A law could be changed to punish people who throw litter out of car windows, putting highway workers' lives at risk.
Welsh councils spend millions of pounds picking litter off the side of roads.
But few people are caught for the crime which local authorities say is impossible to police and dangerous to clean up.
The Welsh Government plans to change the law to fine vehicle owners to make it easier to catch offenders.
The owner would be punished, regardless of whether they threw the litter, or were even in the car at the time.
The BBC has heard stories of cyclists waiting in traffic throwing rubbish back though people's windows to shame them into stopping littering.
Councils are responsible for cleaning roads, with the Welsh Government responsible for major routes including the M4 and a short section of A55.
In Merthyr Tydfil alone the council spent over £2.6m cleaning up litter from roadsides between March 2015 and March 2019, according to figures obtained by BBC Wales.
This includes the costs of closing stretches of road, overtime, equipment to protect workers and lighting.
Despite this, not a single person in the county borough was fined for throwing litter out of a vehicle.
Why are people not being caught?
It is a criminal offence to throw litter out of a vehicle and you could ultimately be prosecuted and fined up to £2,500 if caught.
Most councils issue fixed penalty notices if they believe someone has littered, asking the DVLA for motorists' details.
But if the owner does not pay up or tell the authorities who threw the litter from the vehicle, problems begin.
Under current legislation the council would have to have seen the littering take place, and then identify and prove which person in the vehicle had thrown it in court.
Unlike police, council workers have no powers to follow or stop vehicles and some rural areas are too vast to monitor.
As the registered keeper is not legally required to identify who threw the litter, some councils are not using their powers in the first place.
In response to a BBC Wales freedom of information request, Denbighshire council said it did not fine anyone, as "pursuing the suspected individuals presented insurmountable evidential problems".
What are the changes?
The Welsh Government wants to give councils additional powers so they could fine the owner of the vehicle.
Unlike a fixed penalty - a criminal fine - this would be a civil fine, and the council would not have to prove which person threw the litter.
The registered keeper of the vehicle is legally responsible and they could be fined even if they were not in the vehicle at the time.
A similar system is already in operation in London.
In Cardiff hundreds of bags worth of litter are picked up by council workers every year from the side of slip roads and motorways.
Close to Cardiff City stadium workers picked up piles of empty cans, plastic bottles and coffee cups, in a 20-minute clean-up costing about £4,000.
Matt Wakelam, assistant director of street scene at Cardiff Council, said the cost was so high as they had strict safety procedures and had to hire special buffer vehicles to stop traffic hitting the workers.
Highways staff said they knew of workers in parts of the UK who had been hit by cars - with some killed.
The council has issued 531 fines since March 2014 and pursued 10 prosecutions for the offence.
But Mr Wakelam said these were very small numbers, and people were sending dashcam footage in to help them catch offenders.
"No local authority wants to issue fines; they are a last resort. But we need to take action, because it's so costly to pick up litter," he said.
"We hope that with new legislation and residents providing information we can increase prosecutions, and promote an environment where people love where they live."
'When it's picked up I feel really proud'
Morgan Evans, is a member of the Gurnos Men's Project, in Merthyr Tydfil.
The group, alongside members of the youth group, carry out regular litter picks.
"It makes me so angry when I see it - when it's picked up I feel really proud," he says.
Cyclist John Deeley, a member of Merthyr Cycling Club, said he was "ashamed to be human" after seeing the amount of rubbish on the side of the roads.
Polly Emmott, known as Litter Pickle, is a litter picking tourist, and she has cleared up coffee cups, condoms and water bottles full of urine from the side of Welsh roads.
After spotting some rubbish on a jog two years ago, the 30-year-old started carrying gloves and a bag to collect litter wherever she went.
"You're thinking 'how dare they drop the litter - I cleaned that last month'," she said.
Nia Lloyd, from Keep Wales Tidy, said some people needed to be shamed to stop.
She has spoken to cyclists and pedestrians who had picked up litter and thrown it back in people's cars in traffic jams.
"I wouldn't advise doing that but, unless you shame that person into realising they are doing something wrong, they might not realise it's an issue," she said.
The Welsh Government said enforcement was only part of the solution and behaviour needed to change.
"We are developing a new Wales litter prevention plan and continue to support local authorities and the third sector in tackling this problem," a spokesperson said.