Morgan Valley Road, between the Route 29/Route 53 junction in Lower Lake and the Napa-Lake County line.

This road was paved in 1984 when a new gold mine was built on the Napa-Yolo county line, and the easiest access was from Lake County. The mine is about to shut down now, so the future use of this road is in question. The road doesn't serve many other travelers. Beyond the Napa line, the road becomes Knoxville-Berryessa Road and turns south to end at Route 128 at Lake Berryessa.

Soda Bay Road, between Route 29/Route 175 in Lakeport and the beginning of Route 281 in Konocti Bay. At MP 17, the road becomes Route 281. Supposedly, all of this route is planned to become part of Route 281.

This route was built to provide direct access to Konocti Bay and the "Riviera Heights" housing development.

Gaddy Lane, connects Soda Bay Road and Route 29/Route 175 in Kelseyville.

Bottle Rock Road, between Route 29/Route 175 at Kelseyville southeast to Route 175 at Cobb, near Hobergs Resort.

This route serves the Geysers geothermal plant as well as several homes on Cobb Mountain. # #

From I-5 at Rancho Santa Fe Road in Encinitas, then east through Olivenhain, across the San Dieguito River at Del Dios Highway, to I-15 near Rancho Bernardo.

First planned around 1962. For decades, county planners wanted to build another road linking I-5 and 15, and there was discussion in the 1980s and 1990s about a highway between Encinitas and Mira Mesa. But heavy opposition along the coast drove the county Board of Supervisors to abandon what was called Highway 680 in 1995. Deleted from city general plans in 1989 and 1993. Funds later reallocated to Route 56.

There was a subsequent plan dubbed the Son of 680 that turned the highway to a north-south alignment; this also was shelved.

Camino Del Norte has been extended west of the Camino San Bernardo Overpass for a considerable distance. It ends in the middle of nowhere.

According to the North County Times, in 2005 there was a proposal called the "Grandson of 680". This would be a new arterial linking North County with job and entertainment centers to the south. Planners are said to be including the idea of a new freeway in a countywide transportation plan update due in March 2005. The update may include a discussion on a limited-access route east of I-15 as well as a proposal to put a new north-south freeway between I-5 and I-15.

• Andrew Field's San Diego Highways: San Diego Highways: Unbuilt Select Arterial 680
  (Site Disappeared)