The mystery of the box that paralyzed traffic and puzzled police in Saskatoon Tuesday night has been solved — it belongs to the city.

The police bomb squad closed down one of the city's busiest bridges for almost four hours after someone noticed the box attached to a light standard at around 5:30 p.m. CST.

It turns out the box is used by the city to track vehicle speeds.

Police weren't sure who the box belonged to Tuesday night, however. By 9:30 p.m. they had determined it contained "electronic equipment capable of capturing and transmitting traffic and/or pedestrian data to an unknown location," according to a news release.

According to traffic planning engineer Don Cook, the city has used units like this since the 1960s to monitor traffic. He's not sure why people suddenly thought it might be a bomb.

"I'm not sure why that was a surprise to folks," he said.

"There's obviously a heightened sensitivity out there in the public about things and peoples', I guess, general feeling of security."

The city is now looking at clear labeling on the boxes so that people and the police will know they belong to the city, he said.