In urban areas, they're usually an alternative. If you're going past the city, you could build a ground level highway around the city for a lot cheaper. If you're going into the city, it makes more economic sense to leave your car at the periphery of the city and take a rail system in because of the difference in throughput per $ spent building it (as well as the space occupied by parking for people who need to leave their cars in the city). Plus the people leaving the highway will get onto surface streets, and back up the highway.

Being able to make underground tunnels cheaper and faster is cool. Using them for cars is mostly a boondoggle with clearly superior alternatives.

I think that's reasonable. I suppose I also think it idealist that cities will actually act that way in practice in the short term. I'm specifically thinking of examples like the Corniche highway in Alexandria or Marine drive in Mumbai which shows cities are willing to give up gorgeous public space throughout incredibly dense areas to support car traffic. But there's also examples like Boston's "big dig" which shows cities are willing to spend extra to move those auto pathways underground. At least in the short term it seems that 1) cities aren't giving up entirely on cars, but 2) are willing to pay more to have them underground.

I suspect in practice the actual approach is going to be a mix of all of the above. So my reasoning is primarily that if all cities won't give up cars anyway, it seems objectively better to make it easier to at least move more of them underground. I suppose one case where I would change my mind is if there was evidence that more affordable underground roads reduced the investment in public transit.

> I suppose one case where I would change my mind is if there was evidence that more affordable underground roads reduced the investment in public transit.

It's Friday night so I lack the motivation to go on a stats-finding expedition, but anecdotally this seems like a circular issue to me. Public transportation sucks, so no one wants to fund it and we invest money into car infrastructure. Traffic gets worse, but public transportation is still bad because we haven't improved it, so we dump more money into car infrastructure, and etc.

I do hear you about the practical realities, though. Most people will drive if they can, because it is more convenient (so long as we can keep building more roads, even at exorbitant prices).

I think there would be far less support if people could see what they're actually spending on car infrastructure. At least in the US, it's currently so fractured it's hard to get an idea. Registration fees, gas taxes, federal taxes that get pumped into highway maintenance, etc. There's no clear "we spend $X on car infrastructure, and we could have really good public transportation for $Y".