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Last edited 04 Jan 2022
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 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 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.
 How does a smart motorway work?
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 – queueing 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.
 Hard Shoulder Running (HSR)
HSR uses the same systems, algorithms and mandatory signal settings as controlled motorways in addition to actively managing the hard shoulder. HSR systems dynamically open and close the hard shoulder at peak periods to reduce congestion and increase capacity. HSR allows differing levels of automatic and manual intervention dependent on the traffic conditions.
 What types of technology are used, and what do they do?
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 HSR schemes).
- Pan, tilt zoom (PTZ) CCTV cameras – provide comprehensive coverage of the scheme to allow operators within the control centre to monitor the extent of a scheme.
- Hard shoulder monitoring (HSM) cameras – HSR 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.
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.
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
To ensure motorist compliance and to realise scheme operational benefits, Highways Agency Digital Enforcement Camera System (HADECS) equipment is installed within smart motorways. These 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 was also 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.
This article originally appeared on The Civil Engineer portion of the ICE website. It was written by Stuart Wilson, technical director, Intelligent Transport Systems, Sweco, and published on 4 January 2022.
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