What was the primary factor that caused collisions in California between the years 2009 and 2013?

The primary factor that caused collisions in California between 2009 and 2013 was driver distraction, particularly from mobile device use. This included texting, talking on the phone, and using apps while driving, which significantly increased the risk of accidents. Additionally, factors such as speeding and driving under the influence also contributed to the collision rates during this period.