loon

A loon is a paved bulge or loop projecting from the edge of a road, designed to give articulated vehicles and other large trucks room to execute turns without encroaching on the opposite shoulder or crossing the center line. The loon absorbs the extra width needed when a semi-trailer or rigid vehicle with a long wheelbase pivots at an intersection or junction. Instead of forcing the driver to make a shallow, wandering turn across multiple lanes, the loon provides dedicated space for that tighter arc.

Loons are most common at the approach to industrial sites, ports, quarries, and rural service roads where heavy vehicles regularly turn off main routes. A typical loon extends 2 to 4 meters outward from the edge, sized to suit the turning radius of the vehicles expected to use it. The perimeter is marked with edge lines or reflectors, and the surface must be capable of supporting the concentrated loads of truck tires at slow speed and under full steering angle.

Construction and maintenance

Loons are built with the same materials as the main road surface, most often asphalt or concrete. However, they often develop premature wear and cracking because the turning loads create high lateral stress at the outer edge, and water pools in low spots if drainage is poor. The boundary between the loon and the main carriageway is a point of failure if the base course is insufficient or if differential settlement occurs. Regular inspection for edge breaking and patching is necessary to prevent the edges from spalling.

The term "loon" is used primarily in Commonwealth transport engineering; North American practice typically refers to such features as turning radii or widened bays. Road design standards specify minimum loon dimensions based on the design vehicle type, usually the largest rigid truck or articulated unit that will regularly use the route. A loon designed for 15-meter semi-trailers must be significantly larger than one for rigid 10-meter trucks.

Loons are distinct from simple lane widening because they do not extend continuously; they protrude from one side and return to normal width. This makes them cheaper to construct and maintain than a permanently widened approach, but they add complexity to drainage design and can be a hazard if drivers unfamiliar with the road fail to anticipate them.