A flagship £360 million scheme to ease congestion on motorways in the West Midlands has been branded a safety risk by Labour's Shadow Transport Secretary.
Mary Creagh said she was concerned at the decision to use the hard shoulder during busy periods, saying it increased the risk of accidents.
The shadow minister, who could be in charge of the nation's roads if won next year's General Election, called into question the rationale behind the high-tech active traffic management scheme.
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She was speaking to the Post six months after the latest stretch of the M6 was converted to allow drivers to use the hard shoulder in busy periods - a project
Her comments were condemned by Conservatives who said so-called smart motorways meant safer and faster travel for road users.
More than £360 million has been spent on schemes involving hard shoulder running in the region since the scheme was first piloted in the West Midlands.
Allowing motorists to use the hard shoulder is a common feature of smart motorway schemes, also sometimes called managed motorway or active traffic management.
Electronic signs are used to change speed limits, slowing down or speeding up the flow of traffic to reduce congestion. A red "X" in signs above the hard shoulder show it is closed to traffic except in an emergency.
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In the most recent project, the hard shoulder was opened to traffic during the busiest times between junctions five and eight on the M6 this April, in a £115 million scheme to improve journey times around Birmingham.
Mrs Creagh, who was born and brought up in Coventry, said: "I have a bit of concern about the safety elements of that but that's a cheaper way than building motorways, just hard shoulder running.
"You basically take down the speed, widen it and that's how you do it."
She said: "Nothing's a zero-sum game in transport. You can end up with air pollution and potentially more motorway accidents and you've got no refuge.
"Everything has a potential benefit and a potential cost and what we're not very good at is working out the different costs and the effectiveness of different modes."
The Highways Agency insists smart motorways are actually safer because there is less congestion.
And Conservative Transport Minister John Hayes said: "Smart motorways are central to our plans to triple investment in major roads, reduce congestion and remove bottlenecks.
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"Labour waged war against motorists during their 13 years in government. Now Ed Miliband's transport spokesperson shows they would do it all again with their anti-motorist gimmicks."
Before the latest M6 scheme, the hard shoulder was made available for drivers between junction eight of the M6, near Great Barr, and junction 10, near Wolverhampton, in a project costing £150 million. It was first introduced on the M42 east of Birmingham at a cost of £100 million in 2006.
And it is also to be introduced next year on junctions 11a to 13 of the M6, between Wolverhampton and Stafford, as part of an £87.5 million scheme.
Earlier this year the Highways Agency held a consultation on introducing hard shoulder running on the M5 between junctions 4a, near Bromsgrove, and six, near Worcester. No decision has been made.
It says it is reviewing the possibility of introducing a smart motorway scheme between junctions 13 and 19 of the M6, from South Staffordshire to Manchester.
A Highways Agency spokesperson said: "We know that smart motorways work. They reduce congestion, they improve journey time reliability by smoothing traffic flow - all achieved through using variable speed limits and giving more road space to road users by making the hard shoulder available as a traffic lane.
"We also know that using the hard shoulder as a traffic lane can be done without worsening safety.
"Smart motorways are about supporting the economy by providing much-needed capacity on our busiest motorways, and we are confident that they will tackle congestion on some of the busiest sections of motorway, while at the same time maintaining or improving safety."
Speaking to the Post, Mrs Creagh also stressed Labour's continued support for the planned high-speed rail line, set to run from London to Birmingham, Leeds and Manchester.
She said: "HS2 is very exciting for Birmingham - 50,000 new jobs. I'm very excited about the high speed rail academy, and Doncaster.
"Birmingham will benefit from the fact that the engineering section of High Speed Two is going to be there. You're going to have a whole influx of rail engineers into the city which is going to be wonderful and really transformative for the West Midlands economy."
She added: "HS2 is key for freeing up the West Coast and East Coast main lines for more commuter services and freeing up capacity for freight on the railways as well."