On the day of the collision last month, visibility was good. The sidewalk was not under repair. As she walked, Tiffany Briggs, 25, was talking to her grandmother on her cellphone, lost in conversation. Very lost. “I ran into a truck,” Ms. Briggs said. It was parked in a driveway. Distracted driving has gained much attention lately because of the inflated crash risk posed by drivers using cellphones to talk and text.