HESPERIA - A long-envisioned east-west thoroughfare, now under environmental review, would open a vital link between Interstate 15 and the south side of the city.

Costing about $78 million, it is Hesperia's top public-works project.

"The city and its citizens have been waiting a long time for these improvements along Ranchero Road," said Scott Priester, Hesperia's director of development services. "It finally will be realized."

The project, which will be constructed in three phases, includes a $21 million underpass beneath the BNSF Railway tracks that split the city in half.

A second phase, costing $30 million, calls for building an interchange at I-15 and Ranchero Road.

The $27 million third phase, in which Ranchero will be widened over a six-mile segment from two to four lanes, will connect the underpass and interchange.

"Constructing the underpass is my top priority," said Mayor Rita Vogler. "At last, it is moving ahead."

When completed in two years, the underpass will relieve congestion on Main Street and reduce travel time for residents of the city's southeast area who commute to jobs in the San Bernardino Valley.

Vogler said the underpass would help open Hesperia's southeast sector to new development.

As many as 16,000 new homes are planned at the Rancho Las Flores project in Summit Valley, south of Hesperia.

The 10,000-acre development was approved by the city in 1990, and the area was annexed in 1992, said Dave Reno, Hesperia's principal planner.

"It's an exciting time for Hesperia," Vogler said. "We're growing fast, and a lot will be happening."

Tom Harp, Hesperia's deputy director of development services, said the underpass will divert east-west traffic off Main Street and eventually will open a new route for Apple Valley residents to reach I-15.

It also will provide a new route for firefighters, ambulances and police who now use Main Street to reach the southern perimeter of Hesperia.

The project will be financed with city, state and federal funds, Priester said.

"We now are working with Caltrans to complete environmental studies for the underpass," he said. "Caltrans must approve the environmental report before it is sent to the Federal Highway Administration for review. With approval by that authority, the city can move ahead with design work and acquisition of right-of- way."

The four-lane underpass will be about 200 feet long, with approach lanes extending along Ranchero from Seventh Avenue to Danbury Avenue.

This fall, Priester said, the city will call for bids for construction of the underpass, with work beginning by year's end. It will take about two years to complete the underpass project.

"Meanwhile, we hope construction on the six-lane freeway interchange can start in 2010, and can be completed in 18 months," he said.

Then would come the widening of Ranchero between I-15 and the railway underpass.