Maps.google.com/bayarea

While Google built search dominance on the purported merit of its product — almost entirely without advertising — the company does spend ad dollars to promote Google Maps, a product which will be crucial to its position in the local and mobile markets, reports Advertising Age.

Google Maps ads now appear on buses and trains across the San Francisco Bay Area and Chicago.

The service still falls behind AOL's MapQuest in terms of total use, but its userbase increased dramatically in the past two years. From April 2006, Google Maps' share of US traffic went up 307 percent; MapQuest's decreased five percent.

The increase likely had little to do with Google Maps' outdoor advertising efforts, which began six months ago. But as advertiser money funnels toward local and mobile, the ad push betray's Google's active interest in keeping the product salient.

As a bonus to brick-and-mortar retailers, advertisers can promote local wares across Google Maps. The ads are fewer and less intrusive than those on Microsoft Live or MapQuest, Advertising Age writes.