The speed limit for HGVs has been increased to 50mph on a stretch of the A9 for three years.
The Highland economy has been dealt a blow by average speed cameras on the A9 before the system has even been switched on, campaigners have claimed.
The A9 Average Speed Cameras Are Not the Answer group say increases in journey times between Perth and Inverness are more than 400% higher than predicted by the Scottish Government.
The £2.5million network currently stretches between Perth and Inverness and is due to become live on October 28.
The group believes businesses using the route will suffer and want to see the average speed camera system scrapped.
They say it has anecdotal evidence that the average speed on the A9 is as low as 28mph, and that it is taking an hour longer to navigate the road.
They are calling on the SNP and Transport Scotland to publish an economic impact report into the A9 average speed camera system.
A9 dualling campaigner Murdo Fraser backed calls for the report to be made public.
A Transport Scotland spokeswoman said: “In a recent survey, 78% of drivers who use the A9 road said they believed average speed cameras were ‘effective’ or ‘very effective’ in improving road safety.”
A9 speed cameras: ‘Highland economy damaged – and they’re not even on yet’
