Fossil energy is artificially cheap (the price its consumers pay does not even begin to pay for the negative externalities of producing and burning it -- and most of it goes into private hands that don't intend to use it for any kind of mitigation anyway).

This causes all kinds of problems, has all kinds of causes, and is generally a disaster all around, but at the very least we shouldn't deliberately make it worse by also artificially juicing demand in one of the main fossil-fuel-burning sectors of the economy (any more than we already are). Every petroleum-powered mile not traveled (and energy is to a certain extent fungible, so in principle this also applies to electric cars burning "free" solar energy) is a win for human civilization.

This isn't hippy-dippy environmentalism, just soulless Chicago economics: mispriced commodities do real damage to a political economy, and in the globalized era, there is effectively only one political economy anymore.

The problem with roads and highways is that they are inherently low-capacity, so it's very easy to fill them with traffic.