Why Does My Garmin Gps Send Me Down Back Roads That Are Totally Off The Wall, Especially On Long Trips?

When I take the 300 mile trip from where I live to my parents house I use my GPS to sort of be my little reminder for when to take the left split, right split or whatever, because I tend to zone out while i’m driving. The problem is, no matter how much I mess with the navigation settings, it tries to send me off the highway and on to the “back roads”.