A tip for anyone planning a first-time driving trip in Europe: Don't set off from the airport without GPS!
We've driven all over North America with just road maps, never even using Google Maps. But in Europe I quickly realized that this would be impossible.
The way the roads are designed and marked, you need to change roads, merge and so on many, many more times for the same length journey in Europe compared to the US.
In the US, you could check a map and know that to get from point A to point B, you need to get on first one highway and then switch to another highway, depending on signage to get you to the first highway and then onto the second one. Roads are marked as I95 North or I70 West.
Not once in our European driving did we have an experience going from point A to point B anywhere as simple as that. Roads are marked primarily by the major city destination ahead (which would change as you progress). Highway numbers are much smaller than city names and hard to read. Never would a sign include a compass direction. And many, many times the road would split with both options going apparently to the same major city. Very confusing. Also, just figuring out how to exit a city or town was complicated.
In addition, nearly all transitions from one road to another involve a roundabout with three, four, five or even sometimes six different options. Many times there would be a succession of three roundabouts in a row to transition from one road to another. Without the GPS, I don't think it would have been clear to us which option to take at each rounabout.
With that said, we had a lot of troubles with our car GPS. The biggest problem was that it didn't seem to have some smaller locales where we'd booked hotels in the GPS at all! The second biggest problem was that some hotel addresses were written in such a way that the GPS wouldn't accept them. I coped with this by trying to book only hotels that had a street number and what looked like a conventional street name (rather than a km marker, say, or just the name of a village).
Late in our trip, I figured out that if I used the postal code for a smaller town like Tarifa, Spain, this would work in the GPS when typing in "Tarifa" wouldn't work. I also noticed that it usually worked to just type in the longest word in the street address and not bother with little stuff like "de la" or even "avenida." (Maybe techies already would know this, but I didn't.)
I think some of the problems we had inputting smaller towns into the GPS came from alternate spellings for place names, due to, say, different vowels or stylistic variants in Galician or Basque versus Castillian Spanish or English or Italian. If you didn't use the same variant as what's programmed into the GPS, you're stymied.
I don't understand why our car GPS didn't have hotel names in it, the way Google does, but it didn't. At some points, I would give up on the car GPS and pull out my cellphone to get us to the hotel more easily or to, say, a certain cave. That helped until my cellphone was stolen and we had to depend only on the car GPS.
Overall, I would say that GPS challenges were a major factor in our road trip, more than language problems (we have intermediate-level French and Spanish and we mainly went to France and Spain) and as much as trying to stay on the right side of the toll collection system in various countries. (That would require another post.)
One last point: we found European drivers to be reasonably sane, careful and predictable. But rent the smallest car you can stand, as town streets are often scarily narrow and parking spaces are small.