A standard measure of longitudinal road roughness, usually reported in metres per kilometre. It is not measured directly but computed from a measured profile by simulating how a defined reference vehicle would respond travelling over it at a set speed. Because the simulation is standardised, values from different instruments and different countries can be compared, which is why it has become the common language of pavement condition reporting.

Because roughness is what road users experience and pay for. It governs ride comfort, vehicle operating and maintenance costs and fuel consumption, and it correlates with user complaints far better than surface appearance does. It is also the main condition indicator in pavement management systems used to prioritise spending across a network, and it frequently forms a contractual acceptance criterion on new construction with payment consequences attached.

A profilometer records a continuous elevation profile, from which roughness indices and long-wavelength defects can be computed over long lengths at speed. A straightedge checks local surface regularity, measuring deviation beneath a beam of fixed length, and is a compliance check for workmanship rather than a network survey tool. They answer different questions and most specifications call for both: a straightedge for local irregularity, a profile-derived index for ride.

Vehicle-mounted inertial profilometers are for network survey and long lengths, measuring at traffic speed without closures. Walking profilers are slower but very accurate over short sections, which suits acceptance testing of a completed job, investigating a specific complaint, and providing a reference to calibrate or verify vehicle-mounted systems. Many organisations use both, with the walking instrument as the reference standard for the fleet.

By taking a transverse profile across the wheel paths. The traditional method places a straightedge across the lane and measures the maximum gap beneath it with a graduated wedge. Survey vehicles use a transverse array of lasers to capture the full cross-section continuously. The straightedge length matters, because a longer beam spans more of the profile and reports a different depth, so the specified length must be used for the figure to be comparable.

Because ruts retain water. A depression in the wheel path holds a film of standing water that cannot drain to the edge, and at speed that raises the risk of aquaplaning and reduces steering and braking control, even where the surface itself has good friction. Rut depth thresholds in maintenance standards are set with that in mind, which is why rutting often triggers intervention before the pavement is structurally exhausted.

They answer three different questions about the same road and a pavement can pass one while failing another. Roughness describes how it rides, friction describes how much grip it offers in the wet, and deflection describes whether the structure beneath is still sound. A smooth, high-grip surface may sit on an exhausted foundation; a structurally strong road may ride badly. Condition assessment normally combines all three.