The white panel between the wheels of this truck is called a "Lateral Protective Device", and its purpose is to prevent people, bicycles, motorcycles and small cars from getting sucked under the truck's rear wheels when it swerves into them:

This second safety device must serve a similar function, but I can't quite put my finger on it:

I pass this truck all the time. It seems to spend hours almost every day parked in front of the Flower Mart with its engine running the whole time.