One other comment on the general feature of downloading a route onto a GPS device: I find that I end up taking different routes than if I hadn't.

First an example of where there is no difference: in 1992 I crossed the US including across South Dakota. I believe for the entire state, I followed US 12 for ~400 miles. This year I'm crossing South Dakota. A little more convoluted but the basic navigation can be similar: consult a map, pick major routes and follow them. The "follow them" is not complex. I remember it and even if I didn't, I could write each day in a few lines on a note card.

So where is it different? In 1991 I crossed Chicago using similar techniques to my South Dakota approach: consult a map, pick reasonable route, write some instructions, follow them, stop and consult the map if things got complex.

Advent of smart phone and offline map, makes the "consult the map" step a little easier. Online access and Google maps let's me more dynamically. However all these cases, I would tend to fairly straightforward routes so I wasn't always stopping to consult.

So now this year, I crossed Chicago twice but had a route downloaded into my GPS. By nature it ends up being more complex. I'm on paths a bit more and probably making more turns since there isn't as big of a stop and check cost. So I do find myself on slightly different roads.

Now add uncertainty to things, road closures, construction, map program failures - and I'm back to the stop, consult and reroute steps and probably not as adept at them as before.

Today in the Dakotas there is no difference (today one segment between turns was over 50 miles and I was on the same road all day).