Once you pass Binh Dinh going north, Highway 1A is no fun at all.
If you've got time for it in your schedule, we highly recommend taking a right pretty much anywhere. There are miles of wonderful country side, and as you get closer to the sea, you'll notice red and white cement posts with numbers on them lining the roads. Flooding is so frequent here, the posts mark how high the water is in centimetres, so those trying to navigate the roads will know whether or not their vehicle will make it. You'll see a lot of wooden, ox-drawn carts in this region, and you'll start to see why that still makes sense. An ox's engine never floods. And they can swim. And wood floats.
As long as you keep headed generally along the coast as you go north, you'll meet up with Highway 1A eventually. Heading south towards Qui Nhon, you can do the same thing and bale off the main road when it starts getting nasty, although the section of route one north and south of Sa Huynh is quite beautiful.
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