When California residents voted to brake the state's runaway car-insurance rates last November, alarmed insurers sped into court to overturn the referendum. But last week the California Supreme Court upheld most of the measure, thus increasing the prospect that a revolt against high auto premiums could soon spread to other states. In a unanimous decision, the seven justices affirmed the major provisions of Proposition 103, which slashes car premiums and other types of property and casualty rates 20% below the level of November 1987. Good drivers will get another 20% off.
While the ruling could force firms to give California customers...