Watch the secret filming by Gloucestershire Police
Lorry and van drivers have been caught on camera using their mobile phones while behind the wheel on busy motorways.
One driver was even seen reading from a paper map balanced on his steering wheel.
The motorists were caught by Gloucestershire Police officers who were driving inside an unmarked HGV on the M5 as part of Operation Tramline.
Officers also saw people driving without a seatbelt and stopped people carrying insecure loads.
A total of 23 drivers were stopped by police using the unmarked lorry between 12 and 15 July.
The force says nearly half of the drivers were stopped for using their mobile phone, which police say is one of the 'fatal four' causes of road deaths and injury.
Other offences included people driving at an inappropriate speed, people failing to wear seatbelts or people suspected of drink/drug-driving.
PC Neveu from the force's roads policing unit said: "Operation Tramline allowed us to identify and deal with road traffic offences which had a focus on HGV drivers as part of Commercial Vehicle Week.
"It's another string to the bow of the roads policing unit which allows us to use slightly different tactics to make sure that our motorway network is as safe as it can be."