and also limits the amount of transmittable electrical power. For this reason, for crossings with a span width of more than approximately 2 km, those in charge of construction should consider laying an underwater cable as the more practicable solution. Alternatively, it might be possible to erect one or more pylons in the water to be crossed. Such crossings can be seen occasionally in North America. They are, however, only used when it is more economical and practical to do so than to lay a cable underwater, such as when the water is not very deep and no large passage heights are needed for vessels. Also, such construction can be very problematic as far as getting legal permission to build, because pylons standing in the water are likely to be considered dangerous obstacles for ships, especially in foggy conditions. In some cases on bridges small crossing a wider waterway pylons or crossbars for the conductors can be mounted. Such a solution, which may lead to safety problems at bridge maintenance, was for example realized at the Danish Storstrøm Bridge. It is quite likely that overhead line crossings of broad waters can be replaced with underwater cables. The overhead line crossing the Strait of Messina — which, with a span of 3646 meters, was one of the longest overhead line crossings in the world, with 200-meter pylons among the highest in the world — was replaced by a submarine cable, because of its small maximum transmittable electrical power. Overhead line crossing of valleys Overhead line crossings of valleys consist of two anchor pylons, one at either end of the valley. If the topography of the valley is suitable, these do not need to be very high. In very wide valleys, it is better to use a pylon for each phase in order to achieve sufficient distance between the conductors. In these cases there is frequently a further anchor pylon behind the crossing, used in order to realize the angle change of the conductor cables behind these. The problems associated with large spans also exist in these cases, but these can be easily and economically ameliorated, if the topography does not require high crossing pylons, by using a separate pylon for each conductor. Structures A crossing pylon is used for crossing over a body of water or a valley. Due to the long span, crossing pylons across rivers and sea straits are