The Mackinac Bridge Authority voted unanimously to close the bridge to all traffic during the annual Labor Day bridge walk in September.

The bridge will be closed 6:30 a.m.-noon to all traffic, leaving an estimated 4,000 drivers stranded on either side of the bridge on one of the busiest traffic days of the year.

The action was taken at the request of the Michigan State Police and the U.S. Department of Homeland Security, which are responding to threats posed by terrorists using vehicles to plow into crowds to inflict the most possible damage.

According to the Michigan State Police, there are no credible threats in Michigan at this time.  But as the summer months approach, this event and other open air events is what they're concerned about.   The Detroit Free Press has more.