Edward could have stopped it, but he didn't. He certainly had the speed and the strength to do it. However, being a rational vampire, he decided some random teenager he had only known for about two minutes wasn't worth the risk of compromising his secret identity. That's why he just stood there and watched as one car spiraled out of control across the high school parking lot, crashed into the side of Bella's truck, and turned the unsuspecting girl's head into a crimson s'more sandwiched between two panels of metal.
He couldn't explain why, but he was happy he made this choice. Somewhere, deep down inside, he felt as if he rid the world of a great evil that day.
And when it was over, he simply shrugged and walked away.