The situation of the city on islands has limited modern suburban spread beyond the historic center; its framework of canals and narrow medieval streets has prevented the intrusion of automobiles. The best-known form of transport on the waterways of Venice is the gondola. Today, there are fewer than 400 of these unique boats left, and they have long been outnumbered by other vessels. A number of gondolas still serve as ferries across the Grand Canal, but the cost of maintenance makes their ultimate disappearance likely. The canals are filled with a variety of motor-powered boats. They range from the vaporetti, public water buses run by ACTV (Azienda del Consorzio Trasporti Veneziano), to the private motor-launch taxis. Other specialized craft, such as the barges carrying fruits and vegetables, the garbage barges,