The coastal road that circles the exterior of both Tahiti Nui and Iti is measured in pointe kilometrique markers, known as PKs. PK0 begins in Pape'ete and markers go in both clockwise and counterclockwise directions, meeting in Taravao, which is PK60 counterclockwise and PK54 clockwise.
The west coast has the most sights and the most traffic; it's wise to set out early on a driving tour to avoid peak-hour traffic. Grab a map so you know whether a sight is at PK10 clockwise or counterclockwise. While the distances are not far, there is quite a lot to see on the west coast—beaches, surf breaks, museums, gardens and an excellent golf course—and it may take much longer than you think.
Tahiti Nui also has a handful of short interior roads. While a standard car can access the roads to the Belvedere lookout, the Three Waterfalls, and the start of the walking and 4WD trails in the Papenoo Valley, it's not possible to cross the island from north to south, or south to north, without a 4WD.
The PK marker system also operates in Tahiti Iti, beginning in Taravao, the town at the isthmus. There are two roads: the north road of 18 km (11 mi) from Taravao to Tautira and the south road, also measuring 18 km (11 mi) from Taravao to Teahupoo. The southeast and east coasts of Tahiti Iti can only be accessed by boat, bicycle, or foot. There is one inland road, leading from near Taravao up onto the Taravao Plateau and Taravao's lookout, also called the Belvedere.