What was the cause of Jimmy and Roberta's car accident in Chapter 15?

In Chapter 15, Jimmy and Roberta were driving to her next appointment when Jimmy slowed their truck on Eye Street because the light was red, but it changed to green before he’d completely stopped. Jimmy didn’t look to see if another car was coming into the intersection, as he would generally have but instead reached and touched Roberta’s arm. It was at this instant that their vehicle was t-boned by a medical supply van.