When using verbal authority and mandatory directives, the aspects of wayside signals along the railroad do not control train operations. Instead, train operations are controlled by orders from the Train Dispatcher, who takes responsibility for knowing what trains are located where, and ensures that no two trains are issued authority to occupy the same location of track at the same time. The Dispatcher usually issues orders, mandatory directives, speed restrictions, as well as the location of any wayside work crew via two-way radio to the locomotive crew. The train crew are responsible for ensuring that they obey these orders, speed restrictions, and advisories. This is the traditional means of controlling operations in the United States, and roughly 40% of all tracks in the United States are controlled in this manner.