In 2014, the Michigan High School Athletic Association limited high school football teams to no more than 2 collision practice days in any week. Researchers measured the head impacts to football players at one Michigan high school over the course of a season, before and after the rule change. A total of 15,398 impacts (592 impacts per player per season) were captured before the rule change and 8269 impacts (345 impacts per player per season) after the change. They found an average 42% decline in impact exposure occurred across all players. The largest reduction occurred among linemen. The magnitude of the impacts remained essentially unchanged between the years.
Source: Journal of Athletic Training, 2016