A month or so ago I “witnessed” an auto accident and was the only vehicle to stop.
I use “” marks because I did not see the start actual accident.
I saw the middle of the accident and the aftermath.
I was about 6-8 cars back from the intersection stopped at a red light with my head down looking at the GPS on my phone. When I looked up I saw a large truck bouncing from side to side like it went through a giant pothole. My first thought was damn I have to avoid it. Then when I got near the intersection I realized that it wasn’t a pothole but the truck ran over the front of a car.
The cars in front of me had not stopped and I didn’t know if anyone called the police so I called it in. The person driving the car moved the front of the car out of the street, it was drivable but the fender, bumper and grill came off as one piece.
I told the officer who came what I saw and because I did not witness the actual accident no tickets were issued. The person who’s car was damaged asked me for my name and number and now she has to be in court on Friday and wants me to be a witness for her.
Some people I’ve spoken to today said I should not go because I have to take time off for work. I don’t care about that I will go, missing time from work because it’s the right thing to do. I know I’m not going to be able to say that the other vehcile went through the red light which is what it seems to be because with just the rear wheels it should not have rode uo and over the front of her car.
I hope my honesty does not cause her more grief but I can’t say I saw the entire event when I did not.
Don’t worry about the effect of your honest testimony. Both sides will have lawyers, most likely, and it’s their job to extract from you whatever information you have that could support their client’s case. Just answer whatever questions you’re asked, say clearly and concisely what you saw (and heard, if that’s asked), and don’t try to figure out how your evidence will affect one side or the other. It’s the trier of facts (judge and/or jury) that will take your information, apply the law, and reach a decision.
Don’t be surprised if you’re asked some personal questions – address, age, marital status, education, employment, stuff like that. It’s intended to give whoever is hearing your evidence a sense of who you are, as background for assessing your testimony. The personal questions are nothing personal. 
And props to you – you’re a good person to have stopped, and to be willing to go testify.
That reminds me of a story my mom just told me.
A month or so ago, a family friend who owns a milk trucking business was headed back from the last pick-up of the day, so his milk truck was full to the brim with milk. I don’t know how much a full milk truck weighs, but it’s a lot. He comes to an intersection with a stop for the road crossing his at the bottom of a hill. Just as he reaches the intersection, this woman in a sedan comes flying over the hill with no intent of stopping. He’s going too fast to stop in time, and slams into the driver side of her car. She, of course, dies, as people who are wont to run stop signs do.
There was another woman come from the other direction, who couldn’t wait and make a statement to the police. She didn’t even leave a phone number or her license plate number. Somebody who had stopped since the accident had happened got it, but still.