Weirdly, I think Mazda has a TSB about this -- spiders finding their fuel door area tasty... I've seen a few ultra creepy "albino" spiders congregating around gas pumps in FL and also scuttling around my car at times, so you are not a madman on this one...
If the ones you are encountering are wood spiders (flat, brown, square-ish, no webs, and very fast), I'd ignore them as they don't bite, and as long as an egg was't laid somewhere in your car, not a big deal.
Anything else, while its not necessarily a risk, is a bit unsettling, as many spiders, poisonous or not, leave painful bites.
Also, who wants to be doing 65 on the interstate and find a giant spider land on their lap?
I'd say, try either parking elsewhere and/or getting a half decent car cover. For any spiders that might be lurking in the car, pick up one or two of those sticky traps, and leave one in the trunk and maybe on the passenger floor (be careful not to trap yourself
) and see if anything gets caught.
Good luck!