Why do I see people that aren't there while driving at night?

Sometimes while driving at night, I see people that aren't really there. For example, there will be a fire hydrant on the side of the road, which looks nothing like a person, and I will imagine it's a person crouching there. Or there will be a trashcan by the side of the road with some trash sticking out and I'll see a person in an odd position. This is becoming a more frequent event and is really starting to scare me. Also I am not overly tired when I see these people, usually just ready to go to bed. What's wrong? Is it something in my brain or are my eyes going bad. I'm 18, almost 19, with fairly poor vision and no history of mental disease if that makes any difference.