Vmax in this case is 15 mph.
The radius of the ramp is 50.
For simplicity, let's change Vmax to feet per second:
15 miles / hour (1 hour / 3600 seconds) (5280 feet / mile) = 22 feet per second
Using your formula:
22 fps = sqrt(14.88 f 50 feet)
484 (feet^2/second^2) = 744 feet * f
f = 1.54 ft/sec^2