That's a  or a 7.9 mi/hr increase per second, which is the most
meaningful way to think of acceleration.  It means she speeds up by about
8 mi/hr every second.  However in school they want acceleration in
feet per second increase per second, even though the other is more meaningful.  

Most everybody memorizes that 60 miles per hour is 88 feet per second.

To change  to  multiply by 

So 55 mi/hr = 55(88/60) = 80.67 ft/s}}}

a =  =  =  = 11.52.

Edwin