Smart motorways have been installed across hundreds of miles and feature no hard shoulder and variable speed limits.
Red Xs are also used on overhead gantries to close lanes at certain times.
But continuing to drive in these – even if they’re clear can land you with a £1,000 fine and six penalty points.
On-the-spot £100 penalty charge notices can be handed out but the fee can rise if the case goes to court for more serious offences.
And roads watchdog, Transport Focus, called on better education for drivers to stop them being caught out.
It said Highways England should do more to let drivers know what a red X means and what you should do if you breakdown on a stretch without a hard shoulder.
Emergency Refuge Areas (ERAs) are dotted every 2.5km on smart motorways for cars that become stranded.
Anthony Smith, chief executive of Transport Focus, said: “What’s been missing until now is an understanding of how road users experience smart motorways.
“The message to Highways England is that many millions of drivers successfully use smart motorways, but there is more to do to improve their understanding of how they work and what you should do if you break down.
“Road users tend to trust that ‘the authorities’ would not allow motorways without a hard shoulder if it was unsafe.
“Highways England must remain vigilant that their trust is not misplaced.”
RAC: How to use a smart motorway emergency refuge area
- Emergency refuge areas (ERA) should only be used if it is safe to reach them – they are marked with blue signs featuring the orange SOS telephone symbol.
- If safe to do so, leave your vehicle and contact Highways England via the roadside emergency telephone provided
- After a call has been made, a traffic officer will be sent to assist you, or motorway signs will be set to temporarily close lanes or reduce speed limits.
- If you plan on rejoining the motorway, it is recommended you call Highways England again so that restriction can be put in place to make this as safe as possible.
- If you cannot reach an ERA, attempt to drive your car to the hard shoulder or nearest the boundary as possible.
- Hazard warning lights should be used in all instances.