For me, it's more about time of day than anything. I don't feel safe on any street at 2 in the morning, no matter how nice it is during the day.

Other than that, I mostly judge by the type of people out and about. If it's a crowded area with lots of families (mixed gender, young children, etc), tourists, and people going about daily life, I feel pretty safe. If it's a totally quiet area, with absolutely nobody out on the street, I feel slightly less safe, but not really "in danger" or anything. If it's mostly adult people, who seem as though they've been drinking/smoking, or are just hanging around for no reason I can tell, and there's lots of loud music around (especially when it's just blasting from random parked cars), that's when I want out. To be fair, that last perfectly describes a lot of nightclub districts, block parties, and college towns, that are generally perfectly safe! However, they're generally places I'd rather not be alone. In that type of environment, people often don't pay attention to where they're going, making it harder for me to avoid them. Also, the car traffic is generally harder to deal with, because the foot traffic might not be following the street lights (if any), and the general noise in that type of environment can make it hard for me to hear the traffic flow. So even if I'm in a totally safe college town, I am less safe traveling there. Plus, it can be hard, just by hearing alone, to tell when things get unsafe in that type of environment (bar fights spilling out into the street, etc). But if I'm with someone, I'm perfectly OK. And if it's somewhere I know well, that's fine, too.