Crime is not a major problem but beware of pickpockets especially in major cities and when riding on public transportation. DO NOT leave your belongings in the back seat of your rental car when you park. The Highway system is very good and has improved with each visit. You will have very little trouble going from Lisbon to any of the major towns. Make sure that you have a current map.
New highways spring up all over everyday.
I usually use the map given to me by the car rental firm. I've also driven on some secondary and some non secondary roads and had no problems. Major credit cards are accepted by most establishments but I would suggest a few travelers checks to exchange into local money for the out of the way locations. I highly suggest that you use a travel agent that knows Portugal and specializes in Portugal Holidays. I've used Sun Travel for my trips and have been very happy with their knowledge and the arrangements that they have made.