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Variable speed awareness warning system on the A75 Stranraer to Gretna Green in Scotland

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Strategic importance of the A75
The A75 Stranraer to Gretna Green road is the main route from northern England to the ferry port servicing Northern Ireland. Consequently, traffic tends to come in waves, so it will be heavy when a ferry has just been unloaded.

The A75 forms part of the Euro-route 18, which runs for 1,890 km (1,170 miles) from Craigavon in Northern Ireland, and on through Scotland, England, Norway, Sweden, Finland and Russia. As such, it is an important strategic route for economic connectivity.

Although it is a vital trunk route and has been subject to several upgrades over the years, it remains mostly single-carriageway. There are three sections of dual-carriageway and several long overtaking lanes.
With over 1.5m vehicles using the A75 each year (200,000 plus are Heavy Goods Vehicles) the aim for Transport Scotland is to ensure safe passage – not only for drivers, but also to ensure the safety of pedestrians and cyclists in the towns and villages along its route.

The need for safety improvements
The main objective for Transport Scotland is to address road safety concerns that have been highlighted by various stakeholders. There is a strong interest from local residents and road safety pressure groups, as well as environmental campaigners, the port operators, hauliers, and the local police force.

Stakeholders at the Dumfries and Galloway Transport Summit Report on Ports, Freight and Roads in Sept 2016 raised various concerns in regards to the A75, including the problem of drivers not adhering to the speed limit through the villages of Springholm and Crocketford, whether the speed limit for HGVs should be increased from 40mph to 50mph (in line with the current trial underway on the A9), and demands to make the entire route dual carriageway.

Planning for new road infrastructure can be a lengthy and costly process, so shorter term speed warning solutions are being investigated and implemented, where suitable, to address the immediate safety concerns of regular users of the A75, and residents of villages/towns along the way.

Variable speed limits can cause confusion
In November 2016, Transport Scotland commissioned Clearview Intelligence to conduct speed surveys at twelve locations along the A75 from Stranraer to Gretna Green. The surveys highlighted a recurring problem along the length of the A75 with the 85th percentile speed being unacceptably high, most notably with HGV’s. The 85th percentile speed is a widely-used metric being the speed that 85 percent of vehicles do not exceed in a given location – most drivers behave in a safe and reasonable manner, do not drive at excessive speeds and do not want to get into crashes.

A contributing factor to the issue of speeding, which has been put forward by road safety campaigners, is that varying speed limits can be dangerous on a major trunk road as drivers get confused as to what speed they should be doing on a particular stretch.

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A unique, yet replicable, solution to encourage speed compliance
Scotland TranServ, as the road operator, wished to put in place a solution that would encourage speed compliance, lower the 85th percentile speed overall, and change driver behaviour to improve safety for all road users along the route, especially in built-up areas.

Scotland TranServ engaged Clearview Intelligence to help define an operational solution that could be replicated across the length of the A75 to encourage a reduction in overall traffic speed. Clearview designed a solar powered vehicle count and classify solution – by combining vehicle classification with identification of vehicles travelling above the speed limit, the scheme allows vehicle activated signs to show the appropriate speed limit warning according to vehicle type.

Following extensive site surveys to validate the solution proposed, it was installed in six sites along the A75 and has been designed to improve safety along the route, working effectively within the confines of the existing infrastructure, the variable speed limits, and the differing speed limits for different vehicle types.

As the road has differing speed limits according to vehicle classification, the solution is designed to recognise instances of speeding per vehicle type. This has been achieved through the use of the M680 Traffic Count/Classifier, which classifies vehicles in to nine different vehicle classes whilst measuring the speed of the traffic.

The M680 then communicates that data to Vehicle Activated Signs (VAS) as per the programmed requirements. Each of the six sites consists of two VAS – one sign facing each direction of travel. These signs display ‘SLOW DOWN’ messages and an illuminated speed display, depending on the vehicle class detected and if the speed detected is above the threshold for that vehicle class.

Due to the length and rural location of the A75, the road infrastructure only allows for permanent power supply in places where it passes through towns and villages, or at major intersections. The speeding vehicle problem is not confined to these built-up sections, so the M680s used on the A75 are powered by harvesting solar power from external solar panels, ideal for permanent locations where it is impractical or not possible to tap into mains power.

The benefits of encouraging a positive change in driver behaviour
This type of vehicle activated, dynamic speed warning system serves as a highly visible and immediate reminder to drivers to monitor and manage their speeds appropriate to their vehicle type and is a first of its kind in the UK. It adopts a softer approach to speed compliance than traditional enforcement and penalty measures, but changing driver behaviour for the better is a more effective long-term safety measure.

In summary, the key benefits of this solution are:

  • traffic speed, especially HGV’s, is reduced at key sections of the A75;
  • speed messages are appropriate to the class of vehicle;
  • the system provides a highly visible reminder to drivers to monitor and manage their speeds; and
  • the provision of historic and ongoing records of road usage statistics can inform future decisions on maintenance.
Shona Wooding, Clearview Intelligence

Shona Wooding, Clearview Intelligence