Downed power lines? That's serious business.

If you could visualize the amount of energy in a power line as a fire, you wouldn't even think about driving through it.

A tree took out the 15kV lines to my parents' place a couple of years ago. Even though they were both lying in a trout pond (the perfect earth ground, and no dead/boiled fish in evidence), I was not inclined to get close.

There are ways to detect voltage without contact, but in this scenario you could easily become part of the circuit before you could do the measurement. You can't help someone in distress by adding to the casualty count.

EDIT: To be fair, if I knew the line in question with a high degree of certainty, and had reliable evidence that it was de-energized, I might cross it if I had no other choice. An interruption in the overhead supply line past my house, for example, would be pretty obvious; the neighbours' yard lights would all be off. But in many other places, the line does not feed local residences directly, or at all.