Grand Canal in Venice, Italy (© Juan Romero/Cavan Images)

What if a city's busiest street had no road? In Venice, Italy, traffic flows along the Grand Canal, a 3.9-kilometre waterway that winds through the city in a giant reverse S-shape. Known to locals as the Canalazzo, it is Venice's main water-traffic corridor, carrying vaporetti, water taxis, gondolas, police boats and other vessels. The canal follows the course of an ancient river that once flowed into the Venetian Lagoon.