To begin with you need to understand that with nearly 4m inhabitants, Puerto Rico lays claim to being the 51st state of the US and much of the way of life there is similar to other Hispanic influenced states on the mainland. As in the US, you'll drive on the right in Puerto Rico. With so many people on a small island, the roads can get busy, so in the cities try to avoid driving during the rush hour times of 8 AM – 10 AM and 4 PM - 6 PM as traffic is always bad then.
Roads in the countryside can be quite dangerous because they have narrow bends and local drivers don't always slow down. Driving defensively is a good idea with a warning honk just as you turn a corner. Other roads may only have one-and-a-half lanes so proceed carefully. Always have a good map or rent a low cost satellite navigation system from us as signs can be few and far between. It's highly recommended that you get a car with air conditioning as it is always hot and humid in Puerto Rico.