The Oregon Department of Transportation has implemented an advance warning system that alerts drivers entering a tunnel to the presence of slow or stopped traffic at the tunnel’s exit. The system uses a combination of SmartSensor Advance and Matrix units to detect traffic; if certain conditions exist, the system activates a flashing sign that warns drivers to slow down.Transportation officials in Oregon were faced with a dangerous traffic situation at the interchange between Interstate 84 and Interstate 205 east of Portland: a significant number of rear-end collisions were occurring because drivers were unaware that traffic ahead of them had slowed or stopped. The problem occurred at a tunnel that drivers must enter to merge from westbound I-84 onto southbound I-205. There is a sharp left turn just after the tunnel and traffic often backs up to the tunnel exit, particularly during peak congestion times. Drivers entering the tunnel at full speed were unaware of conditions ahead of them, and could not see stopped traffic around the turn.