Getting a single traffic ticket on your license could raise your auto insurance premiums, in some cases substantially, according to a study released last week by InsuranceQuotes.

This is especially true in California, where your driving-safety record by law must be the most important factor in determining your rate.

Even though Proposition 103 allows drivers to keep a 20 percent good-driver discount if they are guilty of one routine moving violation in three years, insurance companies can and often do impose surcharges for a single ticket.

A typical moving violation – such as failing to stop or improper passing – would raise a hypothetical driver's premium by 32 percent on average in California, according to a study released last week by InsuranceQuotes.com. Nationally, her rate would go up by 19 percent, the study shows.

Going 1 to 15 miles over the speed limit would increase her rate by 37 percent in California versus 21 percent nationally.

Major violations – such as driving under the influence – would raise her premium by roughly 183 percent in California, almost double the national average of 93 percent.

Behavior predictors

Insurance companies charge more for some violations than others because "different actions are different predictors of future behavior. Reckless driving suggests you are more likely to get into wreck. Not wearing a seat belt is not as big a predictor," says Laura Adams, a senior insurance analyst for InsuranceQuotes.com.

The study was based on a single, 45-year-old female driver with a previously clean driving record. Because premiums vary dramatically based on a multitude of factors, an individual's experience could be different.

Some consumer advocates said those increases appear high for going from no violations to a single violation within a three-year period. "They look unrealistic. It's not what we would expect based on the rules in place," says Douglas Heller, an independent consultant to consumer groups.

Harvey Rosenfield, founder of Consumer Watchdog, agreed. "I don't know how they can impose a massively larger surcharge," he said.

Most insurance companies I contacted declined to comment on the study and referred me to their trade associations. Tully Lehman, a spokesman for the Insurance Information Network of California, said via e-mail: "While I can neither confirm nor support the data as collected by InsuranceQuotes.com, an important message or take away about the study is the more serious the infraction, the more serious the financial consequences."

Under Prop. 103, approved by California voters in 1988, auto insurers must use three factors in setting rates – driving record, number of miles driven annually and years of driving experience. Of these, driving record must have the biggest influence.

They may use up to 16 optional factors approved by the Department of Insurance, including severity and frequency of accidents near the customer's home, how long a customer has been with the same company, type of vehicle, gender and marital status.

Criteria for discount

After calculating your premium based on its approved rating factors, the insurance company must give you a 20 percent "good driver" discount if you meet three criteria:

— Have been licensed for the previous three years.

— Not have been principally (at least 51 percent) at fault in an accident that caused bodily injury, death or property damage over $1,000 in the past three years. You will not lose the good-driver discount if the accident caused less than $1,000 in property damage or if you were not at fault.

— Not have more than one point for moving violations in the past three years. Most violations are one point. Major violations – such as reckless driving, hit-and-run and driving under the influence – are two points.

For violations outside of California, the insurer may count one violation point for each point that would have been assessed had it occurred in California. "It may take a few weeks to a few months, but a ticket in another state that is paid or unpaid will show up on your California DMV driver record," says California Department of Motor Vehicles spokesman Artemio Armena.

In California, you generally can get a point for a moving violation (but not an accident) removed from your record if you attend a traffic school approved by the Department of Motor Vehicles.

Although the rules may vary a bit by county, in general you can attend traffic school if you have a valid driver's license, you were ticketed for a moving violation in a noncommercial vehicle, the offense did not involve alcohol and you have not been to traffic school in the past 18 months. If you complete traffic school, the point will be removed and the violation will be confidential and will not impact your insurance rates. You will still have to pay the ticket – and traffic school fees.

'It's all sequential'

So how can your rates go up with one moving violation?

Many companies offer discounts on top of the 20 percent good-driver discount to customers with exceptionally good driving records, such as no tickets or accidents in the past five or seven years.

Say you are an exceptional driver and your base premium is $1,000. With the 20 percent good driver discount, your premium is $800.

Now suppose you make an illegal U-turn, blow off traffic school and get a point on your record. You lose your extra discount and your base premium goes up to $1,200. But because you have only one point, you still get the good-driver discount, which takes 20 percent – or $240 – off your new, higher rate, bringing your new premium to $960.

Allstate, the only insurance company that would discuss rates with me, said it offers a "distinguished driver" discount to customers with a clean driving record for seven years. That discount is 5, 10 or 15 percent, depending on the customer's tenure with Allstate. "You lose that with the first ticket," Allstate spokesman Freddy Santos said.

He could not, however, tell me how a single ticket would affect a customer's rate. "It's complicated. It's all sequential. One thing affects the next and the next. It's all interrelated," he said. However, the "surcharge" for a one-point moving violation "is not even close to the good driver discount."

Losing discount

Customers who want to know how a violation will affect their rates should contact an Allstate agent, he said.

Unlike moving violations, drivers always lose their good-driver discount with an at-fault accident costing more than $1,000.

Although many companies advertise "accident forgiveness" policies, these policies generally are not offered in all states. For example, Nationwide, Liberty Mutual, USAA and Allstate say their accident-forgiveness policies do not apply in California.

Kathleen Pender is a San Francisco Chronicle columnist. Net Worth runs Tuesdays, Thursdays and Sundays. E-mail: [email protected] Blog: http://blog.sfgate.com/pender Twitter: @kathpender