Each auto insurance policy is different and defined by each state law and/or company filing guidelines. For example from a Florida Auto Policy: "Collision" means the impact of your insured car with another object or upset of your insured car.

Collision coverage is intended to cover things that your vehicle strikes while Comprehensive coverage is intended to cover things that strike your vehicle. Comprehensive is supposed to cover un-excluded loss other than collision or overturn.

It can be difficult to determine which occurrences are covered by collision and which is covered by "other than collision" (comprehensive) when discussing objects. If you drive straight off the roadway into a ditch your loss is a collision even though the ditch is a stationary object.

Occurrences like theft, vandalism, falling objects, hail, water, wind and striking animals fall under comprehensive BECAUSE they do not fall under collision coverages.

For business auto, garage or truckers' policies these are covered under "Specified Causes of Loss" which is neither the comprehensive nor collision part of that type of that policy.

If the road you are traveling on causes damage and it isn't defined as an object that your vehicle hit then it would fall under "Other Than Collision" which is comprehensive or on business auto, garage or truckers' policies it could fall under "Specified Causes of Loss."