SAN DIEGO - For commuters who slog through bumper-to-bumper traffic on Interstates 5 and 15 and Highway 76, key milestones are finally approaching.

In inland North County, some work on I-15 is scheduled to wrap up by year's end, officials at the San Diego office of the California Department of Transportation said Wednesday. On the coast, construction is about to start on a new interchange.

And completion is nearing on crucial environmental studies that will shape the long-awaited widening of I-5 and Highway 76.

Meanwhile, the Sprinter light-rail line between Oceanside and Escondido is expected to open in December, said Karen King, executive director of the North County Transit District.

"In North County, this is going to be a banner year," King said. "We're going to be crossing the finish line on a number of projects."

King made the comments at an annual media briefing on the status of highway and rail projects across San Diego County. It was organized by Caltrans in conjunction with the North County Transit District, San Diego Association of Governments and other agencies.

Pedro Orso-Delgado, Caltrans district director, said this is one of the busiest times in recent memory for highway construction, with $20 million a month being spent for that purpose - double the amount of three years ago.

Orso-Delgado predicted the pace will kick up another notch - to at least $30 million a month - once the state starts disbursing money from the transportation infrastructure bond, Proposition 1B, that California voters passed in November.

It's hard to miss all the construction, said Gary Gallegos, executive director of the association of governments, a regional transportation and planning agency.

"As you drive around town, you really can't help but notice the cranes, trucks and construction workers improving our transportation system," Gallegos said.

The work is perhaps nowhere more obvious than on I-15.

Part of that work will culminate in the opening of one new northbound lane on the I-15 mainline between Camino del Norte and Centre City Parkway and one new southbound lane between Via Rancho Parkway and Rancho Bernardo Road in December, said Gustavo Dallarda, Caltrans' I-15 corridor director.

However, Dallarda said, the toll car-pool lanes being built in the center won't open until later; in summer 2008 from Highway 56 to Rancho Bernardo Road, and December 2008 from Rancho Bernardo to Centre City Parkway.

Here are other highlights for North County:

• Construction is expected to begin in March on I-5 auxiliary lanes that will connect Via de la Valle and Lomas Santa Fe Drive. Upon completion in September, Caltrans intends to begin extending the I-5 car-pool lanes from Via de la Valle north to Manchester Avenue and reconfiguring the Lomas Santa Fe interchange.

"We're adding two new loop ramps, which will effectively eliminate the need to make a left turn from Lomas Santa Fe to either onramp from either direction," said Tom Nipper, assistant project manager.

• In March or April, a draft environmental study is to be released for Highway 76 between Melrose Drive and Mission Road. Mark Phelan, project manager, said Caltrans will use the study to decide by late summer whether to build the four-lane expanded highway on the north or south side of San Luis Rey River. Construction is slated to get under way by December 2008 and be completed by December 2011.

"We're going to buy right of way for six lanes and build four lanes," Phelan said.

• In March, North County Transit District expects to finish building 4.7 miles of parallel track along the coastal railroad through Camp Pendleton. In summer, the district plans to construct 1.3 miles of parallel track through Oceanside, between Oceanside Boulevard and Cassidy Street.

Both projects are part of an ongoing effort to ultimately deliver two continuous sets of tracks along the 60 miles between downtown San Diego and the Orange County line, reducing delays and providing room for more trains. Now, less than half of that stretch has a second track, King said.

• In April, North County Transit District plans to begin work on a project to protect the Del Mar Bluffs - and the tracks atop them - from erosion.

• In late summer, Caltrans expects to deliver a draft environmental study that will shape a dramatic makeover of 28 miles of I-5 between La Jolla Village Drive and Harbor Drive in Oceanside.

Crossing six cities and six lagoons, the makeover is almost certain to include four exclusive toll car-pool lanes, said Arturo Jacobo, an I-5 project manager. What's not clear is whether Caltrans will add a pair of lanes - one in each direction - to the mainline.

"Our goal is to begin construction in 2009 or 2010," he said.

- Contact staff writer Dave Downey at (760) 740-5442 or ddowney@nctimes.com.