Would depend on the destination and itinerary .... One of my friend's kids planned a high school graduation trip with his friends backpacking in Glacier NP. (They live in the PNW, so not as long a haul as from here.) I could be on on board for this type of trip .... going to a beach resort, probably not.
Ignoring unexpected COVID related travel restrictions, I would let them go as long as I can generally trust them. Regularly crossed border into Ontario as a teen. At 18 I was traveling to Toronto with small group of girlfriends.
If you feel your 18yo still needs a curfew, then maybe they will be unprepared for so much freedom on a road trip.
If they are reasonably responsible and going with a small group of friends I am familiar with and are also reasonably responsible, I’d be ok with a U.S. trip. International, no way.
I know when I was a high school senior I did spring break and summer trips with my friends without parents. What say all of you these days? 18 years old. But still not all that mature particularly having stayed in during Covid.