I do not think the woman is credible. Mainly because if she truly believed he was drunk I think she would have tried to help the boy in the front seat. I also think her timeline is too broad - if she had indeed seen him, I think she could narrow it down a bit more. I think she is very specific about details when she supposedly only saw him as he drove by.
In fact, I felt exactly the opposite about the timeline. I completely identified with it. Isn't it funny how we each surmise things?
When I am camping the only timeline I care about is dusk. Then I know it's time to prep for the impending dark so I get to work. I don't keep a watch handy or anything. For me, it's like... "who cares?" My camping weekends are filled with eating when I feel hungry, swimming, hiking, relaxation, fishing, sitting around a campfire chatting, or maybe playing dice games or cards at a picnic table. etc.
Therefore, I feel the timeline alone should not be a deterrent. Because she probably didn't know she had to remember anything until she saw it in the paper days later. It seems then, and only then would she piece together her recollection of the glance of a man in a silver BMW driving on the grass.
For example, let's say you were at an event last Friday, and glanced over to something that caught your eye for a brief moment. Then, you moved on about your day and the days following. You would not give that moment or glance a second thought. But, suddenly you see that same person (or whatever) in the news 4 days later, and you suddenly remember exactly that you saw them the days prior. If you weren't wearing a watch or keeping track of time, you might only have a window of time to compare against.
If you are a detailed person, you provide every tidbit of information you can remember, including the window of time you were out doing whatever you had been doing at the moment you saw what you saw.
About the drunk part.... she said he could have been crying. It was memorable, but not so much that she felt she should follow up. I don't think any of us can judge that.