The stationary vehicle detection (SVD) technology being rolled out across the smart motorway network in England could fail to warn operators about thousands of stranded cars per year, a report has suggested.

Smart motorways sacrifice the hard shoulder for an extra lane of traffic, with designated stopping areas 1.5 miles apart. Many drivers and industry figures believe smart motorways are inherently unsafe.

A 2015 trial conducted on an eight-mile stretch of the M25 over six days resulted in 192 stationary vehicle ‘events’ being picked up, but CCTV later revealed that there had been 294 – a success rate of just 65%.

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Of the 102 occasions where the system didn’t highlight a problem, 62 are classified as ‘allowable missed detections’, including one incident where a vehicle was stopped for only a handful of seconds and 16 more that were detected by cameras but fell outside of the range of individual radar units, believed to be 250 metres in either direction.

Highways England now admits that the report contains errors, claiming these 17 incidents should be discounted from the 294 outlined initially. It says a further 45 incidents were merely mislabelled as ‘allowable missed detections’ when the SVD system had worked properly only for the system to automatically suppress the alert.

Automatic suppression of warnings is sometimes necessary so that operators aren’t overwhelmed with false alarms.

Highways England now says the SVD system worked 237 times out of 277, a success rate of 85.6% and above the 80% benchmark required to meet the minimum standard, even allowing for a 3.9% margin for error.

It admits that 40 missed detections were genuine, including seven instances attributed to ‘blindspots’ in the radar system.

Analysis conducted by Highways Magazine – which disputes the explanations given by Highways England – indicates that 2400 stationary vehicles could be missed annually on just a 13km (eight-mile) stretch of the smart motorway network.

An estimate from the RAC suggests there will be 788 miles of smart motorways by 2025. 

A second trial of the SVD system was conducted in 2018, although Highways England says it won’t publish the report on the basis that it contains sensitive information belonging to its providers.

Highways England claims this trial can’t be compared with the most recent study. The first trial focused only on ‘low-flow’ traffic when motorways are quietest, which is when SVD is primarily designed to operate.

The 2018 trial studied low, medium and high-flow traffic: Highways England says it achieved a success rate of 89.6% when focusing only on low-flow situations, hitting 82.4% for medium and high flow combined.

“Stopped vehicle detection is part of a wider system on smart motorways which helps to keep motorists safe,” said a Highways England spokesman. “Initial trials in 2015 indicated that more than 85% of incidents were identified surpassing the benchmark set and the technology has since been developed to become even more effective.

“Drivers and passengers can have confidence that stopped vehicle detection plays its part in keeping motorway journeys safe.”

However, the fact that broken-down cars could escape the attention of operators will be a cause for alarm among some groups. Earlier this year, the Transport Committee began an investigation into the safety of smart motorways after a coroner described them as an “ongoing risk of future deaths”.