Right, the reason it might be underutilized is if you're bad at designing cities for it. Which the US is, so it is.

We design cities for cars, which results in the cities spreading out further and further, which makes transit less desirable and more expensive. Other countries don't have this problem to this degree, because they don't design their cities exclusively for cars.

Also, I don't think most roads would need to exist if the amount of cars decreased. Because of the density problem noted above. Cars are sort of self-eating. The more cars you use, the more land-per-car you need as everything spreads further out to accommodate the cars.

> is if you're bad at designing cities for it

Consider that the transportation system might not be the best fit if it requires designing the rest of the world differently and against preferences (large, detached, single-family homes with a yard).