When I'm deciding where to ride for enjoyment/training (same thing, really), I consider the time, direction, road and traffic volume. There are roads that are horrific at 7:30 AM but are divine at 10:00 or 4:00 AM. That same road might be great northbound at noon but be unbearable southbound if the traffic has a directionality to it. Weekend before last I rode a 116 mile ride and seven cars passed me. Many cyclists in my area rode a third that distance and had ten times as many cars whiz by.

Of course, one doesn't always have the option of choosing times/direction and such. When you've got to be at work at a certain place at a certain time, you've got to play the cards you're dealt. Luckily, many of us do have the option of living in more convenient locations than we have chosen if we prioritized being able to ride with safety and joy.