Active Traffic Management

Smart motorway

Active traffic management (ATM, also known as, ‘smart lanes’ or ‘managed motorway’) is method of increasing peak capacity and smoothing traffic flows on busy major highways. Techniques include variable speed limits, hard-shoulder running (use of the shoulder as a travel lane during congested periods) and ramp-metering (traffic lights at entrance ramps regulating the flow of incoming traffic). Drivers are alerted to changing conditions by overhead, electronic signs.

It is currently in operation in Birmingham, England. The scheme had initially been criticized by some due to possible safety and environmental concerns, however a Highways Agency report into the first six months of the scheme showed a reduction in the number of accidents and the system was expanded to other highways in the UK. It is seen as a less expensive alternative to widening a road.

The section of road subject to ATM is monitored by MIDAS sensor loops placed in the road every 100 meters to observe traffic flows. A computerized system monitors the traffic flows and can set the best speed limit for the current flow of traffic and switch on speed limit signs mounted on gantries up to 2km before an incident. Operators also monitor 150 CCTV cameras along the route and can control both the speed limits and information signs. Overhead variable message signs can direct drivers to use the hard shoulder during busy periods.

When the speed limit has been lowered to 50 miles per hour or below the hard shoulder can be opened as an additional lane. To facilitate this and still maintain safety a series of refuge areas have been created around every 500 meters (1,640 ft) along that stretch of the road. These take the form of lay bys to the side of the hard shoulder and contain the SOS phones within them. In the event of a vehicle breaking down on the hard shoulder, operators can close it or they can close a lane to allow emergency services access to an accident. The hard shoulder is never opened on the sections under a junction between the off and on slip roads. Close to junctions use of the hard shoulder as a lane is restricted to traffic exiting or entering at that junction.

A number of highways in the United States have variable message signs and variable speed limits. 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. A modern implementation of active traffic management was activated in 2010 in Minneapolis and its southern suburbs as part of the Urban Partnership Agreement. The Washington State Department of Transportation (WSDOT) has completed Active Traffic Management schemes on several motorways in the Seattle metropolitan area. It is the first legally enforceable system in the United States. Failure to comply with speed limits and overhead instructions are citable offences. The UK system makes use of Automatic Number Plate Recognition cameras which to monitor traffic flows and tailor the system. Digital enforcement cameras are also mounted on the gantries and are operated by the West Midlands Police to enforce the mandatory variable speed limits.

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. Part of the challenge is allowing drivers more time to adapt to the system, while the other part is poor enforcement from the Washington State Patrol and the absence of automated enforcement such as those used in England. Unfortunately, compliance is vital to achieving successful implementation. ATM in Washington State is unlikely to achieve its full potential without improved enforcement.

Unlike other states in the U.S., WSDOT does not have a time-of-day schedule for ramp metering, nor does it have a rigid meter rate as it is considered too inflexible. 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 algorithm, named the ‘Fuzzy Logic Ramp Metering,’ is the successor to the Bottleneck Algorithm.

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