Most streets are small with 18.5 mph speed limits, bikes and cars mix.
Car traffic is always light because these are not thru-roads for drivers.
Arterials take people quickly (direct without stops) to distant areas. Bicycle freeways!
Fast moving car traffic (18.5mph+) is always physically separated.
my custom edit from the Robert Wood Johnson infographic
Here's an animation explaining "taking the lane" AKA how to get honked at by rude drivers.