5.2 Technical Means of Detection 123 Figure 5.16: Remote visual supervision of a level crossing Figure 5.15: Inductive loop for vehicle detection frequency, L the inductance of the coil and C the capacity of the capacitor), this changes the frequency of the resonant circuit. This shift of frequency is evaluated to detect the vehicle. To compensate inductive effects of traction return currents in the rails, symmetrical double loops are normally used (figure 5.15). By this technology, directions cannot be distinguished (unless using two double loops) and axles cannot be counted. It is applied for initiating the opening and closing of level crossings in some systems. It can also be used to detect road vehicles on level crossings, with the disadvantage that due to lack of iron mass, pedestrians and animals are not detected and cyclists rarely. In road traffic management, such loops are widely used. 5.2.5 Three-Dimensional Detection 220.127.116.11 Visual Observation The simplest and historically oldest form of detection is visual observation of the respective track by staff. The ability of human to also evaluate unexpected observations is the main advantage over all technical systems. Disadvantages are the relatively high probability of human error and the high costs of staffing. Therefore, the usage of visual observation is decreasing, especially in highly developed countries. Another version is remote visual observation via camera and monitor (figure 5.16). The number of people required for observation can be much reduced by this method. It is used especially in situations where not only rail vehicles have to be detected. Examples are the conflicting areas of level crossings with roads, the tracks in platform areas, but also passenger areas for security purposes.