Forthcoming January traffic data will reportedly show that Google Maps has taken the lead from MapQuest as the top mapping site in the US. According to the numbers that I received (I’ve asked comScore to verify), MapQuest’s January monthly unique visitors were 41.5 million and Google’s were 42.2 million.

This is the first time that Google Maps has overtaken MapQuest. However my source said that time on site/engagement, as reported by comScore, is still greater for MapQuest than for Google Maps.

In some respects we have been anticipating this event for a long time, ever since Google removed links to competitors’ mapping sites from its search results in January, 2007.

Some people will argue that this is the inevitable (and potentially “unfair”) result of Google’s ability to refer traffic to its own properties. Indeed, Hitwise has reported multiple times that the bulk of Google Maps’ traffic comes from this sort of self-referring. A year ago in a post on Google’s gains against MapQuest, I quoted Hitwise’s Heather Hopkins who had written at the time:

Google sends more of its own traffic to Google Maps than to Mapquest, a change that occurred last March. This can’t really be attributed to an increase in consumers looking for Google Maps. We can measure this through Internet searches. Searches for “google maps” have increased but the term “mapquest” receives nearly 10x the search volume.

And those Maps referrals apparently increased significantly with the advent of “universal search.” But the fact that Google.com can refer traffic to Google Maps doesn’t fully explain what’s happened. There’s obvious truth in the self-referral scenario, but that would miss other significant factors.

Google has for several years been investing very heavily in Maps (and Earth), as well as distributing Google Maps across the internet via its API. The features and capabilities developed for Google Maps are more extensive and widely known than those of its competitors. And only Microsoft has matched Google’s investment in online mapping; however Redmond has not done as effective a job of promoting Live Search Maps/Virtual Earth’s capabilities to end users. Part of that goes back to Google’s search share dominance. But part of that is Microsoft’s failing.

Among the top mapping providers, Yahoo Maps was actually the early innovator in dynamic mapping more than five years ago with its SmartView offering that allowed users to find and interact with different categories of local businesses on the map. However Yahoo made a decision a few years later to opt out of the “mapping arms race” with Google and Microsoft and not match them feature for feature. As a partial consequence of that decision, use of Yahoo Maps flattened over time. MapQuest, for its part, made a similar decision, instead relying on the strength of its brand and user loyalty as a bulwark against the insurgents.

At one point I heard a former MapQuest executive privately complain that he had to justify everything he did in terms of P&L, while Google could spend freely, in his mind, and “lose money on its Maps.” That may or may not be true.

Regardless, Google steadily built out a large number of features that consumers and the market have found compelling (although not always). More importantly, Google has regarded Maps as a strategic product, arguably its most successful after core search.

Part of the continuing emphasis on Maps is undoubtedly due to its acquisition of Keyhole (in 2004), which became Google Earth. Keyhole CEO John Hanke became head of Maps & Local at Google. Hanke’s thoughtful and mature presence at Google has kept the product evolving and growing. And Maps/Earth are a PR machine for Google — witness the recent launch of Ocean and related press exposure.

Meanwhile, after a long period of near complacency at AOL/MapQuest, new MapQuest VP of Product Mark Law has brought his company’s product “out of hibernation” with a range of interesting new features (i.e., MapQuest Local) and a bigger push into mobile.

MapQuest retains enormous brand strength and may regain the lead. But the momentum is now in Google’s favor.