New York—Radio advertising was essentially flat last year compared with 2004, according to numbers released Thursday by the Radio Advertising Bureau. Combined spot and non-spot radio ad spending totaled $21.50 billion last year, compared with $21.40 billion in 2004. Network revenue dropped to $1 billion, from $1.50 billion. Local ad spending inched up to $15.60 billion, from $15.50 billion. One reason for the lackluster results is that 14% of the radio industry’s billings come from the automotive industry, whose advertising spending is in flux as Detroit shifts more of its ad dollars to the Web. “That’s $1 billion of our revenue, and it’s been down 1%,” said Radio Advertising Bureau President-CEO Gary Fries at the RAB’s annual conference this week in Dallas.