Canberra is home to more than a hundred locked metal boxes, mounted on short poles.

Robert Healy of Calwell has been spotting the little green boxes for years.

"Once you notice one, you start to notice them everywhere," he said.

"I've never been able to work out what they're for. There's no sign on them, no writing… they just appear in odd places."

The retired public servant asked Curious Canberra to look into the green and sometimes grey boxes, and his question won our February voting round.

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I'd seen the boxes Robert was asking about, they stand about a metre tall and can be found beside roads and on median strips.

Robert hadn't just been wondering about the boxes - he'd been photographing them too.

He emailed through a few examples, which helped me track down the person with the key to the boxes in question.

One box opened

Questioner Robert Healy peers into one of the city's mysterious roadside boxes. ( ABC News: Sonya Gee )

Robert and I met Pawel Potapowicz from Roads ACT beside a metal box on Pialligo Avenue, near Queanbeyan.

"We had that question asked before," Pawel said.

"[People have asked] is there a radar in there? Is it taking pictures?"

With a key in hand, the member of the traffic management and safety unit crouched down to unlock the rusty box we had come to see.

Robert and I peered inside.

"They're actually empty, there's nothing inside," Pawel said.

A multi-pin connector can be found inside the box. ( ABC News: Sonya Gee )

The box isn't completely empty though and we find a black power cable that runs down the length of the pole and into the ground.

At the other end is a round multi-pin connector. It plugs into a brick-like device that Pawel pulls from a canvas bag.

A device from the '70s

When in use, the metal boxes contained a traffic counting device. ( ABC News: Sonya Gee )

With the device plugged in and neatly stored in the box, the purpose of the unusual structures is almost revealed.

"The main idea is that they're always located on the major arterial road network," Pawel said.

"They would collect information on the vehicles coming into the ACT and leaving the ACT. They would also collect information on vehicles going into and out of town centres."

Pawel believes they routinely puzzle Canberrans because all of their technology remains hidden.

The counter inside the box monitors traffic flow and type by tapping into sensors that are buried in the road.

"Every time a vehicle drives over it, the sensors detect that and measures the speed and volume," Pawel explained.

Pawel Potapowicz from Roads ACT handles one of the traffic logging devices. ( ABC News: Sonya Gee )

The boxes were originally installed by the National Capital Development Commission (NCDC) in the 1970's.

But even in their heyday, many of the metal boxes remained empty.

"We would basically put the device in certain units and then the device would be removed, the data collected and they would be circulated through the other boxes," Pawel said.

Disappearing fixtures

These days traffic monitoring technology is more portable, economical and accurate too.

At the bottom of the metal box we're inspecting we find a rectangular device that's is connected to two black tubes that stretch across the road.

Today Roads ACT uses portable rubber tubes to monitor traffic. ( ABC News: Sonya Gee )

"They're basically rubber loops with air inside and every time a vehicle drives over them the impulse gets sent to the box and gets recorded with a very simple computer," Pawel explained.

The loops work in a similar way to the buried sensors and the data goes on to be interpreted by software.

Robert's question is timely because the boxes are being decommissioned.

"It is gradually happening over time because there's so many of them," Pawel confirms.

"I would say in the next two or three years you probably won't see them anymore."

But eagle-eyed Canberrans may still spot them elsewhere, in New South Wales and Victoria.

The technology is still used in places where traffic is constantly monitored and in busy locations where the installation of temporary traffic monitors is considered too dangerous.