I calculated this out 2 months ago: The US spent 416 billion dollars on roads in 2014 [1]. At 123 billion gallons of gas [2] and 44 billion gallons of diesel [3], that would require $2.50 worth of tax per gallon. Gas near me is $3.40 with a 60 cent tax so $1.90 short or 24%.
In most places the cost of roads (especially local roads) is being partially covered by general taxes. Where I live is it around 50%.