I took geotech because it was easy. I could have done hydrology if I'd waited till more than a day and a half before the exam to realize that I really wasn't equipped to do the structural option; I needed more than a day to study afternoon-level hdyro and environmental. Likewise transportation, because I didn't have the manuals on hand.
There were other people from our design office taking the exam at the same time, and they also were not taking the structural option even though they were working as structural designers. They'd been warned by structural PhDs to avoid it! It is very weird that one option out of five (or now six?) is significantly harder than the rest, with no different certification to show for it. I should have known structural best and yet I had trouble with it, then got a 96 on geotech after two undergrad courses and a day and a half of specialized preparation. Something not right there. But I digress.
For transpo, get the MUTCD and whatever other manuals you need (isn't there something with runoff tables? It's been a while), work through the Lindeberg text and the PPI2PASS CD--ROM, and you should be okay. I think NCEES also publishes workbooks with sample questions for the various afternoon sessions. The questions in the books and the CD-ROM will give you a good idea of which manuals you'll need. If you get time to take a real-life prep class on top of that, you should be golden.
It doesn't hurt to prep for more than one of the different afternoon options and then leaf through them day of the exam and pick the one that looks most agreeable with your current state of mind.
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