Is There Really a "Cushion Effect"?: A Biomechanical Investigation of Crash Injury Mechanisms in the Obese
-- Kent, R., Forman, J., & Bostrom, O. (2009). Obesity DOI: 10.1038/oby.2009.315
Short answer: No. "the mechanics of the trauma seem to vary between normal weight and obese individuals, there seems to be not much evidence supporting the concept of obesity being protective against injury during a car crash. With the exception of a possibly lesser chance of head injury, obese subjects may actually be more injury prone during an automobile collision."