I see people nonchalantly saying they've done these cross-country bike touring trips, and I have an ignorant question: how do people feel safe while actually riding on the road?

Riding bikes around neighborhoods I can understand when cars are only going 20-25mph, but to travel between cities you'll certainly have to go on either very fast, very busy, or very fast and very busy roads, right? I'd like to understand this because I imagine long distance biking must feel very freeing and exciting.