What is a Kerb in Road Design?
A kerb (also spelled curb) is a structural boundary element that marks and reinforces the edge of the carriageway — the transition zone between the paved road surface and the adjoining shoulder, footpath, or planting strip. Beyond its visual role as a boundary marker, a kerb serves critical engineering functions: it provides lateral confinement to the pavement edges, guides surface water toward drainage inlets, defines pedestrian areas, and — depending on its type — restricts or prevents vehicles from leaving the carriageway accidentally.
The type of kerb chosen for a particular road section depends on the nature of adjacent land use, pedestrian traffic intensity, vehicle speed, and local drainage requirements.
Three Main Types of Kerbs
1. Low (Mountable) Kerb
The low kerb, also called a mountable kerb, is the gentlest of the three types. It rises approximately 10 cm above the pavement edge and has a gradual, sloping face that vehicles can ride over without damage if they stray slightly from the carriageway. The design intention is not to physically prevent vehicles from leaving the lane but rather to guide drivers back to the carriageway with a gentle nudge while still allowing access to the shoulder when genuinely needed (such as for emergencies or turning movements into driveways).
This kerb type is ideal for rural through roads, highway sections with adjacent shoulders, and anywhere that vehicles need the option to drive onto the adjacent surface.
2. Semi-Barrier Kerb
The semi-barrier kerb is an intermediate type, rising about 15 cm above the pavement edge with a nearly vertical face. While a vehicle can technically mount it under severe circumstances (such as emergency braking), the height and face geometry make it significantly more difficult and damaging to cross than a low kerb. It creates a psychological and physical deterrent to inadvertent lane departure.
This kerb is deployed on the periphery of roadways with significant pedestrian activity — market areas, institutional zones, or arterial roads through towns — where protecting pedestrians is important but not at the extreme level required in a dense urban core.
3. Barrier Kerb
The barrier kerb represents the maximum protective design, standing 20 cm above the pavement edge with a steep, near-vertical face. Its purpose is to completely prevent vehicles from crossing onto the footpath or pedestrian area — it will stop or severely damage a vehicle attempting to mount it. The barrier kerb is the appropriate choice when pedestrian safety is the absolute priority.
It is used in densely built-up areas, near schools and hospitals, adjacent to footpaths, and in areas with very high pedestrian footfall where the consequences of any vehicle encroachment would be catastrophic.
Kerb Type Comparison — Quick Reference
| Property | Low (Mountable) | Semi-Barrier | Barrier |
|---|---|---|---|
| Height above pavement | ~10 cm | ~15 cm | ~20 cm |
| Vehicle mounting | Easily mountable | Difficult to mount | Cannot mount |
| Pedestrian protection level | Low | Moderate | High |
| Typical application | Rural roads, through lanes | High pedestrian periphery roads | Built-up areas, schools, footpaths |
| Drainage impact | Minimal obstruction | Moderate guidance | Strong channelisation |
Additional Functions of Kerbs
- Edge Support: Prevents edge break-off and ravelling of flexible pavement layers at the carriageway edge.
- Drainage Guidance: Channels surface runoff toward drains or inlets, preventing spread across the carriageway.
- Traffic Delineation: Provides clear visual guidance for drivers, especially in low-visibility conditions.
- Parking Control: Marks where parking is permissible and prevents vehicles from encroaching on footpaths.
Kerb vs Guard Rail — When to Use Which
Kerbs are appropriate where the road is at or near ground level and adjacent to footpaths or shoulders. Guard rails, on the other hand, are used at the edges of elevated embankments, cuttings with steep drops, or bridge approaches — situations where a vehicle going off the edge would face a significant fall hazard. Guard rails deflect errant vehicles back onto the road rather than stopping them, designed to absorb impact energy progressively.
Key Takeaways
- Kerb = boundary between carriageway and shoulder/footpath
- Three types: Low (10 cm) → Semi-Barrier (15 cm) → Barrier (20 cm)
- Higher kerb = more pedestrian protection, less vehicle accessibility
- Selection based on: traffic speed, pedestrian density, land use, drainage
- Barrier kerb essential near schools, hospitals, dense urban footpaths
- Guard rails (not kerbs) used at embankment edges and bridge approaches