Answer:
a. The students had both been riding at high speeds which increased the damage to both of them once they collided. Neither of them had been wearing protective gear. They had both been on a pedestrian only sidewalk, and shouldn't have been riding there anyway. Before they went out into the corner, neither of them looked for someone coming from the other side of the building, where the other student was. Finally, the fact that they were going so fast made it so that neither of them could stop in time before they hit the other.
b. This could have been prevented if they had worn the proper safety gear, had followed the rules of the pedestrian only sidewalk, had been moving slower, and had looked for other people before going out into the interception of the two sidewalks.