In Prisoners, after the disappearance of the girls there was a first clue, which is "Alex Jones driving the campervan" and hitting a tree. Why was he driving the campervan? If his aunt (Holly) wants to get away with it, she could leave no clue. But why did she make him do so?
I think that the answer is simpler than it looks.
He is mentally very backwards and at the same time he is in a very hard situation to solve, his whole life he lives with his "aunt" who is his kidnapper and at the moment he has to deal with another kidnapping of two kids by his aunt (he just wanted to play with them).
I strongly believe that it was suicide attempt or at least some try to escape, not from the police, but from reality. After all the same did Bobby when he was in similar situation (he was cornered with no way out).