The profilograph is a device used to measure pavement surface roughness. In the early 20th century, Profilographs were low speed rolling devices. Today many Profilographs are advanced high speed systems with a laserbased height sensor in combination with an inertial system that creates a large scale reference plane. It is used by construction crews or certified consultants to measure the roughness of in-service road networks, as well as before and after milling off ridges and paving overlays. Modern profilographs are fully computerized instruments.
The data collected by a profilograph is used to calculate the International Roughness Index (IRI), which is expressed in units of inches/mile or mm/m. IRI values range from 0 (equivalent to driving on a plate of glass) upwards to several hundred in/mile (a very rough road). The IRI value is used for road management to monitor road safety and quality issues.
Many road profilographs are also measuring the pavements cross slope, curvature, longitudinal gradient and rutting. Some profilographs take digital photos or videos while profiling the road. Most profilographs also record the position, using GPS technology. Yet another common measurement option is cracks. Some profilograph systems include a ground penetrating radar, used to record asphalt layer thickness.
Another type of profilograph system is for measuring the surface texture (roads) of a road and how it relates to the coefficient of friction and thus to skid resistance. Pavement texture is divided into three categories; megatexture (roads), macrotexture, and microtexture. Microtexture cannot currently be measured directly, except in a laboratory. Megatexture is measured using a similar profiling method as when obtaining IRI values, while macrotexture is the measurement of the individual variations of the road within a small interval of a few centimeters. For example, a road which has gravel spread on top followed by an asphalt seal coat will have a high macrotexture, and a road built with concrete slabs will have low macrotexture. For this reason, concrete is often grooved or roughed up immediately after it is laid on the road bed to increase the friction between the tire and road.
Equipment to measure macrotexure currently consists of a distance measuring laser with an extremely small spot size (< 1 mm) and data acquisition systems capable of recording elevations spaced at a mm or less apart. The sample rate is generally over 32 kHz. Macrotexture data can be used to calculate the speed-depending part of the friction number between typical car tires and the road surface. The macrotexture also give information on the difference between dry and wet road friciton. However, macrotexture cannot be used to calculate a relevant friction number, since also microtexture affects the friction.
Lateral friction and cross slope are the key reaction forces acting to keep a cornering vehicle in steady lateral position, while exposed to exciting forces from speed and curvature. Since friction is strongly dependant on macrotexture, and texture, cross slope as well as curvature can be measured with a road profilographs, road profilographs are very useful to identify improperly banked curves (see banked turn) that are posing a great risk to motor vehicle accidents.