This is for everyone, but especially those that live in The South (Tainan, Kaohsiung and Chaiyi) and should know better!
When planning your big road-trip to the East coast, please, please do not confuse the incredibly scenic highway 20 ('nan-heng') with the foul boring no 9 ('nan-hwei') further south. The no 9 may be quicker (that's why it's full of trucks) but no 20 is one of the major pluses of living in the South. Make a full day of it - better still 2 or 3 days and stay at the top/middle at Yakoo. Easy hiking/stunning clear views first thing in the morning. Prepare for the cold.