Phuket does have more than its share of traffic congestion from too many cars on the road, but it is much easier to navigate than many other large cities. (Ex.Phuket airport to Rawai, Kata, Karon.,)
Streets is usually well marked and the majority of drivers are well mannered. If you use your turn signals, someone within 4 or 5 cars will generally let you change lanes, even in bumper to bumper traffic.
Patong is cross a mountain also a driver must best experience.