I agree that you should just be honest. And while I understand the desire to provide some form of a palatable, but honest, explanation as to why you don't want to ride with this woman, I also think a simple "no, I can't" will suffice, too. If you really would like to explore a friendship with this woman, then feel free to suggest meeting for coffee or something else as an alternative, but I'll be honest that there are already a couple of red flags with this woman IMO, so be careful with how you proceed.