That is a huntsman spider and they are the worst. They like to hang around in cars sometimes. It does happen and if one fell on me I'd be freaked and I don't know what I'd do at all. I would probably scream a bit but pull myself together until I got to the nearest pull over place and then I'd freak out fully.

Also, it's probably not going to drop onto you like that, there's a higher chance it's going to climb up the back of your chair and onto your shoulder, or crawl across you in some other way.