Gallo, 23, has pleaded not guilty to three counts of second-degree murder.
Rivera testified he and Gallo drank several pints of beer and at least one shot before the crash around 12:30 a.m. on Sept 9, 2009.
"He's significant for that situation to show and demonstrate how irresponsible and selfish these two were when they began their night of indulgence," said prosecution attorney Susan Price. Rivera said Gallo was celebrating a new job.
"He seemed pretty excited. He was going to start a new job the next day," Rivera told jurors.
Rivera said that he drove his parents' minivan on the day of the crash while he and Gallo ran errands during the afternoon. The two later wound up at a bar where Gallo's girlfriend worked.
At a previous grand jury hearing, Rivera testified that he and Gallo visited three bars that night and spent hours taking tequila shots and drinking beer.
Rivera, who was supposed to be the designated driver, testified he was blacking out. He remembers Gallo didn't want to keep drinking but Rivera egged him on.
At the time of the crash, Gallo's blood-alcohol level was nearly three times the legal limit. If convicted, he faces a maximum sentence of more than 50 years to life in prison.
An accident reconstruction expert also testified on Wednesday, confirming that Gallo, not Rivera, was the driver of the minivan. Data from the van showed the car was traveling around 65 miles per hour at impact, which is nearly twice the posted speed limit. The car that Adenhart was in was traveling at 43 miles per hour.
"The defendant never touched his brakes," Price said. "He never applied the brakes at all before this collision."
Rivera testified Gallo told him to run. Rivera stayed at the scene. Gallo fled, but police eventually found Gallo walking against traffic on the side of a freeway two miles from the accident scene and arrested him after he was identified by witnesses.
Gallo was convicted of DUI in 2006. He was on probation.
Prosecutors said they took the unusual step of charging Gallo with second-degree murder - and not the lesser charge of manslaughter - in part because of his prior drunken-driving conviction. Jurors do not have the option of finding Gallo guilty of manslaughter if they decide to convict.
"If he ran the red light, he's responsible for these deaths," defense attorney Jacqueline Goodman said Tuesday. "It's tragic. Nobody left home that night thinking they wanted to kill anybody. Nobody even left home thinking that was a possibility."
The Associated Press contributed to this report.