Between 2000 and 2006, U.S. hybrid sales increased from 3,000 to 250,000 vehicles. Why? The rise in gas prices? Government incentives like income tax credits and deductions, state sales tax waivers, single-passenger access to HOV lanes, and waivers of fees for registration, emissions testing, excise and parking? Or changing consumer tastes and preferences for a clean environment and an energy security policy?