Darin Barton was holding a sign asking drivers for help near an I-70 exit when he heard the crash. He told KDVR a semitrailer crashed into cars and the cab of the truck rolled over.
"As soon as it rolled over, it just caught on fire. And I just dropped my sign, took off running," Barton told the television station.
Rogel Lazaro Aguilera-Mederos, 23, of Texas, was charged with four counts of vehicular homicide, police said.
Barton, who has been homeless since 2014, said he ran down the embankment toward the wrecked vehicles and saw several vehicles on fire.
"As soon as I (saw) flames, I headed under the bridge, grabbed three or four people out of a couple cars," Barton told KDVR. "I just did what I hoped anybody would've done if I was sitting down there.”
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