Over all, participants in the program get an average of 10 to 15 percent off their premium, and everyone gets at least something for playing along. At Progressive’s Snapshot program, people who do qualify for the discount tend to save an average of 10 to 15 percent. Some other companies require you to keep the in-vehicle device for continuous monitoring (and possible price adjustments later) and may make you pay a fee for it plus some ancillary services; others ask you to send it back after six months but allow for a retest later upon request.

At the moment, State Farm and Progressive are not raising rates on people who sign up for monitoring and prove to be terrible drivers. Participation is voluntary, and Progressive, the early adopter in usage-based insurance, says that close to 15 percent of its customers are already enrolled.

Still, as more people sign up, the standard rate will start to feel like a penalty for those who decline to participate. And if all of the good drivers pile into the programs and qualify for lower prices, the companies may eventually raise rates on the holdouts. One bonus for parents who are on the fence: You can often use an insurer’s usage-based insurance or related tracking programs to monitor your teenager’s driving. And Safeco Insurance has an interesting twist, in which people who have gotten speeding tickets or been in accidents can use a monitoring device to requalify for the rate they had before the black mark went on their records.

Given all these incentives, why might someone hold out?

First, not every company offers a usage-based insurance policy. Our insurance company, USAA, does not, although it has started a pilot program to gather data. Geico has no program either, and a spokeswoman declined to comment on the reason. Progressive, State Farm, Allstate and others have policies or pilot programs, but people in certain states or who drive vehicles that are incompatible with the companies’ hardware may not be able to sign up or may not be able to share as much driving data.

But privacy is the biggest concern. While the major players are not yet tracking exact latitude or longitude, they would like to, and Progressive is testing it. “A mile driven on a highway is safer than a mile on a city street with lots of intersections, but in today’s world we don’t know which road you’re on,” said Dave Pratt, general manager of usage-based insurance at Progressive. Some customers are already asking for the company to track them in this way.

The companies probably have little interest in who you’re visiting or patronizing on any given day, but divorce lawyers and others may one day subpoena the information if insurance companies store it. The data can work in drivers’ favor too, though; one Progressive customer used his driving data to prove that he did not kill his infant daughter, who died of asphyxiation at her home. The company can also pull the data if you think it might help you avoid fault when you’re making a claim; so far, it is not doing this unless customers ask it to, though it’s hard to imagine that insurance companies won’t eventually be grabbing for the information while examining future claims.