Controllers can't use controller-applied visual separation in lieu of wake turbulence rules. If a pilot agrees to maintain visual separation or follow another aircraft, then he assumes the responsibility for wake turbulence separation.
Landing behind an equally-sized or heavier aircraft, it is best to descend above the preceding aircraft's flight path. Departing behind an equally-sized or heavier aircraft, it is best to try and climb above the preceding aircraft's flight path, which isn't always possible, hence using time or relying on a particularly strong crosswind to take care of the wake turbulence is a pilot's best bet.
More and more research is being done on the topic of wake turbulence separation. The FAA has developed something called RECAT, which is a program designed to recategorize the wake turbulence separation standards between aircraft that will be rolling out in the next few years.
