I moved back to Fairview, TN last May, which is where 840 used to end, and I drove it the other day and realized that they finally extended it. But to my dismay it has yet to be fully completed even though it was supposed to be done in 2008. There were some environmental issues with it's placement and they had to re-plan it after they had already bought all the land. Pretty messy stuff in the end. "in late 2002, Gene Cotton, founder of the South West Williamson County Community Association, was quoted in the Tennessean: 'They're building an interstate and calling it a state road so they don't have to perform in-depth environmental studies.'" Basically Williamson County has a lot of money to waste on blocking projects like this. Good news though - there are definitely alternate routes you can take to avoid Nashville. If you look at 840 on a map you'll notice the break between pinewood road and Thompson Station. Just take the back roads between the gap and hop back onto 840 which takes you to I-24 and straight to Manchester. The gap is only about 25 to 30 mins.
I'm going from Germantown.....Go I-40 to 840, go south...when it ends, go left, which will take you into Franklin, then I think it is 98 to Murfreesboro and you are on I-24, go south to Manchester.
Or, take Hwy 64 over to Fayetteville, turn left on 53 (I think, verify) through Lynchburg (Jack Danials) to Tullahoma, then 55 to Manchester.
Lots of ways over, but if you want interstate the entire way, you have to go either through Nashville, or Birmingham (longer)....
You can expect 5 - 6 hours over, and then at least that long sitting in line to get in.....
That's basically the way I came back, except I took 64 all the way back because the cops directed me down to the Pelham exit (127), 64 was there and it looked like a straight shot.
As I said before, that was a huge mistake. From Pelham to Fayetteville 64 is fine, but from Fayetteville west 64 absolutely sucks all the way to Shelby County. The speed limit is mostly 45 because of construction.
When they finish 64 it may be an awesome drive, but this year I will stick with I-40 to I-440 to I-24. That's about four hours solid despite the long detour to Exit 127, and last year there was no delay at all at the gate on Thursday at noonish.