The fourth time Christopher Hinnard was caught with a DUI, he killed someone.

Now Allen Phillips' family wishes Hinnard would've learned his lesson -- before it cost someone's life.

On Tuesday, Hinnard agreed to serve 15 years in prison for aggravated vehicular homicide.

"A life is gone. An innocent person was killed. And that happened to be my son, whom I adored," said Elizabeth Wilhoit. "I hate for anybody to have to go through what I've gone through."

For nearly two years now, Wilhoit deals with the daily pain of losing her son.

"He was out doing his job, and he was just run down," she said.

Right before Mother's Day in May 2013, the 45-year-old Phillips was driving his cab down Rossville Boulevard when Hinnard, who was traveling in the opposite direction, hit Phillips head-on.

Hinnard tested positive for oxycodone plus a handful of other prescription drugs.

It was his 4th DUI.

"I just wasn't aware this was all happening today," Hinnard told Judge Rebecca Stern before pleading guilty.

"It doesn't bring my son back. It doesn't bring back our loss. We feel it tremendously," Wilhoit explained. "It's been a terrible loss, and it was just so unnecessary."

Hinnard's three previous DUIs happened between 2000 and 2007. After each time, he received one-year sentences that were wiped away after probationary periods of good behavior.

Under Tennessee law, a DUI is considered the lowest-class felony at the fourth offense.

But for Phillips' family, that fourth offense has left them with permanent pain.

"He was such a good man," Wilhoit said. "We miss him tremendously. But it's over with today."

On top of Hinnard's 15-year prison sentence, his driver's license will be revoked for 10 years.