What percentage of U.S. highways are greater than 6% grade?

by Tihamer Toth-Fejel
(Maryland)

My only experience with really steep grades is Old Priest Grade (just outside Yosemite). A white knuckle experience with smoking brakes if you're lucky. Now, I'm sure no trucker in his right mind is going to go down an 18% grade with an 80,000 lbs load, but I'm pretty sure you've had to deal with 6% and 7% grades. I believe that such grades need to be posted for federally funded highways.

I am curious: About what percentage of U.S. federal highways have a grade of 6% or more?

Thanks!