What are smart motorways and how do they work?

Smart motorway technology is deployed to actively manage traffic flows and optimise the motorway network. Stuart Wilson, technical director at Sweco, explains the types of smart motorway technology and how they are used to achieve operational outcomes.

Smart motorway technology allows traffic flow to be actively managed, for example by imposing a temporary speed limit. Image credit: Highways England/Flickr account
Smart motorway technology allows traffic flow to be actively managed, for example by imposing a temporary speed limit. Image credit: Highways England/Flickr account
  • Updated: 04 January, 2022
  • Author: Stuart Wilson, technical director, Intelligent Transport Systems, Sweco

Smart motorways - a quick overview

A smart motorway is defined as a concept that uses technology and procedures to monitor and respond to fluctuating traffic conditions on our motorways. Smart motorways which are being currently designed and installed have evolved from several years of feedback, lessons learnt and improvements since their first deployment on the M42 motorway in 2006.

Why do we need smart motorways?

According to several studies, the financial impacts of congestion on the strategic road network is estimated to cost £2 billion per year, an amount likely to increase further due to the predicted traffic growth up to 2035.

The key aims of smart motorways is to reduce congestion and improve journey times by better managing the traffic using roadside technology infrastructure, associated control centres, systems and operational regimes.

Smart motorways support the economy by providing much needed capacity on the busiest motorways, while maintaining safety for motorists and those who work on the roads.

Other benefits of differing types of smart motorway have included reductions in accidents and reduced impacts on the environment associated with emissions from stationary or slow-moving vehicles.

How does a smart motorway work?

Smart motorways function by adopting various operating regimes to meet scheme operational requirements including:

Controlled motorway

Variable Mandatory Speed Limits (courtesy of Highways England Smart Motorways material)
Image credit: Highways England/Flickr

Variable Mandatory Speed Limits are used on all types of smart motorways. A Mandatory Speed Restriction refers to the use of a red ring speed restriction which is legally enforceable.

Traffic conditions are monitored using vehicle detection equipment installed in or adjacent to the motorway at strategic locations. The vehicle detection equipment is linked to a Motorway Incident Detection and Automatic Signalling (MIDAS) system which analyses the data and recognises two differing traffic conditions – Queuing Traffic or Congestion.

Queue Protection and Congestion Monitoring algorithms within the MIDAS system recognise differing traffic conditions and automatically set appropriate signs and mandatory speed restrictions on signals to actively manage traffic conditions. Safety specific intervention is provided by the Queue Protection algorithm which protects the back of queueing traffic and Congestion is managed by introducing reduced speed limits to increase traffic throughput.

In addition to the MIDAS system, additional technology is being added to smart motorways schemes for the purpose of detecting stationary vehicles.

Hard Shoulder Running (HSR)

Hard Shoulder Running (courtesy of Highways England Smart Motorways material)
Image credit: Highways England/Flickr

Hard Shoulder Running uses the same systems, algorithms and mandatory signal settings as Controlled Motorways in addition to actively managing the hard shoulder. The Hard Shoulder Running systems dynamically open and close the hard shoulder at peak periods to reduce congestion and increase capacity. Hard Shoulder Running allows differing levels of automatic and manual intervention dependent on the traffic conditions.

All Lane Running (ALR)

All Lane Running (ALR) (courtesy of Highways England Smart Motorways material)
Image credit: Highways England/Flickr

Following lessons learnt, feedback, driver surveys and simulations, the Smart Motorways All Lane Running concept was introduced.

All Lane Running schemes permanently convert the hard shoulder into an additional running lane, while operating with all of the features, systems and signalling associated with Controlled Motorway schemes.

What types of technology are used, and what do they do?

Message signs and signals

Message signs are used to provide information to motorists in relation to the road conditions ahead.

Where they are not co-located with individual signals, some types of message signs can also display Variable Mandatory Speed Limits.

Signals display mandatory variable speed limits above carriageway lanes and can also be used to open or close any of the lanes, including the hard shoulder (for Hard Shoulder Running schemes).

Closed Circuit Television (CCTV) Cameras

Comprehensive CCTV coverage is provided within smart motorways schemes to supports operation of the scheme. Different types of cameras include:

  • Pan, Tilt Zoom (PTZ) CCTV cameras – provide comprehensive coverage of the scheme to allow operators within the control centre to monitor the extents of a scheme.
  • Hard Shoulder Monitoring (HSM) Cameras – Hard Shoulder Running schemes require 100% coverage of the hard shoulder which allows an operator to ensure it is completely clear of breakdowns or debris prior to dynamically opening the hard shoulder.

MIDAS vehicle detection

In order to understand traffic conditions (such as speed and flow) the MIDAS system gathers traffic data from vehicle detector sites which are positioned at strategic points along the motorway network.

There are two main types of detectors deployed within smart motorway schemes which support the MIDAS system as follows:

  • Inductive Loops – the traditional form of vehicle detection on motorways. They are cut into the road surface and measure the change in inductance as a vehicle travels across them to determine vehicle speed and traffic flow.
  • Radar Vehicle Detection – used increasingly to gather traffic parameters. They are positioned on poles adjacent to the carriageway, reducing construction requirements, removing the requirement for complex traffic management during loop cutting and also reducing ongoing maintenance requirements.

Stopped vehicle detection

In addition to the above MIDAS detectors, Stopped Vehicle Detector (SVD) radar devices are also being installed on all smart motorways, including retrofitting to previously completed schemes.

This radar operates by monitoring the entire carriageway and identifying stopped vehicles to the control centre and alerting the operators.

This allows for quicker identification of stopped vehicle incidents and supports the process of implementing associated interventions, such as lane closures and speed restrictions.

Variable mandatory speed limit / Red X enforcement

Variable Mandatory Speed Limit Enforcement (courtesy of Highways England Smart Motorways material)

To ensure motorist compliance and to realise scheme operational benefits, Highways Agency Digital Enforcement Camera System (HADECS) equipment is installed within smart motorways.

The latest cameras can monitor all lanes of traffic simultaneously and adjust the speed thresholds at which they capture offences to align with the varying speed restrictions.

The system has also recently been upgraded to provide enforcement of red ‘x’s by identifying and recording vehicles that illegally pass under a red x displayed on the associated gantry.

About the author

Stuart Wilson ([email protected]) is a technical director within the Sweco Intelligent Transport Systems team specialising in transport technology and smart motorways. He is based in Teesside office.

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