Determining if a road is safe to bike on
I consider several factors.


Traffic density
Traffic speed
Road shoulder or not
Bike lane or not
Route efficiency
Road condition
Bus and truck traffic density
Streetcar routes


The main combination that I really work hard to avoid is high density, high speed, and no shoulder or bike lane, with poor road condition thrown in as a bonus. I will ride on high density, low speed roads without a lane/shoulder most any time. 

For intersections, if possible, I tend to avoid ones that are poorly signaled; but on some routes I just have to be extra careful on those. At night, high speeds and poor road conditions gets extra weight.