The family of a teacher whose body was found in the River Severn are calling for more CCTV cameras to monitor the crossings to and from south east Wales.
Gemma Garn's car was parked on the second Severn crossing for five hours before it was spotted.
They body of the 26-year-old from Cwmbran, Torfaen, was not found for another six weeks.
One assembly member said people would be "staggered" to learn there is no full camera coverage on the bridges.
An inquest on Thursday recorded an open verdict on the death of Ms Garn, who went missing after a night out with friends in December last year.
The coroner, David Bowen, said the reason for her death will "remain a mystery", after hearing how she had been celebrating a promotion at Croesyceiliog Comprehensive School in Cwmbran where she taught.
She was last seen driving a hire car toward the second Severn crossing.
"There is insufficient evidence to show Gemma took her own life. She left no note and had not talked about suicide," stated the coroner at the inquest.
"There may be a simple explanation - she could have become ill and stopped to get air or be sick and then fallen from the bridge."
But speaking after the hearing, Ms Garn's sister, Theresa, said the lack of CCTV cameras on the bridge had left them unable to uncover why she died, despite an intensive investigation by police.
"We don't know what happened. There is no evidence. The police have done their damnedest - but there are no cameras on that bridge," she said.
"Their efforts have been restricted by the lack of CCTV cameras on the bridge, and this must be addressed so that no other family goes through this heartache."
The South Wales East assembly member, Mike German, has echoed the concerns of the family.
"People must be staggered to find that you can have a vehicle parked on one of the Severn Bridges for five hours without being spotted by the police or by CCTV," said Mr German, speaking on BBC Radio Wales' Good Morning Wales programme.
"Remember this is our important artery into Wales, and if there is a security risk, a safety problem you would expect the services to know, wouldn't you?
"If she had been seen - and there are a couple of eyewitness reports that she was seen walking, climbing through the barriers - then it might have been possible for someone to reach her in time."
Mr German said he felt that part of the issue was the number of agencies responsible for traffic on the crossings, including two police forces and government bodies in Wales and England.
He said there should be "one single point of contact" for ensuring security on the bridges, suggesting that role should fall to the UK Department of Transport.
"We do need to make sure that these bridges are secure," he added.
"They are our arteries into Wales and you would have expected that there would have been CCTV across the whole of both of the bridges, so you can see what is going on."
Traffic Wales, the assembly government's traffic management service, said it monitored seven CCTV cameras on the second Severn crossing.
The spokesman said in addition to CCTV cameras, Highways Agency-funded traffic officers and Gwent Police also carried out vehicle patrols across the bridge.
The Highways Agency said it was jointly responsible for routine patrols with the police.