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.