Optical fibers are commonly used for telecommunications. However, during data transmission, a small proportion of the coupled light is reflected back to the source at each point on the optical fiber. By analyzing the reflected signal, the strains, temperatures or vibrations along the entire route can be recorded seamlessly. This contribution shows how distributed fiber optic sensing (DFOS) can be used in structural health monitoring, e.g. to detect cracks in bridges and to determine natural frequencies. In some cases, existing communication lines can be used for these applications, which means that no installation effort is required on site. This is especially true for traffic flow analyses, as communication lines are often either embedded directly in the road structure or laid just next to the road. Distributed acoustic sensing (DAS) can be used to detect and classify individual vehicles. The speed and direction of travel can also be derived from the measurement data. The challenge here is the large amounts of data of several gigabytes per minute.