In 2006, the U.S. DOT initiated the UPA (Urban Partnership Agreement) program to demonstrate congestion reduction through strategies consisting of combinations of the 4Ts: Tolling, Transit, Telecommuting/TDM, and Technology. The Seattle/LWC UPA projects focused on reducing traffic congestion on SR 520 between I-405 and I-5, a heavily-traveled, east-west commuter route across Lake Washington. The lake separates Seattle from the eastern suburbs (including Bellevue and Redmond). These strategies included tolling all lanes of the SR 520 bridge, adding bus service in the SR 520 corridor, implementing active traffic management systems, and real-time traveler information signs on highways and at transit stops and stations. The suite of projects had a combined benefit-to-cost ratio of 1.76 due to travel time savings and reduced emissions.
WSDOT uses two freeway loop detector configurations to record volume, lane occupancy, and speed every 20 seconds on a lane-by-lane basis. The traffic sensors are generally located in each lane, including any HOV lanes, and entrance and exit ramps. WSDOT archives the traffic sensor data for research and evaluation purposes. The archives contain 5-minute aggregations of the raw traffic sensor data.
Morning peak mean travel speeds on SR 520 increased from about 45 mi/h to 56 mi/h eastbound and from about 47 mi/h to 58 mi/h westbound. Afternoon peak mean travel speeds increased from about 47 mi/h to 60 mi/h eastbound and from 27 mi/h to about 45 mi/h westbound. Mean travel speeds on SR 520 remained above 45 mi/h in both directions of travel throughout the entire morning peak period post-deployment, whereas pre-deployment the mean travel speeds dropped below 45 mi/h for a major portion of the morning peak period.
Comparatively, I-90 (parallel, non-toll route) saw morning peak mean travel speeds decline from about 60 mi/h to 57 mi/h eastbound and 57 mi/h to 56 mi/h westbound. Afternoon peak mean travel speeds declined from about 57 mi/h to 56 mi/h eastbound and about 50 mi/h to 41 mi/h westbound.
Faster travel speeds on SR 520 and lower travel speeds on I-90 indicate a shift in use patterns in after deployment of tolling.