A signature element of the Sonoma-Marin rail project and a key selling point for many of the voters who approved it -#8212; a 71-mile bikeway, postponed due to a funding crisis -#8212; is moving forward in places, but not fast enough for bicycle advocates.

The planned bike path, which eventually could be one of the longest of its kind in the nation, was crucial in winning voter support for a quarter-cent sales tax to pay for the Sonoma-Marin Area Rail Transit commuter line.

But faced with slumping sales-tax receipts during the recession, the rail authority was forced to build the line in stages and cut a third of the pathway funding.

With revenue now on the rebound, the agency has been laying track throughout Sonoma County. But construction on the bike path -#8212; envisioned as the spine of a growing bike network in the two North Bay counties -#8212; remains largely suspended, troubling bike advocates and enthusiasts.

The agency has yet to build one segment of the pathway and has no estimate for when it expects to finish the project, though officials have said they are aiming to complete the first phase of the rail project, from Santa Rosa to San Rafael, by the end of 2016.

"The fact that they haven't established a date for completion (of the pathway) gives me concern about a lack of commitment," said Gary Helfrich, executive director of the Sonoma County Bicycle Coalition. "We feel that the bike path should be happening quicker. We want to see more of a -#8216;can do' attitude."

SMART officials say they can't do work on most of the pathway until federal environmental regulators give the go-ahead. That approval should happen later this year, SMART spokeswoman Carolyn Glendening said.

In some places, the pathway will follow the rail; in others, it will veer off onto existing bike paths and city streets. Where the path runs along the rail, construction can happen only after the rail line is built, SMART officials say. Otherwise heavy machinery used to lay the rails would tear up the asphalt path.

A total of 17 miles of path will be on bike lanes along city streets.

Cities and other jurisdictions will build some segments of the trail. The city of Santa Rosa finished 0.3 miles of the path from West College Avenue to Eighth Street in November, and Caltrans is working on part of the bike path as it widens Highway 101 from Novato to Petaluma.

This summer, Sonoma County Regional Parks will construct a mile-long segment through Roseland south of Santa Rosa with federal funds and money from Santa Rosa's former redevelopment agency that the state agreed to release.

Recent action on the bike path has excited some cycling groups. In November, SMART awarded a contract to construct seven segments of trail, including three in Sonoma County. Those segments, including stretches in central and southwest Santa Rosa and Cotati, are not part of the federal environmental process that has delayed the rest of the path.

"This is extremely exciting," said Andy Peri, advocacy director for the Marin County Bicycle Coalition. "Overall, we're optimistic about having a lion's share of the pathway completed in 2016."

When it is finished, the system will be one of the longest so-called rails-with-trails systems in the country, said Barry Bergman, trail development manager for the Rails to Trails Conservancy.