(S)AFETY
More likely than not this Michigan State University fan is just a fan of safety. Or there is a chance this driver is a fan of the technical football event that results in a “safety” which would make this vanity license plate pretty damn smart.
The cleanest explanation can be found over at Wikipedia. The cliff notes version is:
In gridiron football, the safety or safety touch is a scoring play that results in two points being awarded to the scoring team. Safeties can be scored in a number of ways, such as when a ball carrier is tackled in his own end zone or when a foul is committed by the offense in their own end zone. More at Wikipedia