Oahu: If you want to travel around the island on your own schedule, renting a car is a must. Heavy traffic toward downtown Honolulu begins as early as 6:30 am and lasts until 9 am. In the afternoon, expect traffic departing downtown to back up beginning around 3 pm until approximately 7 pm.
Maui: Driving from one point on Maui to another can take longer than the mileage indicates. It's 52 miles from Kahului Airport to Hana, but the drive can take three hours. As for driving to Haleakala, the 38-mile drive from the mountain’s base to its summit will take you about two hours. Traffic on Maui's roads can be heavy, especially during the rush hours of 6 am to 8:30 am and 3:30 pm to 6:30 pm.
Big Island: It's a good idea to rent a car with four-wheel drive, such as a jeep, on the Big Island. Some of the island's best sights (and most beautiful beaches) are at the end of rough or unpaved roads. Most agencies make you sign an agreement that you won't drive on the path to Mauna Kea and its observatories. Keep in mind that, while a good portion of the Saddle Road is smoothly paved, it is also remote, winding, and bumpy in certain areas, unlighted, and bereft of gas stations.
Kauai: A rental car is the best way to get to your hotel, though taxis and some hotel shuttles are available. From the airport it will take you about 15 to 25 minutes to drive to Wailua or Kapaa, 30 to 40 minutes to reach Poipu, and 45 minutes to an hour to get to Princeville or Hanalei. Kauai roads can have heavy traffic.