Kauaʻi looks small on a map, but one big fact shapes every trip: there is no road around the whole island. The Na Pali Coast blocks the northwest, so the main highway is a giant out-and-back, and almost everything routes through Līhuʻe. Here are realistic drive times so you can plan a day without surprises.
Kauaʻi's main highway runs from the north shore (ending at Haʻena, past Hanalei) all the way around the east and south to the far west side past Kekaha. The paved road ends there; the rough dirt stretch out to Polihale has no lifeguards and dangerous water, so treat Polihale as a sunset spot, not a swim. The highway does not connect across the top. The Na Pali Coast in the northwest has no road and never has. So you cannot loop the island. If you are staying on the north shore and want to see Waimea Canyon in the west, you drive all the way back through Līhuʻe to get there. Plan trips as out-and-backs from your home base.
Līhuʻe is roughly the center of the road network, so most times are measured from here. To Kapaʻa is about 25 minutes (around 12 miles). To Poʻipū is about 25 to 30 minutes (around 15 miles). To Princeville is about 50 to 60 minutes (around 30 miles). To Waimea Canyon is about 75 minutes (around 36 miles). To Haʻena and Keʻe Beach at the very end of the north shore is about 1 hour 40 minutes (around 39 miles), partly because of slow one-lane bridges.
This is the long one people underestimate. Princeville or Hanalei down to Poʻipū is roughly 1 hour 10 minutes to 1 hour 15 minutes without traffic, around 42 miles, because you go all the way through Līhuʻe. From the north shore out to Waimea Canyon is closer to 1 hour 45 minutes. If your trip mixes north and south shore activities, group them by region on separate days rather than zig-zagging.
There is real rush-hour traffic on the east side around Kapaʻa, especially morning and late afternoon, and the times above assume open roads. Add 20 to 40 minutes through the Kapaʻa corridor at peak times. One-lane bridges on the north shore past Hanalei also slow things down and require taking turns. Leave early for popular spots, both to beat traffic and to get parking before it fills.