[Potironette]

Is it reasonable for a car to accelerate at -18 mi/s^2 while it's braking?

(Context: I have a homework problem that asks me to find the acceleration of a car while it's braking: "According to the recent test data, an automobile travels 0.250 mi in 19.9 s starting from rest. The same car, when braking from 60.0 mi/h on dry pavement, stops in 146 ft. Assume constant acceleration in each part of the motion, but not necessarily the same acceleration when slowing down or speeding up. (a)Find the acceleration of the car when it is speeding up and when it is braking.
I found 1.51 mi/h^2 while speeding up so I figured maybe braking would be similar.
Tried to use x_f = x_i + v_i*t + .5at^2 and got -(60.0 mi/h)^2/(2*.0277 mi) = 6.50 * 10^4 mi/h^2. I tried to turn it to mi/s^2 because clearly the car isn't braking in a whole hour (still the number's huge?), but it was -18 mi/s^2. I'm not sure where I went wrong on this problem. The car couldn't possibly be accelerating that fast right..? Even earth rotates slower than that, right..?)