A friend of mine and I did it. I documented our adventure in my blog,
http://www.maxtower.com/panama
We drove from Portland, OR to Panama. It took us 2.5 weeks of solid driving. You don’t want to do it unless you really just want to for the adventure. Otherwise its just a huge hassle. I had a lot of fun on my trip, but if you don’t care about finding all the undiscovered beaches between the US an CR, then it won’t appeal to you. Crossing each border takes about 2-3 hours. The roads are not ideal for long trips. Actually the roads in CR were the worst that we experienced. So if you can handle CR driving, then the other countries are fine.
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