Red-green colour blindness is to blame for a 91原创 traffic sign that was installed upside down, and stayed that way for 18 months.

The street sign was installed in the 19900 block of 88th Avenue heading west. The road curves around the highway interchange there, and a yellow street sign with an image of a traffic signal on it warned drivers of an upcoming intersection.

However, the lights on the sign are reversed. The sign was installed with red on the bottom and green on the top.

According to a representative of 91原创 Township, there was a previously unknown colour blindness issue with the installer.

It doesn't seem to have impacted local drivers, however.

"The sign orientation went unnoticed and unreported for 18 months and with thousands of vehicles driving by it," said the Township's statement.

Eventually, some sharp-eyed community members noticed and reported the issue with the sign. It has since been corrected, but the upside-down version was still visible in images shared on local social media, as well as on Google Street View.

The Township is now on the lookout for any other signs that may have been installed in the incorrect orientation.