I wonder how the numbers would break down if you factored in that a lot of coasters are only open seasonally, like Cedar Point and other Ohio/Pennsylvania parks. If you were to compare a month-to-month or some other statistic, I'd bet the results would be a bit different.
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Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/index.html