However, a former head of traffic engineering in the ACT government's Urban Services directorate questioned the road safety rationale for dropping the speed limit from 60km/h to 40km/h on the arterial road.

Graeme Shoobridge said pedestrian traffic signals to cross Northbourne Avenue did an adequate job of protecting people on foot, and there was little reason to drop the speed of traffic on the road to 40km/h.

"If it's road safety they're interested in, the difference between 50, 60 and 40 from a pure transmission on an arterial road point-of-view is not very much," Mr Shoobridge said.

Mr Shoobridge said any safety issues should address whether pedestrians were ignoring the crossing lights, and lowering the speed limit could lead to more walkers attempting to take the traffic on, creating more of a risk.

Transport Minister Chris Steel said more pavement markings and signage would be installed to improve visibility of the limit.

"Lower speed limits reflect the changing use of streets in the city over time, like Northbourne Avenue. The centre of the city is frequented by more public transport users, pedestrians and cyclists as well as motorists. The city has also grown, and there are many more people living in the city centre now together with a mix of recreational and commercial activity," Mr Steel said.

Rising community frustration over the speeding fines, which have raked in more than $5 million in the space of a month, has prompted an online petition, which attracted nearly 200 signatures in less than 24 hours.

Narelle Slarke, who started the Change.org petition, said the ACT government should rescind the fines and dramatically improve signage on Northbourne Avenue. She suggested large highway-style signs telling drivers to reduce speed for the new zone.

"I'm absolutely and utterly livid. I've started the petition because I don't believe I'm an unsafe driver. I was doing 54km/h on my way to work, and I get fined," Ms Slarke said.

Ms Slarke, who lives in Downer, said the ACT government should also write to every household in Canberra to advise them of the change. She said she received a fine on the road less than 6km from her home.

"Every other change [on Northbourne Avenue] was advised to me by mail, apart from this one. So I sailed down Northbourne Avenue, as 20,000 other people did, at 60km/h. They shoved up these signs in really weird places," she said.

Large temporary variable messaging signs were in place for four weeks in March and April, and again in June and July, to inform motorists of the speed limit change.

Updated permanent signage was installed in March, and displayed at each intersection within the 40km/h zone. The ACT government also wrote to 2000 businesses and residences in the Braddon and Civic area in March.

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Mr Steel said reducing the speed limit was part of the ACT government's commitment to improving road safety.

"If you're in the city or a town centre, there is a simple message: slow down to protect other road users, or you may be issued a fine," he said.

Lower speed limits were outlined in a joint city and gateway urban design strategy prepared by the ACT government and the National Capital Authority in 2018.

Better Regulation Minister Tara Cheyne and Chief Minister Andrew Barr were compelled to defend the rollout of the new speed Civic limits in the Legislative Assembly during the week.

Ms Cheyne said there were 17 signs on the road, variable messaging signs were in use and the government had run radio advertisements in peak hour to alert drivers to the change.

"With 20,000 vehicles detected committing an offence, sending a warning letter to every single driver would have been a manual process which would have been a significant diversion of resources," Ms Cheyne said.

The speed limit was changed in March, and cameras caught 20,000 motorists speeding in a two-week grace period from June 21. The cameras began issuing infringements from July 5.

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