As explained yesterday on why California has the highest gas prices in the nation, next to Alaska, stringent pollution requirements and high taxes were the main reasons.
This California Energy Commission has a chart (click here to enlarge) explaining the history of gas tax increases and where they're at now, which is $0.378 between state and federal taxes. But that's not all, there's the sales tax and the gas production increase for companies to stay within state environmental requirements.
(h/t Bottleneck Blog for the chart)