Smart motorways with hard shoulders only used at busy times are "too complicated for people to use" and won't be built in the future, Highway England's boss has said.
Jim O'Sullivan told MPs the government-owned company will not build any more "dynamic" smart motorways, where drivers use hard shoulders at busy times, because too many motorists do not understand them.
Parts of the M1, M4, M5, M6, M42 and M62 have already been converted but it is not clear what the cost has been.
Mr O'Sullivan said drivers are confused about when they can use the hard shoulder and when it is closed to non-emergency traffic.
He told the Commons' transport select committee that some hard shoulders on dynamic smart motorways are only open to traffic during morning and evening peaks, but this catches out some drivers when their routine changes.
"People whose normal daily commute takes place at 8am or 9am, if they've been to the dentist and come out at 11am they drive down the hard shoulder," he said.
"When we close it at other times of the day, people still drive down it.
"We get people who stop there when it's a running lane.
"What we also find is, because people aren't sure if it's a hard shoulder or a running lane, even when it is open, the usage of that running lane is much lower because people aren't sure whether it's a hard shoulder or not.
"I don't think we will be building any more dynamic hard shoulder smart motorways. They're just too complicated for people to use."
Other smart motorways include a type where all lanes, including the hard shoulder, are open apart from when there is an accident.
Controlled motorways have variable speed limits but retain the hard shoulder for emergencies.
Concerns about smart motorways have been raised over the past few years, with a number of drivers being killed after stopping in live running lanes.
In March, Derek Jacobs, 83, was killed after his car was hit when he stopped in the fast lane on a section of the M1 in Derbyshire converted to a smart motorway.
A woman was killed months before on the same stretch of road after leaving a broken down car.
However, Mr O'Sullivan insisted smart motorways are "as safe or safer than conventional motorways".
He added: "I would prefer to break down in the live lane of a managed motorway or a smart motorway than I would prefer to break down on a live lane on a conventional motorway or on a dual carriageway."
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