Strøget, a pedestrian shopping street in central Copenhagen, is the longest of its kind in the world. It is in fact not a single street but a series of interconnected avenues which create a huge auto-free zone.
Technically speaking Venice on the Adriatic sea offers the largest auto free zone in any urban area in the word. However, its canals are filled with motorized boats of all sizes which offer many of the inconveniences (and conveniences) of automobiles.
There are also great quantities of auto free zones in the interior of Italy since in Italian hill towns and villages many, if not most of the streets are too steep and/or narrow for automobile circulation.