- Unsafe neighborhood - whether you live there or are just visiting, be realistic about your 'hood and go to a gym or safer location if it might be unsafe.
- Risky time of day - even the safest neighborhoods have dangerous times of day. If your area doesn't have sidewalks, forcing you to run in the road, your danger zones are dusk, dawn and nighttime. If your neighbors roll up their sidewalks when the sun goes down and your streets don't have lights, don't be the only one out in the dark within shouting distance.
- High traffic and heavy pollution - exercising involves deep breathing but more cars on the road means more pollutants in the air. Bad combo. Putting all that nasty stuff in your lungs for an extended period of time can lead to serious health issues. Stay inside where the air is more likely to be filtered.
- Rough road - cobblestone and broken or dirt roads shouldn't stop you from running the streets. Just wear a headlamp if you're going to brave uneven paving while exercising in the dark. No headlamp? Head to the treadmill.
In the meantime, dress for the weather and get out there! It's spring or fall in almost every part of the world and both are wonderful seasons for outdoor exercise.