After years of being prevalent in places like Japan and South Korea, QR codes are finally showing up all over the place in the United States. In magazine ads, on public signs and even on vehicles, these two-dimensional barcodes are popping up more and more. But how effective are they?

About 72% of smart phone users say they would be likely to recall an advertisement that contained a QR code, according to a recent study by Baltimore advertising agency MGH. Of course, that’s just people who own smart phones, which is only a fraction of the overall population (about 27% according to Comscore).

Recalling an ad with a QR code in it is one thing. Actually using the barcode is quite another. According to the survey, only 32% of smart phone users say they’ve used a QR code. Of those that have, they tend to use them to enter contests or to access extra information or other content.

It’s worth remembering that QR codes only really start popping up in the United States within the last year or two. Perhaps the biggest obstacle to their widespread adoption is simply the fact that most people don’t own smartphones. Of those that do, they may or may not know what a QR code is or how to use it.

For iPhone users, for example, activating them requires one to seek out and download an application for that explicit purpose (or use Google Goggles if they have the Google iPhone app).

Until QR code readers come built-in natively on a majority of smart phones and those devices are being carried around by a majority of consumers, the technology probably won’t have an enormous impact. In the meantime, they appear to be headed for ubiquity. It’s just a matter of time.