Well, I was hit from behind by a car going at around 50MPH (I was stationary) and I suffered no injuries besides a little whiplash.
My father had a head on collision at a similar speed (35-40) when he was younger, going over a hump-back bridge. He was in a mini. The other driver had a much larger car.
The other car driver walked away with just a few bruises. His car was a write off.
My Dad’s airbag deployed with such ferocity that he broke his nose and suffered whiplash. The front of the mini was pushed forwards so that the engine ended up inside the passenger compartment. He struggled to free his legs, but ultimately only suffered bruising. He was fit enough to leave the car (which was obviously a wreck) and walk to a nearby house, where the farmer’s wife made him tea and tried to stem the bleeding from his nose, and the cuts in his head (caused by his broken windscreen). They were worried that he was concussed, but he turned out not to be.
That story happened in the 1970s, I think. 35-40 isn’t that fast, especially with today’s modern, safety conscious cars. I’d be surprised if she didn’t walk away with a few minor injuries. Now, to get the engine to ignite, you would have to be very unlucky indeed! I’ve seen motorway crashes at 80MPH plus where the engines don’t ignite… in fact, of all the crashes I’ve seen in person, I’ve only ever seen 2 where the engine ignited.
That said… what country is this in? Because of the MOT system in the UK, our cars tend to be super safe. I’ve driven down motorways so many times and seen multi-car pile ups. The cars are completely trashed, but everyone is OK… they’re all stood on the hard shoulder nursing bruises and cuts, waiting for the emergency services.
On the other hand, when I lived in Asia then I’d go past accidents and think “meh, only a small dent in the car… I’ve smashed mine up worse!” but then I’d see all the passengers being driven off with sheets over their faces. They have no MOTs there, and their cars become unsafe quickly as a result.
EDIT: Sudden thought… I think my Dad didn’t have an airbag, actually. I think he broke his nose when it impacted with the steering wheel. I’ll have to ask him. My airbag didn’t deploy during my accident either… although I did hurt my nose on the steering wheel as well. It also leaves you a bit stunned, when you smack your face like that.