From the many definitions of a suburb it is here assumed that a suburb is a built-up area situated beyond the reach of or developed beyond the capacity of 'soft transport' (pedestrian, bicycle and horse-drawn vehicles). Considering the mean speed of a horse-drawn tram (5 km/h) the furthest distance from the city centre reached is approximately 3 km, while, with a capacity of about 30 persons per carriage, the greatest traffic flow is about 1,500 persons per direction per hour. More advanced forms of mechanized transport opened up new territories for urban spread, increasing the distances and density of outer development. These areas constitute the phenomenon of suburbs of the industrial and post-industrial epoch, regardless of the type of built-up structure and its shape.