Can't say about kangaroos, but many (did I say many?) years ago I was on a car when we hit a wallaby by accident.
Wallabies are like 1/5 or less in size/weight when compared to kangaroos, they are small animals I would say comparable in size/weight to a large rabbit.
The car was a not so small car, a Holden Torana, a V8 4.2 liters, if I recall correctly, still the hit was very hard and the australian guy that was driving didn't manage to keep the car on the road and we ended up in the field nearby.
No physical harm to anyone on the car, but the fender and front of the car were seriously damaged.
I wouldn't have wanted to have hit a full-sized kangaroo instead of the poor little wallaby.