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Video: Highway sign bounces in severe weather

Staff writers
August 15, 2012 — As a strong thunderstorm roared through the Calgary area, one resident captured dramatic video of a highway sign bouncing on Deerfoot Trail.

Sign 'gallops' on Alberta's Deerfoot Trail
Sign 'gallops' on Alberta's Deerfoot Trail

It's a phenomenon known as "galloping," and it created quite the scene on Alberta's Deerfoot Trail this week. 

A video captured Tuesday shows a highway sign bouncing over the major roadway as cars drive below. Around the 39 second mark of the video, the sign for the 32nd Avenue exit can be seen swinging free of the metal post before dropping to the road. 

Despite numerous drivers traveling on the road at the time, no injuries were reported. The video, however, quickly went viral on social media platforms like Facebook and Twitter. 

"What's happening there is a really rare phenomenon," says Matthew Spratlin, a bridge design engineer with Alberta Transportation. "It's called galloping, and it's basically a commotion induced by the wind hitting the sign panel. And when the wind hits the sign panel, it causes the sign to move up and down." 

Spratlin says the phenomenon is caused by a combination of wind speed and wind strength. Engineers build the signs knowing that strong winds and severe weather are possible in certain locations. 
"For example, a sign structure in Calgary would be designed to withstand the maximum winds that would typically be expected in Calgary," Spratlin explains. 

It's more typical to see things like traffic lights bouncing in a storm, but highway signs can do the same. Spratlin says there's also anecdotal evidence of one sign galloping in the wind, while another only 100 metres away is standing still. 

The storm that brought powerful winds to Calgary on Tuesday also uprooted trees and triggered widespread power outages. Large hail came crashing to the ground in nearby Airdrie, as well.

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