Trying to figure out exactly what you mean here, and this was why I asked what does the plot need, it seems like you want the mother to die in an accident in which she might be committing 'suicide by auto.'
Let's go back to the basic, could it be a 'suicide by auto' type accident and how that conclusion is arrived at. First point is we're looking at single-vehicle accidents. Second point is a collision with a solid object such as a major tree or a bridge abutment. Telephone poles tend to not cut it since people realize you can shear the pole in half. It's got to be a seriously immovable object. Third is a high speed with a virtually direct trajectory. In other words, the vehicle can be aimed at a particular point, and smashed into something with sufficient speed to be likely to kill a person. Fourth is the vehicle only contains the driver. Although I'm sure that it's been tried with a buddy, or as a murder/suicide, this is very unusual.
Going through your accident, you're fine on one, bad on two, bad on three, and iffy on four. Four is easily dealt with, however, so I'll ignore that. Once you are in a skid, your ability to aim a vehicle is non-existent, absent a very skilled driver. You can slide and hit nothing, you can slide and bounce off snow banks, or you can slam into a great big oak tree. It's not predictable at all and for someone trying to commit suicide, you need some level of predictability in the result.
To give you an example, people know that to kill yourself with pills, you need to "overdose." So people take an entire bottle of tylenol to commit suicide. Unfortunately, not in the short term usually a fatal dose (but long-term you're going to really screw up your liver). For a writer, however, we've got to deal with both readers' reality and reality. So if I was to write a scene in which a person only took six pills to commit suicide, it would be hard to be convincing. Yet there is a pill that is tiny, easy to swallow, and six of them will leave you deader than a doornail.
Your accident, as described, however, won't work with any driver who's skidded in icy weather. People almost always describe the feeling as being out of control and having no idea where their vehicle was going to go. So your reader isn't going to go where you want them wondering whether she's trying to kill herself or not.
Now any good suicide by auto always have a level of doubt in them. And it will be difficult with snowy/icy weather, but hey, there's nothing like a challenge. And we won't forget about keeping the daughter safe. And since we're writers, let's not forget to make sure we've got some drama.
I like Jeseymour's snowplow. But for the purpose of a suicide by car, it won't work, as too many people run into the backs of plows because they're going to fast and don't judge the closing speed right.
So we've got a plow going down the highway, plowing the right side of the road, with a wing plow extended to the right. Plow driver reaches the end of his district and needs to cross the median to go back the other way. Plow driver raises his plows, including the wing, but does not swing the wing to the side of the truck, leaving it extended, with the end about four feet off the ground. Plow driver moves over to the high speed lane prior to entering the crossover.
Road is slippery, but traffic is doing about forty to forty-five other then dealing with the plows. Mom's driving is iffy, but not too bad. Mom sees plow and moves to the right to get around it. But instead of clearing the plow, the top of the vehicle hits the extended plow, hitting from one to three feet into the car. (This depends upon how dead at the scene you want mom. One foot probably won't kill her, three feet in will take off her head.) Car would then start to spin, and could spin merrily down the road and not hit another thing until it came to a stop in a snowbank. Daughter would very reasonably be able to walk away without a scratch.
Driver of the vehicle behind stops and waits for police to arrive. When they do, he says, "At first I thought she was going to miss the plow, but then it seemed like she almost aimed to hit it." At this point, police (and more importantly, the reader) are going to wonder whether she was trying to commit suicide by car or not.
The only thing freak about the accident is leaving the wing plow extended. As a plow operator, he should know that when not in use, the wing should be swung against the side of the truck, but who's perfect in this world? But if you want a good suicide by car scene, where the reader is left wondering, this should accomplish it.
Best of luck,
Jim Clark-Dawe
