Today California, tomorrow America, and the day after tomorrow, the world. So it has been in matters ranging from fashion and music to high tech and healthcare. But nowhere has the state's role as trendsetter par excellence been as great as in the field of the environment.
The deal announced between Governor Arnold Schwarzenegger and the California legislature, ordering a reduction of greenhouse gas emissions to 1990 levels in the state by the year 2020, is but the latest in more than half a century of trailblazing measures that have frequently become the national, and on occasion the international, norm.
Back in 1947, Los Angeles set up its own Air Pollution Control District, the first in the country, to tackle the smog created by wartime industry. Not until 1955 did the US government follow suit at a national level.
By then, Los Angeles in particular and the state in general had become the land of the freeway. After smog forced the shutdown of LA's schools and industry in the autumn of 1954, California became the first state to establish a board to set standards for car emissions.
Other moves followed. In 1967, the state had already inaugurated Carb, a state air resources board, three years beforesetting up a national equivalent, the Environmental Protection Agency. In the mid-1970s, it led the way in requiring cars to be fitted with catalytic converters. Next, it made unleaded petrol compulsory.
New decades brought new initiatives. In 1990, Carb took its most ambitious step yet, the Zev, or Zero Emission Vehicles, programme, stipulating that 2 per cent of vehicles for sale in 1998 should be totally clean, and 10 per cent in 2003.
In July 2002, Democratic Governor Gray Davis (who the following year would be replaced by Mr Schwarzenegger) signed a landmark state bill requiring carmakers to limit emissions, starting in the 2009 model year. Before long, the cream of Hollywood - the likes of Leonardo DiCaprio, Natalie Portman and Cameron Diaz - were going nowhere without aToyota Prius.
Quirky it may be, but California cannot be ignored - the state accounts for 14 per cent of the entire US gross domestic product. The Bush administration may have scorned the Kyoto agreement. California, however, is another matter - as Tony Blair demonstrated when he skipped Washington and went straight to the Golden State in July to announce a deal with the Governor for Britain and California to co-operate in fighting global warming.
Mr Schwarzenegger seems an improbable environmentalist, known for his fondness for Hummers. But many of his nominees to key posts in the environmental area have solid green credentials, and few doubt his personal support for hydrogen as a replacement for petrol to power cars.
But even California's powers are limited. As so often in the US, sweeping measures are whittled away in the courts and elsewhere. Car companies waged a ferocious action against the Zev law, arguing they were technologically unfeasible. The 2002 initiative has also been bitterly contested.
Despite years of efforts, the automobile remains the state's chief pollutant, accounting for 60 per cent of California's total emissions. Unarguably however, Californian initiatives have forced the pace of research into clean technologies.