Thursday's fire simulation drill will allow Caltrans to test out the new tunnel's safety systems, including a series of state-of-the art jet fans on the bore's ceiling meant to blow smoke outside and pull in fresh air in the event of a fire.
The new bore will also be connected to the third bore via cross-passages that can serve as emergency exits in the event of a fire in either tunnel, according to Caltrans.
The drill will take place from about 2 p.m. to 3:30 p.m. today, and passing motorists may see smoke but they shouldn't be alarmed, project spokeswoman Ivy Morrison said.
Drivers should expect traffic backup on Highway 24 between 8:30 p.m. Friday and 10:30 a.m. Saturday as crews re-stripe traffic lanes and make other last-minute preparations, according to Caltrans.
"Motorists should plan for major delays at night" over the weekend, since only one bore will be open in each direction of the tunnel, Morrison said.
The nighttime delays could continue throughout the weekend and possibly into next week, depending on how long it takes for the final testing and prep work to be completed, Morrison said.
There have been reports that the new bore could open as soon as Saturday, but Morrison declined to say when the opening would occur, explaining that the date depends on how the work goes, weather and other factors.
The $417 million fourth bore project is intended to relieve traffic congestion on Highway 24 in the off-peak directions, Morrison said.
She explained that having a fourth bore should speed the flow of traffic through the Caldecott Tunnel in the eastbound direction during the weekday morning commute, and in the westbound direction in the evening.
The additional bore should also make traffic on the roadway more predictable on weekends, she said.