Complex sentence: In the first test, forty-five participants were randomly assigned to watch different videos of a car accident, in which the separate videos had shown collisions at 20 miles per hour, 30 miles per hour, and 40 miles per hour.
Split into simpler sentences: In the first study, 45 participants were randomly assigned to watch videos of a car accident. The first test randomized forty-five participants to watch different videos of a car accident, in which the separate videos had shown collisions at 20 miles per hour, 30 miles per hour, and 40 miles per hour.