Mostly underutilized? Have you driven between Dallas and Austin on I-35? Add a few hundred Mexican trucks a week thanks to NAFTA, and you'll have folks more than willing to pay the cost of a tank of gas to ride a tollway at 85 mph.
There certainly will be people willing to pay to get out of the traffic - but will there be enough people to do that, for this company to get anything close to a reasonable return on investment? I just can't see how - unless the state removes the freeway option for the same route ($7.2 Billion is LOT of money).