Motorists are reaching speeds of 99mph approaching the toll booths on the Second Severn Crossing, despite the introduction of speed cameras.
Speed restrictions have already been implemented on the bridge
High-speed drivers have been filmed by mobile cameras searching for change and on mobile phones.
The cameras were brought in to get drivers to slow down, but 50 were recorded driving between 75 - 99mph in one two-hour period.
A 50mph bridge limit has been imposed by the Welsh Assembly Government.
It follows 13 collisions resulting in casualties at the toll booths, with top speeds in excess of 120mph on the approach.
It was hoped that speed cameras, fitted inside special vans, would encourage traffic approaching the toll booths on the westbound carriageway to slow after concern was raised by staff working at the tolls.
Health and safety officials had complained about the high numbers of speeding vehicles, and so it was decided that the camera vans would park on a flyover above the approach to the bridge.
John Rowling, manager of the Safety Camera Partnership, which is responsible for static, mobile and red light camera enforcement in the Dyfed-Powys, South Wales and Gwent police force areas, said the speeds were totally unacceptable and putting lives at risk.
"The driving behaviour we've seen during the first week of enforcement is nothing short of appalling," he said.
"Despite plenty of signing to give advance warning of the new speed limit, motorists are still driving at up to double the legal speed limit.
"The approach to the toll booth on the second crossing is particularly dangerous as drivers have to select one of nine lanes while approaching stationery vehicles."
The £300m bridge is more than 5km (3 miles) long
He said the speed limit on the bridge had to be reduced for safety reasons.
"The 50mph limit was introduced because there were unacceptable speeds resulting in accidents.
"Twenty people were injured in this collisions and something had to be done.
"We recorded 50 people approaching the tolls over 75mph, some were taking their seat belts off, looking for change and on their mobile phones. It just isn't acceptable," he said.
Superintendent Nigel Russell, head of roads policing at Gwent Police, added: "During this week's enforcement, mobile camera operators witnessed motorists approaching the toll booth at high speeds while either on the mobile phone or reaching in their pockets for change.
"Coupled with the bad weather conditions, this kind of driving puts innocent road users at risk.
"The majority of accidents on this stretch have occurred because motorists fail to slow down on the approach and then collide with stationery vehicles.
"The 50mph limit has been introduced to reduce these accidents and we are appealing to all motorists to slow down and consider the safety of themselves, other road users and those working at the tolls," he added.
In August, the assembly government introduced a lower speed limit on the bridge, which opened in 1996 to ease the pressure on the original crossing.