of friction between typical car tires and the road surface in both dry and wet conditions. Microtexture affects friction as well. Lateral friction and cross slope are the key reaction forces acting to keep a cornering vehicle in steady lateral position, while it is subject to exiting forces arising from speed and curvature. Cross slope and curvature can be measured with a road profilograph, and in combination with friction-related measurements can be used to identify improperly banked curves, which can increase the risk of motor vehicle accidents. Road profilometery Road pavement profilometers (aka profilographs, as used in the famous 1958-1960 AASHO Road Test) use a distance measuring laser (suspended approximately 30 cm from the pavement) in combination with an odometer and an inertial unit (normally an accelerometer to detect vehicle movement in the vertical plane) that establishes a moving reference plane to which the laser distances are integrated. The inertial compensation makes the profile data more or less independent of what speed the profilometer vehicle had during the measurements, with the assumption that the vehicle does not make large speed variations and the speed is kept above 25 km/h or 15 mph. The profilometer system collects data at normal highway speeds, sampling the surface elevations at intervals of 2–15 cm (1–6 in), and requires a high speed data acquisition system capable of obtaining measurements in the kilohertz range. The data collected by a profilometer 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/mi (a very rough road). The IRI value is used for road management to monitor road safety and quality issues. Many road profilers also measure the pavement's cross slope, curvature, longitudinal gradient and rutting. Some profilers take digital photos or videos while profiling the road. Most profilers also record the position, using GPS technology. Another quite common measurement option is cracks. Some profilometer systems include a ground penetrating radar, used to record asphalt layer thickness. Another type of profilometer is for measuring the surface texture 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, macrotexture, and microtexture. Microtexture cannot currently be measured directly, except in a laboratory. Megatexture is measured using a similar profiling method