Hollywood Freeway Is Reopened to North, Southbound Traffic : Tunnel fire: The grand jury may be asked to investigate the Metro Rail blaze. Probers still don’t know what caused it.

For the first time since fire ravaged an underground Metro Rail tunnel last week, all but one lane of the Hollywood Freeway “slot” opened to traffic Tuesday, finally putting the downtown Los Angeles commute on the road to recovery.

“It’s kind of a good-news day,” Dave Roper, deputy district director of the state Department of Transportation, said as he announced progress by construction crews to shore up the gutted tunnel under the freeway.

Meanwhile, the county Board of Supervisors will consider asking the grand jury to investigate the cause of the fire, which raged unchecked for hours and caused the tunnel to cave in.

Such a probe would come amid separate investigations by the district attorney’s office, which is looking into possible safety code violations, and the Fire Department, whose arson squad was summoned to inspect the tunnel damage.

Arson investigators Tuesday examined photographs and samples taken from the tunnel, but they have yet to determine the cause of the blaze or the point where it ignited, officials said.

Investigators face a number of obstacles, fire officials said. For one, intense heat and the possibility of further collapse prevented firefighters from entering the tunnel for nearly 24 hours after the blaze erupted, meaning that a great deal of potential evidence could have been destroyed. The same danger of collapse has limited the time investigators can spend inside the tunnel, and the caved-in portion is completely inaccessible.

“They are not able to probe every inch and spend a great deal of time in there,” Battalion Chief Lon Pursell said. “It’s a very difficult fire to investigate.”

The 150-foot segment of the tunnel that sits underneath the freeway has been shored up sufficiently to open most lanes of the Hollywood Freeway, part of U.S. 101.

Three inside lanes of the southbound 101 between the four-level interchange and Mission Street opened Tuesday afternoon, while the northbound lanes opened gradually Monday and early Tuesday.

Roper, the Caltrans official, said that the fourth, outside southbound lane will not be reopened until the ground near it can be better stabilized. The collapse of the tunnel a few feet from the lane has created a huge hole in the ground.

“We don’t want that No. 4 lane to collapse into that hole,” Roper said.

The opening of the southbound lanes took on an almost ceremonial air. First, traffic barriers were removed from all three of the lanes on the top of the four-level “stack” where the Hollywood, Pasadena and Harbor freeways converge just west of the downtown Civic Center.

Then, a convoy of commuters following a contingent of California Highway Patrol motorcycle officers streamed down the highway, which had been deserted since the fire began at about 1:50 a.m. Friday.

In addition to a continuing ban on trucks, Roper said, ramps leading from the Harbor and Pasadena freeways to the southbound Hollywood will remain closed. This is intended to ease the crunch a few hundred yards south of the stack, where the Hollywood normally would widen to four lanes.

Several downtown ramps leading to the southbound Hollywood also remain closed.

Roper said CHP officers would remain stationed at the west end of the San Bernardino Freeway, the north end of the Santa Ana Freeway and the southbound Hollywood Freeway at the stack to make sure no big trucks enter the slot.

Roper said the San Bernardino Freeway busway, which had been opened to all traffic since midmorning Friday, will once more be restricted to car pools and bus traffic, beginning early this morning.

Two monitoring systems have been set up to make sure the freeway stays where it belongs, Roper said. One calls for the measuring of carefully calibrated survey points on the surface of the pavement. The other involves stress gauges placed on the scaffolding in the tunnel to measure any deflection.

“Neither has showed any changes,” Roper reported Tuesday afternoon.

Highway Patrol officials predicted that, despite the opening of most of the 101 “slot,” many commuters will use other routes downtown.

“They’ll probably continue to car-pool or take alternate routes until the whole thing clears up, until they have full access,” CHP Officer Harold Daily said.

Daily reported the Tuesday evening commute was moving smoothly through the newly opened lanes, with “hardly any backup.”

The “alternate-route” strategies, along with car-pooling and staggered work schedules, have been credited with preventing worse-than-normal traffic jams downtown since the Hollywood Freeway was closed.

But the use of alternatives was also beginning to wear thin, CHP officers reported.

” . . . Some of the alternate routes, like the southbound 405, got a little slow,” CHP spokeswoman Lydia Martinez said of Tuesday morning’s commute. “People (from the Valley and the Westside) were afraid of getting stuck downtown, so they went over to the 405 and they started getting stuck over there.”

Times staff writer Richard Simon contributed to this story.