Edited because I admit original statement below is incorrect.
"You could say they are a flat tax since every driver pays the same per usage. You could even argue it is a progressive tax since richer people use toll roads more. The only way you CAN'T describe a toll is a regressive tax. Words have meaning."
This is completely incorrect. A flat tax has a constant tax rate, which is why it's often referred to as a "proportional tax." Under a true flat tax system, everyone pays the same percentage of their income.
A toll is absolutely regressive because the burden it imposes is constant, irrespective of income; poorer individuals will pay a proportionally higher percentage of their income than wealthier counterparts. As income increases the "effective rate" asymptotically approaches zero, which is regressive by definition.
If you read the literature[1], they're regressive - less regressive than sales tax, but still regressive despite being utilized more by higher income drivers.
"You could say they are a flat tax since every driver pays the same per usage. You could even argue it is a progressive tax since richer people use toll roads more. The only way you CAN'T describe a toll is a regressive tax. Words have meaning."