The new cameras, which are now in place on the approach to both bridges, are there to enforce the temporary speed limits on the narrow lanes which will be in place until work is finished next year.
Now anyone who does not follow the rules of the new lanes and exceeds the new 50mph limit could be slapped with a £100 fine and three points on their driving licence.
There are currently three free-flowing lanes in the centre of the crossing are open to motorists to drive through.
But with the introduction of the new lanes, a new speed limit came into force. Motorists now face a speed restriction of 50mph to ensure the safety of drivers and road workers.
Highways England have said the lanes are likely to be in place for several months, with further work being carried out in 2019 to return both routes to a three-lane motorway with the usual 70mph speed limit.
Hannah Milliner, general manager Severn Bridges at Highways England, said: “This is a significant piece of work which is far more complex than simply removing toll booths.
“We want to make sure that journeys can be completed safely and free of charge by the end of the year and will then focus on the next stage of work.”
But Gloucestershire Live reported that eagle-eyed motorists spotted speed cameras being installed before work to remove the tolls started.
Cameras were also spotted on the A48 bridge, the first Severn Crossing at Chepstow, which was closed on Sunday for the toll booths to be removed.