An accident waiting to happen.

'An accident waiting to happen' is how you describe a situation that is dangerous and could cause an accident. Everyone knows it's a dangerous situation, but usually no one does anything to fix it until an accident has already happened.

Example:
-They really need to get a lock on this pool gate. It's an accident waiting to happen!
-You're so right! I'll go tell the pool managers.

Hitchhike

To 'hitchhike' means to travel via car with people you don't know. To do it, you stand on the side of the road and hold out your thumb. When someone stops, you get in. Of course, this can be extremely dangerous, but it is cheap.

Example:
-I can't believe your sister actually hitchhiked!
-Yeah, she could have gotten kidnapped or worse!