Active traffic management

[9] When the speed limit has been lowered to 60 miles per hour (97 km/h) or below the hard shoulder can be opened as an additional lane.

[10] To facilitate this and still maintain safety[5] a series of refuge areas have been created around every 500 metres (1,640 ft) along that stretch of the road.

[12] ATM involves converting the hard shoulder into a normal lane during periods of high traffic flow to expand the capacity of the road[13] and may reduce the need to widen motorways.

The New Jersey Turnpike has been using active signage since the 1960s, though systems have evolved over time as they have been deployed in other areas of the country.

[17] The Washington State Department of Transportation (WSDOT) has completed Active Traffic Management schemes on several motorways in the Seattle metropolitan area.

The ATM schemes build upon WSDOT's existing arsenal of Intelligent Transport Systems (ITS) which is supported by traffic sensor loops embedded in the pavement approximately every 800 m (2640 ft) apart.

The primary ATM strategies used by WSDOT is ramp metering, queue protection, hard shoulder running, junction control, and lane-specific signalling.

WSDOT's ramp metering strategy is aimed at reducing the volume of traffic entering the motorway and has been in operation since the early 1980s.

Dedicated operators monitor traffic conditions visually through CCTV and switch the meters on and off manually.

Once turned on, the meter rate is automatically determined and updated every 20 seconds using a local traffic-responsive algorithm based on fuzzy logic.

The length of the queue on the slip road and the mainline occupancy immediately surrounding the slip road are fed as inputs to the algorithm, which determines a meter rate that allows as few vehicles to join the motorway mainline as possible without overflowing queuing vehicles onto nearby arterial streets.

Variable message signs (VMS) accompany the reduced speed limit to warn drivers of slow traffic.

[19] As part of the SR 520 Bridge Replacement and HOV scheme, WSDOT plans to implement junction control through hard shoulder running.

[19] Although WSDOT has not published data on the performance of the ATM implementation, low compliance with variable speed limits and overhead instructions is noticeable among Washington drivers.