Active Traffic and Demand Management (ATDM) is a strategy used by the Washington State Department of Transportation to reduce collisions associated with congestion and blocked lanes. About 25% of traffic congestion is due to events like collisions or disabled vehicles.
How does it work?
The system uses overhead lane signs to provide advance notice of traffic conditions:
- Variable speed limit signs direct drivers to incrementally reduce their speeds.
- Symbols direct drivers to change lanes when a lane is blocked.
- Overhead message signs warn drivers of slowdowns, backups and collisions ahead.
The system decreases last second avoidance maneuvers and panic braking, which are both primary factors that contribute to collisions.
Current Active Traffic and Demand Management Corridors
- Interstate 5
- Interstate 90 Seattle/Bellevue
- Interstate 90 Snoqualmie Pass
- State Route 520 Seattle/Bellevue
WSDOT is one of the first state transportation agencies to use this type of Active Traffic and Demand Management system in the United States. Considered a work in progress, analysis of existing traffic management corridors for safety and congestion relief could lead to additional locations like Joint Base Lewis McChord in Tacoma.
These applications are consistent with WSDOT’s Moving Washington strategy which emphasizes the efficient operation of existing highways by deploying strategies that reduce collisions and maximize traffic flow.
Active Traffic and Demand Management signs allow WSDOT to quickly alert drivers to changing roadway conditions and direct drivers to reduce speeds and move away from a lane closure or get around a collision.
Emergency responders report high compliance with the “lane blocked/lane closed” symbols. These control symbols, including red X’s and yellow merge arrows provide extra time for vehicles to move over and provide a gap between emergency responders and moving traffic. WSP troopers patrolling the corridors report feeling safer when working on the roadway.
The system is helpful in managing weather events as well as managing other emergency situations.
For example, during the week of January 16, 2012, WSDOT traffic engineers used ATDM to give drivers advance warning of the major snow storm forecast for Western Washington. When the snow fell, the signs were used to lower the speed limits for snow conditions. This same approach has been used on Snoqualmie and Stevens passes with success and is now used in the Seattle metropolitan area.
There is not enough data to adequately estimate congestion reduction at this time. Congestion, like collisions, can vary greatly from year to year. Several years of travel time data may be needed before we can measure whether there is a change in congestion levels attributable to ATDM.
The SR 99 Alaskan Way Viaduct Replacement Project paid for the signs on I-5 to help manage increased traffic on the interstate while the new tunnel is built.
The ATDM system was used extensively during a nine day closure of SR 99 in October 2011, when the southern mile of the viaduct was demolished.
During the closure, I-5 experienced a significant increase in traffic. To manage the increased volume, the electronic signs notified drivers of the heavy on-ramp traffic, particularly from Spokane Street. The signs advised drivers to use caution.
Observations showed many drivers moved out of the lane to provide gaps for merging traffic. The improved merge action aided in the overall management of construction traffic congestion. ATDM signs also gave drivers information on alternate routes that could save them time. Management of traffic during the closure was successful with no major collisions.