indian Roads Congress) has carefully calibrated standard widths based on decades of Indian traffic experience.

Factors Determining Carriageway Width

1. Vehicle Width

As per Indian Motor Vehicles Act, standard vehicle widths are: 2.5 m for non-transport vehicles (cars, jeeps) and 2.7 m for transport vehicles (trucks, buses). The lane must safely accommodate the widest expected vehicle.

2. Lateral Clearance

Beyond the physical vehicle width, each vehicle needs lateral clearance — space between its sides and lane boundaries — for comfortable, safe operation. This accounts for driver steering imprecision and the psychological discomfort of tight spaces.

3. Number of Lanes

Total carriageway width = lane width × number of lanes. The number of lanes required is based on the design hourly volume and capacity analysis.

IRC Standard Carriageway Widths

Urban Road Design Speeds (IRC)

What is Intermediate Carriageway?

An intermediate carriageway (5.5 m wide) is a configuration where the road is wider than a single lane but narrower than a full two-lane road. It provides just enough space for two vehicles to pass each other with care or for one vehicle to overtake a slower one at low speed. It serves as a practical compromise on roads with moderate traffic where a full 7.0 m width is not yet economically justified — offering manoeuvring space without the full cost of two-lane construction.

Width Transition (Taper) Between Different Sections

When the carriageway width changes along a road — for example from a two-lane section to a four-lane section at a town entrance — the transition must be gradual to allow drivers time to adjust their lateral position. IRC specifies that such transitions (tapers) should be provided at a ratio of 1 in 15 to 1 in 20 (i.e., for every 1 metre of width gain, 15–20 metres of road length is used). Abrupt transitions are dangerous because they force sudden lane changes.

Bridge Carriageway Widths

For bridges with a clear span exceeding 6 metres, IRC prescribes specific carriageway widths between kerbs:

• Single-lane bridge: 4.25 m clear width
• Two-lane bridge: 7.5 m clear width
• Multi-lane bridge: 3.5 m per lane + 0.5 m per carriageway

Design Principles for Lane Width Selection

1. Wider lanes → better safety, less stress on drivers, more lateral clearance for large vehicles
2. Narrower lanes → more lanes possible within same road width → higher capacity per unit width
3. Lane width affects operating speed — wider lanes encourage slightly higher speeds
4. On curves, extra widening (beyond standard width) must be added because rear wheels of vehicles take a shorter radius path

Quick Summary

• Carriageway width = lane width × number of lanes
• Single lane: 3.75 m | Two lane (no kerb): 7.0 m | Two lane (kerb): 7.5 m
• Intermediate carriageway: 5.5 m (one lane + manoeuvrability)
• Multi-lane: 3.5 m per lane (expressways, urban)
• Width transitions must follow taper of 1 in 15 to 1 in 20
• Bridge widths are narrower — IRC specifies clear kerb-to-kerb distances