Work to turn the hard shoulder into an extra traffic lane on part of the M4 has been approved despite criticisms from the Commons’ Transport Select Committee.
Similar initiatives are already in place on a number of motorways up and down the country, including sections of the M42, M1, M6 and M5.
They are seen as a way to boost capacity and ease congestion without causing excessive disruption to day-to-day traffic.
Much of the concern centres on the spacing of emergency lay-bys, which are said to be too far apart to allow motorists to stop safely in the event of a breakdown or accident. Find out what to do if you break down on a smart motorway.
The Government said that it will review the distance between the lay-bys to reduce the likelihood of cars stopping in live lanes.
But the Transport Select Committee expressed disappointment that work on the M4 would go ahead before a full safety assessment could be carried out.
Louise Ellman, chair of the cross-party group of MPs, said ministers were “blatantly ignoring” many of the fears surrounding all-lane running.
“We had barely received the response to our report before the Government endorsed an all-lane running scheme on the M4,” she said.
“In the course of our inquiry, there were genuine concerns raised by the emergency services, road workers and recovery operators. The Government cannot ignore them.”
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Emergency lay-bys on smart sections of the M4 will be spaced an average of 1.85km apart.
It is thought this is too far to be visible to drivers who run into difficulty when driving on the motorway and need to pull over.
In research carried out by the RAC, only 28% of the organisation’s members who had broken down on all-lane running motorways said they could see an emergency lay-by.
Additionally, only one in four knew the lay-bys existed, while many respondents said they would feel more at risk if they broke down on a smart motorway compared to dynamic hard shoulder motorways, on which the hard shoulder is used as a running lane at busy times.
RAC chief engineer David Bizley said the “flat refusal” of the Government to listen to genuine voices of concern was “very worrying”
He said: “The Government’s response to the Transport Select Committee says that Highways England will review ERA spacing as part of a broader approach to reduce the frequency and risk associated with live-lane stops.
“We hope this will be applied to the M4 scheme in the interests of safety and cost efficiency as it would of course be far more expensive and more disruptive to have to put in more ERAs after the scheme is built.”