On closer inspection, many of them turned out to be wearing tiny earpieces that connected wirelessly to their smartphones.
What's next? Perhaps throngs of people in thick-framed sunglasses lurching down the streets, cocking and twisting their heads like extras in a zombie movie.
That's because later this year, Google is expected to start selling eyeglasses that will project information, entertainment and, this being a Google product, advertisements onto the lenses. The glasses are not being designed to be worn constantly - although Google engineers expect some users will wear them a lot - but will be more like smartphones, used when needed, with the lenses serving as a kind of see-through computer monitor.
"It will look very strange to onlookers when people are wearing these glasses," said William Brinkman, graduate director of the computer science and software engineering department at Miami University in Oxford, Ohio. "You obviously won't see what they can from the behind the glasses. As a result, you will see bizarre body language as people duck or dodge around virtual things."
Mr. Brinkman, whose work focuses on augmented reality or the projection of a layer of information over physical objects, said his students had experimented on their own with virtual games and obstacle courses. "It looks really weird to outsiders when you watch people navigate these spaces," he said.
They have not seen the Google glasses. Few people have, because they are being built in the Google X offices, a secretive laboratory near Google's main Mountain View, Calif., campus where engineers and scientists are also working on robots and space elevators.
The glasses will use the same Android software that powers Android smartphones and tablets. Like smartphones and tablets, the glasses will be equipped with GPS and motion sensors. They will also contain a camera and audio inputs and outputs.
Several people who have seen the glasses, but who are not allowed to speak publicly about them, said that the location information was a major feature of the glasses. Through the built-in camera on the glasses, Google will be able to stream images to its rack computers and return augmented reality information to the person wearing them. For instance, a person looking at a landmark could see detailed historical information and comments about it left by friends. If facial recognition software becomes accurate enough, the glasses could remind a wearer of when and how he met the vaguely familiar person standing in front of him at a party. They might also be used for virtual reality games that use the real world as the playground.
People flailing their arms in midair as they play those games is a potentially humorous outcome of the virtual reality glasses. In a more serious vein is the almost certain possibility of privacy issues and ubiquitous advertisements. When someone is meeting a person for the first time, for example, Google could hypothetically match the person's face and tell people how many friends they share in common on social networks.
This month, the Electronic Privacy Information Center, a research and advocacy group for Internet privacy, asked the Federal Trade Commission to suspend the use of facial recognition software until the government could come up with adequate safeguards and privacy standards to protect citizens.
Mr. Brinkman said he was very excited by the possibilities of the glasses, but acknowledged that the augmented reality glasses could pose some ethical issues.
"In addition to privacy, it's also going to change real-world advertising, where companies can virtually place ads over other people's ads," he said. "I'm really interested in seeing how the government can successfully regulate augmented reality in this sense. They are not really going to know what people are seeing behind those glasses."