newtonian mechanics - Acceleration of a car in a collision?

For example because of newton law $F=ma$ force is acceleration times mass but if I go at a high constant velocity ($900\: \mathrm{miles/hour}$) in my car for examples I have $0$ acceleration but if I hit an obstacle by newton's law it will have no effect on me because $F=ma$ and mass of my car x 0 = 0

But if I do the experiment I will find that I actually hit the obstacle and it does an force on me why?