More complex systems use a variety of sensors. Inductive sensors embedded in the road surface can determine the gaps between vehicles, to provide basic information on the presence of a vehicle. With clever software processing of the inductive data a wide range of vehicle classes can be derived by careful analysis of the inductive profile. Treadles permit counting the number of axles as a vehicle passes over them and, with offset-treadle installations, also detect dual-tire vehicles. Light-curtain laser profilers record the shape of the vehicle, which can help distinguish trucks and trailers. In modern systems simple laser light curtains are being replaced with more technically advanced Lidar systems. These safety critical systems, used in autonomous vehicles, are less sensitive to environmental conditions.