[Serra Britt]

Without doing the actual calculation to check your answer, I'm looking at it in reverse. At an acceleration of -18 mi/s^2, it would mean you would stop nearly instantly even going 60 mi/h. So your initial thoughts are correct that the acceleration is too fast. It does still need to travel the 146 ft at least.

Using formulas here http://www.dummies.com/education/sci...-and-velocity/

A speed of 60 mi/h is 88 ft/s.
146 ft = (88 ft/s)x(time) so time to stop is 1.659 seconds.
So then:
1.659 sec = (-88 ft/s)/(acceleration) so acceleration would be -53 ft/s^2, which is a lot less than -18 mi/s^2