Google has filed a patent for an ad format that would give online browsers free or discounted travel to an advertiser's store.
The ads would involve real-time analytics to compare the cost of different transport options and the potential profit margin for the journey.
The calculation would take into account location, the most likely route, and the user's daily agenda - perhaps informed by Google Now. Google notes that its ad tech would be 'identifying a particular user', which may cause issues around data privacy.
Google will sell the ads in an RTB auction based on competing advertisers' desire to capture the sale.