The only time I've seen a flag was when my friends and I drove from Boston to Florida two years ago. Discover called my friend due to the fact his card was being used as we drove down the east coast of the U.S. (only stopping for gas/food) leading to a few transactions in a relatively short amount of time over a couple states.
I did that a few times driving from LA to Portland. I would have been in trouble had the fillup times been reported to the CHP/OHP
I just noticed you can let capital one know of travel online, just click modify features than slect your services than select travel
Better safe than sorry, though, I've never called when traveling within the United States. I did call when I was traveling internationally.
I was driving from NY to Cali and charged my WF card in 3 states all in the same day. I woke up the next morning unable to gas up, and had to call fraud dept. to get unlocked. It was a PITA. I would definitely make the call, just in case it saves a little time and hassle during your vacation. You could also, or instead, ask them for a direct number or extension to the fraud department, program that into your phone. That way if something does happen you can get it right away without going through the customer service maze.
It must depend on card company. I recently took a driving trip from boston to DC and used my citi AA card in MA, CT, NY, NJ, DE, DC, VA, MD in 2 days and had no issues. This was my experience, but I agree with your point about having the phone number of fraud department handy. I always carry atlear 2-3 cards when I travel coz if my card was to get blocked, the fraud departments don't work round the clock.