We have received some questions recently about the traffic volume for the Alaskan Way Viaduct found on our 2012 Traffic Flow Map. So we asked our traffic data team to help explain why the volume seems significantly different from previous years.
In years past, the Seattle Department of Transportation (SDOT) calculated the viaduct’s traffic volume using a measurement point just south of the Columbia Street on-ramp. This is the location where the viaduct achieves its highest volumes. The number was calculated by combining data from one week volume counts of the on- and off-ramps along SR 99. The volume reported was the Average Annual Weekday Traffic (AAWDT), which is a measure of typical traffic across a weekday (Monday through Friday).
In 2012, due to the ongoing construction of the South Atlantic Street overpass, we were not able to collect valid data for the SR 99 on- and off-ramps located near the stadiums. As a result, SDOT was not able to calculate our own number for 2012 volumes on the Alaskan Way Viaduct.
Instead, a volume from a Washington State Department of Transportation (WSDOT) counter on SR 99 south of the stadium ramps was used. While the WSDOT data is of excellent quality, it measures traffic volume at a different location from where SDOT has historically calculated the viaduct’s volume. The WSDOT figure also represents the Average Daily Traffic (ADT) volume, a measure of daily traffic for the full week (Sunday through Saturday), which in the case of the viaduct is lower than an AAWDT volume.
These changes in the data should have been noted on the 2012 Traffic Flow Map, but were not. To clear up any confusion created by a change in data sources for the viaduct’s traffic volume, SDOT has published a revised version of its 2012 Traffic Flow Map that no longer shows a value for the traffic volume on the viaduct. If you are looking for 2012 traffic volume data for SR 99 and the Alaskan Way viaduct, please consult the WSDOT Traffic Volume Map or WSDOT’s Annual Traffic Report.