In any situation like this where an accident happens and someone is severely injured or dead and the police don't make any immediate arrests because again it's an accident they don't know who's culpable, you should immediately begin to contact lawyers if you don't know of one already.
As you contact lawyers they will take 10-15 minutes to listen to what you describe and decide whether or not they're willing to assist you or they'll give you some immediate general advice. In this case they will probably say something like if the police contact you back call to come in for additional questioning contact me and I will be your attorney and they'll have a fee arrangement and so forth. And this is your contingency plan because the police are going to call a cooperating suspect and ask you to voluntarily come in to answer questions.
The other situation is when you're not immediately arrested perhaps the police have gathered enough in the initial investigation that the district attorney is going to press charges. Again the response is the same immediately after the fact contact a lawyer explain the situation and get their advice and they can work with the district attorney to see if charges are planned. Then you voluntarily turn yourself in for a rest and processing and you already have your attorney.
Now the situation where you are immediately placed under arrest the advice is always don't talk to the police mention that you're specifically exercising your right to remain silent. If they process you and release you on your own recognizance that's fine get an attorney at that point If they want to hold you tell them you need to contact an attorney and you don't know of one. Because you are legally allowed access to an attorney they must assist you in some minor way allowing you to make phone calls allowing you to look up attorneys to contact and so forth. Call a couple explain the situation that you're under arrest pick one that you like doesn't have to be the one that you stick with. But they'll usually be the one that can get you out so you can then find a better attorney if needed.
Then once you get home look up defense attorneys online and talk to a few over the phone. Select one and pay them for a few hours of their time. Based on their assessment you can then proceed to have a conversation with law enforcement if your attorney advises you to.
One example is U.S. Law Shield. The idea is that if you shoot someone in self defense, they have a number that you can call to get a lawyer.