(Reuters) – Progressive Corp already has hundreds of thousandsof auto insurance customers who willingly let the company tracktheir driving behavior in exchange for the chance at substantialdiscounts, and on Monday the company opened the door tocompetitors' clients as well.

Progressive, the fourth-largest U.S. auto insurer, said it wouldopen its “Snapshot” usage-basedinsurance program to anyone who wanted to track how well theydrive.

Based on 30 days of driving data, people can earn a prospectivediscount on their insurance premiums, which Progressive would thenapply if the user decides to become a customer. Historically, theinsurer said, roughly 70 percent of people who sign up for theprogram end up earning a discount.

The Snapshot device, which plugs into a port in cars built after1996, tracks behavior like miles driven, braking patterns and evenwhat time of day a car is used. The new trial program will beavailable in 35 of the 42 states where Progressive currently offersthe service to its existing clients.

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

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

“With the test drive of Snapshot, you do the 30 days, you're nota customer, you can just see what your discount is,” said RichardHutchinson, general manager of usage-based insurance forProgressive.

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

“Flo will be everywhere,” Hutchinson said. SNL Financialrecently ranked Progressive as the fourth-largest advertiser in theproperty insurance industry, spending more than $500 million onmedia in 2011.

One of the most obvious fears for consumer advocates is thepossibility that insurers would use the data they gather fromtechnology like “Snapshot” to raise rates on customers, especiallyas Progressive's peers move to catch up with the insurer and launchtheir own usage-based programs.

But one industry consultant recently said it will likely end upbeing another way around – customers will have to pay extra to notbe tracked by their insurer, given how aggressively the industry isexpanding in that direction.

“Whatever the math will say the rates will have to start goingup on the rest of it,” said Robin Harbage, global marketing andsales leader for consultancy Tower Watson's auto insurancepractice, in an interview that pre-dated the Progressiveannouncement.

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

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