What is a mountable kerb?
A mountable kerb — at a roundabout often called a roundabout kerb block — is a heavy precast concrete unit that marks the edge of the carriageway clearly but can be driven over. It keeps the normal driving line obvious for cars while letting a lorry or bus track its rear wheels across the edge without damage.
The design problem it solves
A roundabout has to do two things at once that pull against each other:
- A tight radius, so speeds stay low. That is the core of a roundabout’s safety.
- Still be negotiable by long vehicles. An articulated lorry cuts in with its rear wheels through a bend — the swept path. On a tight radius that path does not fit on the carriageway.
The answer is an overrun area around the central island: not carriageway for cars, but a surface that will take a semi-trailer. The mountable kerb forms the transition. It is high enough to read as a boundary, and low and rounded enough to be driven over without damage or alarm.
Types
- Internal and external. Units come with a concave and a convex curve, for the inside and the outside of the roundabout respectively.
- Splay or wing unit. At the start and end of a run, a wing unit closes the row and runs the height difference out gradually into the road surface.
- Bend protection. The same units are used away from roundabouts wherever tight bends are cut by heavy traffic, for instance at commercial accesses.
- Large radius blocks covering a quarter of the circle, so a whole roundabout can be laid with few elements.
Points to watch when laying
- Dimensions follow the design guidance. Island diameter, carriageway width and overrun width belong together and cannot be chosen independently.
- Bed the units in concrete. A mountable kerb takes heavy, shoving loads. Without a concrete bed and haunching it works loose within a year.
- Mind the curvature. These are curved units with a fixed radius; order to the radius actually set out on site, not the drawing radius.
- Water has to get away. The edge is the low point of the profile; without a channel or gully water stands there and freezes in winter at exactly the point vehicles track over.
- Keep the upstand low. A high, sharp-edged kerb in this position damages lorry tyre sidewalls, which is precisely why the rounded profile matters.
Related terms
- Kerb
- Channel
- Concrete block paving
- Paving bond
- Roundabout
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