A falling weight deflectometer, or FWD for short, is a testing device used by civil engineers to evaluate the physical properties of a stretch of pavement. This could include (but is not limited to) highways, local roads, and airport runways. The machine is usually contained within a trailer that is driven to a location by another vehicle. At a predesignated change in distance, the FWD device is deployed. A metal plate on the device is set up to strike the pavement at a given force, and sensors are placed around the plate and in a straight line radiating from the plate. These sensors record the deflections in the plavement (analogous to ripples in a pond) induced by the falling weight, hence the name "falling weight deflectometer".

This device has several practical purposes in determining the properties of pavement. For one, voids or empty spaces can be detected underneath the roadway. Joint testing determines how two abutting slabs of concrete or asphalt are fitting against one another. Other statical analyses determine the general strength of the structure, and can even predict what materials and in what combination were used to build the pavement if such information is unknown.