Determining if a road is safe to bike on How do you determine which roads are safe to ride on and which should be avoided? What traffic patterns do you feel comfortable riding in, and which ones set off alarms? What sorts of intersections do you avoid? How about for nighttime riding; are the same roads safe at night, or do you take a different route at night than you would during the day? Are there any statistics on ratio of accidents to total bicycle usage on certain roads to help inform this decision?

There are several streets in my area that I will always avoid. There are a few parkways with no shoulders whatsoever, narrow lanes, winding roads, fast moving traffic, and very few stop lights and side streets to calm traffic. There are several that I feel pretty confident on, with reasonably wide bike lanes, low traffic, and frequent intersections that mean that traffic doesn't usually get too fast and drivers are on the lookout for people turning in. I'm just wondering if there are any more objective criteria I can use, or good rules of thumb, for where it's safe to ride.
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.