The jury deadlocked in deciding Jorene Nicolas' fate. The vote was 11 guilty to one not guilty.
At least three jurors mouthed "I'm sorry" to the mother of Deanna Mauer, the 23-year-old woman who died in the crash, as they left the courtroom.
Prosecutors alleged Nicolas was distracted and did not notice traffic had stopped when she slammed into the back of Mauer's car in April 2011 on the 405 Freeway in Westminster. According to prosecutors, her vehicle was traveling about 80 mph when she crashed into Mauer.
The defense alleged Mauer swerved in front of Nicolas, and that the victim's brake lights either weren't working, or she didn't apply them right before the crash.
Jurors deliberated for about 16 hours, beginning Wednesday afternoon, before announcing they were unable to break their impasse.
Senior Deputy District Attorney Jennifer Walker told the court that she would be looking into possible juror misconduct after hearing from other jurors that one person may have used physics formulas to re-evaluate testimony. Juror 6, the lone holdout, may have brought in a magnifying glass to take a closer look at evidence, Walker said.
Prosecutors asked for a new trial, and a trial setting conference has been set for May 16.
Nicolas' defense attorney was granted a request to be removed from the case due to a conflict. Nicolas was appointed a new public defender.
City News Service contributed to this report.