Insurance companies have different incentives for giving discounts. One of them is to give discounts based on the driver being accident free for 1 year, 3 years, etc. Companies have different policies.

I used "accident-free discount class" as a general meaning. The discount class can be 1 year, 2 years, etc....and the longer, the greater the incentive.

There are many auto insurance websites that talk about their "accident-free discounts"

http://www.statefarm.com/sfca/insurance/auto/ab.asp
Accident-Free Discount/Accident Forgiveness
This discount applies to State Farm policies that have been in-force for at least 3 years and there have been no chargeable accidents during that 3 year period.
The discount increases as the number of accident free years increases. It works as follows:
• Policy that is 3 years accident free obtains 12% Accident Free Discount
• Policy that is 6 years accident free obtains 17% Accident Free Discount
• Policy that is 10 years accident free obtains 22% Accident Free Discount & qualifies for a "Forgivable Accident"

http://answers.yahoo.com/question/index?qid=20080615093729AA...
Can a comprehensive claim cause me to lose my accident free discount?

http://www.smrt.com.sg/taxis/join_smrttaxis.asp
Accident free incentive of $800 per year

http://www.tn.gov.in/gosdb/gorders/par/par-e-204.htm
Award of incentives to Drivers of Government Vehicles who have rendered accident free service of 10/20 years-Further orders-Issued.

http://www.carinsurancecomparison.com/