An avenue is usually larger that a street. A street usually has only two lanes whereas an avenue has four or more. John

As John said very well, an avenue would normally be expected to be wider than a street. The problem is that towns and cities grow and develop, so the once tiny road could become a bustling thoroughfare. I remember a city where I once resided had a major artery named Sheridan Road (a road should be even less significant than a street). In addition, some cities in the United States, designate streets as only north-south and avenues east-west (or vice-versa). In other words, there could be an East Main Avenue, but not an East Main Street. Size distinction is lost in such case.