More here. I think this is basically a HAWK signal, but that may be too specific for this.
The George Washington Memorial Parkway, in consultation with the Federal Highway Administration, will be temporarily installing a flashing beacon warning system at the trail crosswalk of the northbound lanes of the Parkway just prior to Arlington Memorial Bridge, one mile north of I-395. The temporary installation is planned to be in place for approximately thirty days between early September and early October as a trial to help inform the upcoming environmental assessment process that will evaluate long-term safety and traffic improvements in the area.
The warning system is designed for bicyclists and pedestrians who want to cross the Parkway and to alert motorists of the presence of trail users. Trail users can activate the flashing beacon and notify motorists of their intention to cross the Parkway. The purpose is to improve safety and increase awareness by slowing down traffic when pedestrians and bicyclists want to cross the Parkway.