The 11 page study is available online at http://www.ibtta.org/files/PDFs/win08_Campbell.pdf.The 35 percent is TRN's estimate based on a fatality rate of 8.1 per billion vehicle miles traveled on all the nation's expressways which we get from FHWA's Highway Statistics 2006 publication and on Campbell's survey result of 5.2 fatalities per billion miles traveled.
Using the all expressways 8.1 fatalities/bVMT as the denominator the Campbell estimate for tollroads facilities of 5.2 gives 64% as the safety factor for being on a toll road as opposed to a tax road of the same general expressway class. Toll bridges and tunnels don't really have any large counterpart sample on the tax side since there are very few untolled major bridges and tunnels.
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