Public comments are being accepted for a proposed new state highway that would connect state Highway 4 in eastern Contra Costa County to either Interstate 580 in Alameda County or a location further east at Interstate 205 in San Joaquin County.

State Highway 239 would connect from Highway 4 at Marsh Creek Road south for about 17 miles to one of the two endpoints being considered under the project. The new state highway proposal has been years in the making and is sponsored by Caltrans and the Contra Costa County Transportation Authority.

An online-only public scoping meeting was held Tuesday on the project, but public comments are being accepted through Jan. 18.

Two proposed alignments for a new state Highway 239 are shown. (Graphic courtesy of Contra Costa Transportation Authority via Bay City News)

The agencies proposing the project, which had its study phase starting back in 2012, say the four-lane highway is needed to relieve increased commute traffic between eastern Contra Costa County and western San Joaquin County, as well as to improve access to the Byron Airport and to reduce non-local traffic through Byron itself.

However, sustainability advocates have said the highway would enable more suburban sprawl and take away money needed for other projects.

The public agencies plan to release a draft environmental document for the project next year, with final environmental approval targeted for 2024. Funding would come from county and federal sources and Regional Measure 3, a bridge toll hike approved by voters in the nine Bay Area counties in 2018.

People can comment through Jan. 18 via the project’s website, by email, by phone at (925) 255-5466, or by mail to Caltrans District 4 Environmental ATTN: Lily Mu, P.O. Box 23660, MS: 8B, Oakland, CA 94623.