that u-turns were not permitted, that would be a sufficient defense.
if you made a u-turn to try and avoid getting hit or causing an accident, then a judge may take this into consideration. while it may not result in the charge being dropped, it could lead to a lower fine or no points off your license.
making a u-turn for an emergency purpose such as your car breaking down may be an appropriate defense. although it would be much more understandable to pull off onto the side of the road that you are traveling down, if your only reasonable option was to make a u-turn, then the court will take that 