Lucky you going to the Azores! I was there last April, and absolutely loved it. My friends and I split our 10 days between Terceira (4 nights) and Sao Miguel (6 nights). We stayed in Ponta Delgada for the 6 nights that we were in Sao Miguel. I didn't mind driving the main roads on the island, which make travel from places like Ponta Delgada to Ribiera very quick (30 minutes, I think). My friends and I all agreed that if we returned, we would spend a couple of nights in Furnas. Let me explain why.
Furnas is in a valley, and has much geothermal activity. This gives the option for the beautiful Poca de Dona Beija thermal pools and also the Terra Nostra park. Furnas is easy to get to in daylight, but when it's dark and foggy due to the thermal activity, the roads in and out of town can be treacherous. We went to the thermal baths one night after having dinner in Furnas. By the time we left, my friend looked up a route out of town back to Ponta Delgada on her iPhone. We ended up going over a mountain, in the dark, complete fog, and a road with at least 25 switchback turns. I think in the daytime, that drive might have been beautiful, but at night, it was like driving in a blizzard whiteout. I was driving, and without a doubt the most calm person in the car, because I could see the middle line on the road. My friends were completely terrified. As I'm writing this, clearly we made it back to Ponta Delgada with no issue, but I would not want to do that again.
Driving on Sao Miguel, you must be very careful. The highways are fantastic, but as you get off of them, roads become very narrow, and it's not unusual to come around a corner in a village and have a car completely stopped in your lane. You’ll see many mirrors near intersections; use them to help see oncoming traffic that may otherwise hidden from your view! Use caution and you'll be fine. By all means, do as much of your driving as possible in the daylight hours.
