Atlanta has become a surprise success for electric car makers and the reasons—state subsidies and unfettered access to carpool lanes—offer a telling lesson in what it takes to lift demand for the vehicles.

Georgia provides more than $4,000 in income-tax credits on average for an electric-car purchase, cut-rate electricity, employer support of recharging stations and, in Atlanta, access to high-occupancy vehicle lanes in the city's congested roadways.

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