As gas prices climb to seven-year highs in America, drivers in some states are hit much harder than those in others.

The average gas price in the U.S. was $3.378 per gallon as of Oct. 21, according to data from GasBuddy.

But that average conceals a pretty wide spread among the 50 states. Oklahoma has the lowest average gas price among states at $2.982 per gallon, GasBuddy data shows. Gas prices are at their highest in California, at an average of $4.531 per gallon.

One factor driving this price disparity is regional access to oil refineries. For example, gas prices in the Gulf Coast may be lower than in the West Coast because that region has access to more refineries.

The West Coast is also hindered by the Rocky Mountains acting as a barrier separating Western states from others. This limits the number of pipelines connecting the region to the rest of the country.

Policies and taxes also play a role. California’s high prices are attributable in large part to higher taxes to the state’s carbon management program and to its unique fuel blend requirements.