Cars crash, of course!
Well, let's think about this a moment. At some point in time, a traffic engineer took the time to analyze a stretch of roadway; perhaps because of a high incident rate on that particular stretch of roadway, perhaps for a raising of the sped limit..or a dozen other "perhaps". The point is, he or she made a decision to place a guardrail right about........here....and voila, they were right. People aren't going off of the road in open areas, they are hitting the guard rail, which is, by definition, a "rail" to "guard" you from going off the roadway.
And, by the way, CARS don't crash into guardrails..people drive them into the guardrails. As a non-sentient being, the car has no idea what the shiny thing by the road is...but YOU do! Pay attention to WHY it was put there, and you might just NOT drive yourself into it.
However, in conclusion, the following are the main common reasons that causes car crashes:
1. Driver inattention-talking, texting, reading, eating, daydreaming, putting on makeup, and so forth.
2. Driver intoxication-drunk driving, driving while on illegal or legal drugs.
3. Driver loss of vehicle control-excessive speed, following too close to other vehicles, over correcting to animal, object, or roadway condition.
4. Driving too fast for conditions-weather, icy roads, wet roads (hydroplane), avoiding traffic or person, and so on.
5. Driver has medical emergency, et al.
6. Mechanical failure.