Nearly 1,400 Low Emissions Zone (LEZ) breaches escaped fines because Glasgow City Council took too long to issue them.
Newly released figures show that 1,388 LEZ penalty charge notices (PCN) were not served after officials broke the 28-day limit for sending fines out.
The issue in sending out fines happened after the council switched to recorded delivery instead of normal post.
The switch comes after a tribunal cancelled fines for dozens of drivers because they were not sent using recorded or registered post.
“During the switch to the new delivery process, a number of penalties were not issued due to a time bar,” a spokesperson for the council said.
With initial PCNs costing £60, that could represent £86,000 worth of uncollected fines, although fines can be halved if paid within 14 days.
But the number also doubles each time a driver breaks the rules up to £460 for cars and £960 for larger vehicles.
Data from the council has also shown a declining trend in the total of LEZ fines being issued – even when the 1,388 figure is taken into account.
Some 2,236 fines were issued in total for the month of October while 3,439 were handed out in November.
That means the two previous months were the lowest on record since the system started in June – which had 2,897 fines – where vehicles received a maximum of one PCN.
It is much lower than the statistics for September (5,341), August (5,757) and July (6,139).
The council made nearly £500,000 out of fines in its first four months.
Glasgow City Council said all the money earned by LEZ would only be spent to reduce air pollution within the city or meet the council’s climate change targets.
A spokesperson said: “Poor air quality presents a serious risk to public health and so Glasgow’s LEZ is a crucial and proportionate measure to tackle the harmful pollution that has blighted the city centre for decades.
“We know that the vast majority of vehicles driving into the zone meet the emission standards, however the LEZ addresses the most polluting vehicles which disproportionately create the harmful concentrations of air pollution in the city centre.
“Scottish LEZs operate by way of a penalty system, set in legislation to discourage non-compliant vehicle entry and to maximise the air quality benefits that can be delivered.”
The zone covers the city centre bounded by the M8 to the north and west, the River Clyde to the south and Saltmarket/High Street to the east.
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