is to realize that in 1953, Dwight Eisenhower, having spent WW2 as the Supreme Commander of the Allied Expeditionary Force in Europe came back to the US and was like:
"Your roads are all shit. if Pearl Harbor was like Omaha Beech(sic) it would take months to get tanks over there through Panama, and at least a week by rail. we're going to make GOOD roads, that can carry our VERY HEAVY tanks. Why? because Russia. And to make the Russians underestimate these Good Fucking Roads that i know we need from fighting in EUROPEAN SWAMPS for my entire forties, we're going to say they're for launching bombers, even though our bridges preclude transporting them." (paraphrased)
and so he did.
and because there were now highways connecting cities, suburbs became a thing, it wasn't just cities and rural. the white(pejorative) wealthy and middle class fled the cities because they were afraid of Black people reacting justifiably to how they were treated.
Now, because they were wealthy, they demanded the suburbs have the same amenities they came to depend on in the cities. Municipal sewers instead of septic tanks, municipal water instead of wells, well paved streets with sidewalks and street lighting, good highways, police, fire dept, etc.
But most of the costs of those things do not scale as much with volume, they're a heavy initial cost, so no matter how small the town is, the upkeep goes up.
But those wealthy people? wanted to pay as little as they did in the city. they railed against taxes, they railed against people who aren't like them moving in. bills still have to be paid, though.
How did states solve this? A pyramid scheme, mostly, as far as I can tell. You get the money from new house purchases, so you're basically taking the money from people's loans. which is great, except... a lot of those loans default and you lose that AND whatever piddly property tax they were willing to pay. Sure, some of it comes from sales tax, except... there's no local business anymore and wages are suppressed, so more money is leaving your town than the taxes imply
and more people move out
and you're stuck paying for sewer, water, electricity, etc etc in a very sparse area with very little money and a high maintenance burden. you keep trying to attract rich people to stay afloat, but they only give lump payments and keep fighting you on property tax. so they're unsustainable, especially since they cause the most duty cycles on the stuff that's costly to maintain.
so, the broader point: highways haven't been funded heavily with federal money since around the 60s in most places. they rely on state funding, which relies partially on who lives where. who have all been spending down your money bailing out their towns every time the sewer breaks, but not actually paying for it. no more highways will be made, and maintenance is only done when things get dire and cost 4x as much, because often they couldn't afford it the first time. because suburbs require so much upkeep but generally have such low property taxes on the wealthy people who fled to them.
theres more reasons. but imo that's the one at the core of everything.
also the cross streets requiring bridges over highways is primarily racism in terms of where they chose to cleave, but a big part of the reason they went through cities is so they could blow the bridges to make blockades if hostiles were using the highways themselves. Sucks how much racism and the cold war hold hands, doesn't it.