t2p

large / long sections of roads in the mountains of yosemite have no guardrails

gave me the creeps

RCMoeur 

I've ridden for 70,000+ miles and driven more than 10 x that. Almost no roadway makes me feel uncomfortable - except US 550 south of Ouray, Colorado. The highway climbs a cliff face that gets heavy snowfall and avalanches in winter, and all year around the only thing between the southbound edge line and a very long near-vertical fall is a few inches of rarefied mountain air. No thanks.

StarBiker

Speed. Looks like a big drop. Pic of where you are talking about in Tennessee with speed limit?

50PlusCycling

Highway 491 (formerly Route 666), which runs from Shiprock to Gallup in the state of New Mexico. It was (and still is) called the "Devil's Highway" for obvious reasons. For most it its existence, it was also America's most dangerous highway. Most of it runs through the Navajo Reservation, and it is the main road from the various villages and towns on the Big Rez to the liquor stores in Gallup. There are countless little monuments along both sides of the road marking the places where people have died in collisions, most of them alcohol related. I've been up and down this road a few times, it's safe enough earlier in the day, but you don't want to be on it at night.

Steel Monkey

Terry Barentsen (yoob tube bike vidder) survived 17 but would never do it again. Other vidders doing the Key West destination refused to return on this route. Having watched a few vlogs, I didn't think Georgia's portion was much better. And it's STILL part of the "East-Coast Greenway." If only the promised infrastructure improvements hadn't been just promises.
What jumps out at me is that ANY road is safe for cyclists IF motorists practice patience and safety. Just sayin'.