The company, which has already analyzed more than 5 billion driven miles, says its driving-behavior data is twice as good as any other factor in predicting risk, and that loss costs for bad drivers turn out to be roughly 2.5 times higher than they are for good drivers.

Yet even for the highly competitive auto insurance market, Progressive's move is aggressive. The company claims it is the first time drivers can objectively compare the rate they would be charged with the rate they currently pay their carrier.

“With the test drive of Snapshot, you do the 30 days, you're not a customer, you can just see what your discount is,” said Richard Hutchinson, general manager of usage-based insurance for Progressive.

Already a heavy advertiser via the persona of the perky sales clerk “Flo,” Progressive is planning what Hutchinson called “a big, big campaign in all dimensions” to promote the expansion of the program.

“Flo will be everywhere,” Hutchinson said. SNL Financial recently ranked Progressive as the fourth-largest advertiser in the property insurance industry, spending more than $500 million on media in 2011.

One of the most obvious fears for consumer advocates is the possibility that insurers would use the data they gather from technology like “Snapshot” to raise rates on customers, especially as Progressive's peers move to catch up with the insurer and launch their own usage-based programs.

But one industry consultant recently said it will likely end up being another way around – customers will have to pay extra to not be tracked by their insurer, given how aggressively the industry is expanding in that direction.

“Whatever the math will say the rates will have to start going up on the rest of it,” said Robin Harbage, global marketing and sales leader for consultancy Tower Watson's auto insurance practice, in an interview that pre-dated the Progressive announcement.

“It's almost inevitable that the people who aren't using will have to pay more because the people who are the best drivers are being self-selected into these programs.”

The technology also has the strong backing of environmentalists. A 2010 MIT study bankrolled by the Conservation Law Foundation found converting all Massachusetts drivers to a mileage-based insurance platform would reduce accident costs and fuel consumption by 9.5 percent.