While they might look like the makeshift work of a vigilante residents' group, they have actually been installed by the ACT Government, slated for 11 streets across the capital.

The solar-powered signs are being trialled for 12 months "to promote road safety in residential areas through driver awareness of travelling speeds".

The portable signs are being rotated around the selected streets, set in place for three months at a time.

The green smiley-face flashes if motorists are adhering to the speed limit. If they go over the speed limit - no, the emoji poo does not show up.  But a red "SLOW DOWN" does appear.

As Transport Canberra acknowledges, "they are not enforcement devices, they are a friendly reminder to motorists to watch their speeds, slow down and drive safely".

One of the signs, in Stonehaven Crescent, Deakin, has piqued the interest of residents, with a mixed response

One resident called the sign "quite jolly".

"As a mother of teenage sons, it's nice to come home to a smiling face and it's so unusual for that to happen," she said, with a laugh.

But another resident said the signs were unattractive and actually encouraging people to speed as motorists challenged themselves to try to get the red SLOW DOWN sign to appear.

"It's like having the Eiffel Tower outside your house," the resident said.

"I think it's an eyesore and out of character with the streetscape."

The resident agreed Stonehaven Crescent was "a rat run" for morning commuters and speeding was an issue.

But he did not believe the smiley face was having much of an impact.

"As a resident, I'd like to know, 'How much did they cost and why weren't we consulted?'," he said.

The Canberra Times reported earlier this year the trial was costing $40,000. The trial is due to end in early 2019 and an evaluation of how effective it was due by the end of that year.

Another resident said while speeding was a problem in the street, especially with people heading home in the early evening, he disagreed the signs were making a difference.

"I think they are a complete and utter waste of money," he said.

The signs are being rotated between the following residential streets, chosen for either having a speed problem or lots of residents' complaints about speeding: