Be sure that you word your topic as a question like this:
What is the main cause of car accidents in the U.S.?
That essay idea would be an explaining essay, and you can probably research the main reasons why people have car accidents. However, if you are trying to write a "cause" essay, this one probably isn't a good idea even though it uses the word "cause" in the question. The reason for this is that Cause Essays are argument essays which speculate on the most important reason for some situation. A good Cause Essay topic will not have a simple answer that you can research and find a definitive topic. It will be something that people have different opinions about. Here are some examples:
What is the most important cause of teens who start driving at 16 having more accidents than those who start driving at 18?
What causes people to tailgate?
What causes some cars to be less safe than others?