Highways Agency - Safe roads, Reliable journeys, Informed travellers

Introduction

The Network Active Traffic Management Supervisory Sub-System is a tool for the tactical control of motorway networks.

The primary objective of this subsystem is to reduce motorway congestion and improve safety by means of better operation of tactical control systems through real-time traffic modelling.

The model is capable of predicting the onset of flow breakdown on the motorway for a future period. Then, by means of controlling existing motorway systems, it puts into effect speed limits, ramp metering, hard shoulder running or other tactical control systems to help prevent its occurrence.

What are the basic principles?

The system will use a traffic model to predict the build up of congestion and then run a number of possible corrective traffic management plans. The resultant output from the model will enable the system to ascertain which plan or combination of plans will be the most beneficial to ease traffic congestion on the network.

Having selected the most beneficial plan(s), the system is capable of automatically requesting it's implementation to new and existing traffic control systems.

How will it be implemented?

• There are four types of users that may access the system. Workstations are installed in the control room and engineers room.
• The system provides information via the Traffic Information Highway and to the Urban Traffic Management Control.
• The system connects to the West Midlands control office via the Control Office-Based System LAN.
• The system implements Traffic Control Plans in the non-Active Traffic Management Area via the Control Office-Based System.
• The sub-system is connected to inductive loops on the network.
• The system is informed of the devices status (i.e. if a sign or signal is set).
• The system sends out alerts when fog is detected.
• The system receives traffic flow data from the Motorway Incident and Automatic Signalling
• The system connects to the 'Active Traffic Management' LAN to receive traffic flow information and state of the network (which signs and signals are set).
• When traffic control plans are to be deployed, the system will implement them via the Active Traffic Management control system.
• A printer is available to all users.
• The engineer can access the system remotely via a laptop.

How will it operate? A possible scenario

During the early hours of the morning traffic flows freely, and NASS simply monitors the network.

At 06:00am the system predicts that the slight increase in the number of vehicles using the network will result in a queue forming at 06:30 on one of the links.

Vehicles continue to join the network and demand increases to a level where the system now detects that the queue predicted to start at 06:30 will be significant.

The system interrogates its databases for any predetermined plans designed to relieve congestion on this particular link. For each plan, the system models the effect of implementing them, looking for the optimal solution.

The winning plan in this instance is one that implements ramp metering at two consecutive junctions upstream from the affected location. The system calculates that the winning plan is best to be implemented 06:25 to smooth the traffic flow, and therefore reduce the probability of flow breakdown at 06:30.

The system continues to monitor traffic flows and occasionally re-tests the winning plan. At this time of the day, no other queuing is detected in any other part of the network.

At 06:23 am, the operator is informed of the queues developing, and that the system has determined a plan to implement ramp metering to relieve the expected traffic congestion. No operator intervenes to reject the plan and at 06:25, the plan is set.

The system continues to monitor and includes the effect of ramp metering. The system predicts that at 06:40 the plan is no longer needed and that it can be removed at that time.

At 06:40 am, the plan is removed.