The Pittsburgh Tribune-Review makes a very interesting observation in an article discussing the ramp-up in competition among personal auto insurance writers:
Competition certainly shows in the number of companies writing premiums. In 2005, 317 insurers wrote nearly $7.1 billion of vehicle-related premiums, according to [Pennsylvania] state Department of Insurance data. Eighty insurers wrote at least $10 million in premiums that year.
Compare Pennsylvania’s numbers to Massachusetts. Since 1990, that state has lost 35 auto insurers, and has 19 companies writing policies for 4 million drivers. New Jersey, prior to legislative reform in 2002-03 lost 20 insurers, including five of the top 10.