sentences:  A familiar view of Saint Petersburg is a drawbridge across the Neva.Every night during the navigation period from April to November, 22 bridges across Neva and main canals are drawn to let ships pass in and out of the Baltic Sea into the Volga-Baltic waterway system.A calculated schedule with precise time of consecutive opening and closing for each bridge is maintained to guarantee passage of cargo ships and tankers at a precisely controlled speed, in order to have at least one bridge at a time staying connected to ensure passage for firefighters, police, ambulances and other ground transportation.