Signs of impairment: unable to maintain lane position, speed/braking problems
Also inattention problems like driving w/o headlights at night; judgment problems like unsafe lane changes
Prepare to take evasive action if see identifying factors but don't intervene personally
Keep distance, get out of way, never try to pass
If vehicle coming head-on in your lane, move to shoulder, stop, honk, blink lights
Get license plate number if possible but don't get too close, call 911