What constitutes a "Busy Street"?

[Bob C.]

On one end of the street spectrum there is the cul-de-sac in a closed development, and on the other end of the spectrum there is a superhighway running through the front yard. 

Somewhere in between those two points we go from quiet, to busy.  Where is that demarcation?  

For streets in the middle of that spectrum,  what defines "a busy street"?