Each of us has roads that are bicycling favorites, and least-favorites.
One of my favorites, and I've mentioned it before, is Adams Street. It doesn't have striped bike lanes, but it's wide with excellent visibility, traffic is light, and the speed limit is 25.
The most uncomfortable I've ever felt on a road was years ago, on the infamous Eagle Road. I still remember it vividly. At the time, believe it or not, it was a narrow 2-lane road. So narrow that the "fog stripe" at the edge of the road was painted partly on pavement, partly on dirt, where chunks of pavement had broken off. And the speed limit was 55. I really felt like an idiot one day, during rush hour, with cars blowing by at 50+mph, 18 inches from my elbow. (Nobody had cell phones back then, or I probably wouldn't be here now! My route choice was legal, but not very smart.)
Alex Sorton is a traffic engineer at the Nortwestern University Traffic Institute. A few years back, he came up with an interesting way to measure road comfort. He calls it "Level of Service."
In a nutshell... L.O.S. is a combination of:
DISTANCE (of motor vehicle traffic from rider's elbow), plus
SPEED of motor traffic, plus
VOLUME of motor traffic.
Obviously, the skill level of the cyclist is also an important consideration. Usually, public-road cyclists are classified into one of three groups: advanced, beginner/casual, and children. A road that might be fine for an experienced adult cyclist could be totally unsuitable for the other groups, particularly kids.
If the road is wide enough, pretty much any speed is OK. I feel more comfortable riding on the shoulder of I-84, speed limit 75, than I do on Orchard Street, speed limit 30. But as motor traffic and bike traffic get squeezed closer and closer, and/or vehicle volume increases, something's gotta give. Too often it's "rider comfort" that gets compromised. (Although I believe ACHD does an admirable job of trying to incorporate bike-friendly infrastructure whenever feasible.)