A good contender should be Russian ?-297 Amur, from Chita to the Svobodnyi fork. It's 1400 km of road accompanied by Transsib, but it is barely inhabited and barely traversable. Maybe it got better in the last years, but it should be a worthy competitor for Brazilian rainforest roads all year around.

There are derelict settlements here and there, still you will never know where the next one will be and what sort of civilization will it feature.

There are some roads in northern Saskatchewan that would be in the same league as well. SK 905 from Sucker River to Points North Landing, SK is 433 km of gravel with no towns en route, although there is a small camp with an outfitter along the way where you could probably get fuel if you had an emergency (it isn't for general sale to the public though). No cellular coverage - satellite phone required.

On questions where all answers are nearly equally good it is hard to pick the "winner" for the "accept". So, I'm accepting the most upvoted answer.

Also, as I promised, here is a compiled list (from the answers) plus some other roads from other sources. For most roads I googled some links, included some citation from the source (as italic) and/or added some comments.

This answer is converted to community wiki answer, which on the one side means that me gaining no reputation from the upvotes and on the another side everyone is welcomed to edit this answer. So, please, extend and improve it :).

Roads

• Australia

  • Any road in the autstralian outback! - Could be dangerous or even deadly. (Especially for stupid people)

• Brazil

  • BR-319 Bad road, but already cycled. If you want try it, hurry because the Brazilian gov. is thinking about its reopening.

• Canada

  • James Bay Road and the Trans-Taiga Road - the "winners" :)
  • Saskatchewan Highway 905 also The highway is approximately 456 km (283 mi) long. ... Highway 905 is entirely unpaved. Points North Landing is about 200 km south of Stony Rapids and marks where the road used to end. Points North Landing serves as a permanent camp providing services for the many exploration companies searching for uranium in the area.
  • Trans-Labrador Highway - as the page shows, here are many "help points" on the road in +-100km range
  • Road to Kwadacha - There are some campsites along the road, relatively dense traffic and the road has constantly monitored radio-channel, so getting help not so hard

• USA

  • Dalton Highway Even if the wiki said: Anyone embarking on a journey on the Dalton is encouraged to bring survival gear it also said Despite its remoteness, the Dalton Highway carries a good amount of truck traffic - so, it isn't as "bad" at all...

• Russia

  • Old Summer Road - abandoned part of the Kolyma Highway / Road of Bones Kyubeme and Kadykchan via Tomtor. The Eastern 2/3 of this road is completely unmaintained. There is only 1 population centre along the 420 km route, Tomtor, population 1000

• Argentina

  • National Route 40 (ruta 40) - ... the most remote part of its route. 124 kilometres (77 mi) south of Perito Moreno is the junction with a side road to Cueva de las Manos, and 3 kilometres (1.9 mi) farther is the tiny settlement of Bajo Caracoles (population 100) This doesn't sounds very scary - but yes 124Km is enough far...

Technically not roads (at least not at full length)

• Australia

  • Canning Stock Route - In 2005 Jakub Postrzygacz became the first person to traverse the entire track without either support or the use of food drops, travelling alone by bicycle for 33 days. With large tyres and a single-wheel trailer, he carried all his food with him and replenished his water at wells.

• Afghanistan

  • Wakhan corridor - Life is hard there. The eastern reach of Afghanistan has the highest infant mortality rate in the world, and yak dung is the only source of fuel available during the brutal winters

• Mongolia

  • Great Gobi A Strictly Protected Area - There are some "roads" and "routes" thru the desert and even if you can get help from nomads and also from many tours crossing the desert, but still - it isn't best to get lost.

From the answers not included to the list:

• Russian ?-297 - there are more villages near the highway (even if the highway not crossing them directly, they're in close range on older roads)

• UK - North Coast 500 - even if the route is spectacular, here there isn't any danger :)

National Route 40 (Argentina)

It's 5.194 km long, it's highest point is at 5.000 m and the lowest at sea level

I don't know exactly the longest uninhabited sections, but traveling south from Perito Moreno National Park you must travel 235 km of uninhabited road until you can find a very small town of 100 inhabitants.

To reach the community of Kwadacha/Ware, British Columbia, Canada, one must drive nearly 450 km. Although that's not a huge distance, it's all on poorly maintained logging roads. Openstreetmap reckons it takes more than 18 hours to drive, Google Maps is a bit more optimistic at 14–15 hours. I remember reading a blog/website by a local, who says he does it in 10 hours. If "longest" is measured by driving time, it's certainly a contender for the most remote road that actually ends somewhere inhabited.

Happy Valley-Goose Bay, or Labrador City. Two years ago I planned a drive (by car -- still crazy) for a team of people from Montreal, Canada to Happy Valley-Goose Bay (Canada). There is a barren stretch of over 400 kilometers (250 miles), but I think there is a settlement midway at Churchill Falls (sorry!). You go an additional 300 miles to Labrador City on a road that is 2/3 gravel and 1/3 pavement. There is a road, but it's packed gravel (which IMHO could still be done by bike). You are advised to bring a satellite radio because there is no cell phone reception. We were unable to go, but our plan included bringing our own spare tires.

How about Fairbanks to Prudhoe bay including all 666km of the Dalton Highway: https://en.wikipedia.org/wiki/Dalton_Highway

It doesn't quite qualify as "uninhabited" as it has three towns along the way, but the TOTAL combined permanent population of all three towns is only 57.

The U.K. can’t compete with countries like Canada, Russia etc in terms of scale, however the North Coast 500 - a 500 mile route around the north coast of Scotland - has been named one of the top coastal routes in the world and is probably the most challenging route the U.K. has to offer cyclists. http://www.northcoast500.com/home/about-the-route.aspx