Why are Texas interchanges so tall?

This is not true and is one of the problems with getting mass transit to work in the US.

Most of the employment in the US is now smeared across the city. I'll give Austin as an example.

Austin employment has spaced out clumps. Downtown. The UT campus. The Domain (old IBM/NI area). The Arboretum. Oak Hill. The "new" airport.

Austin housing has similar clumps, generally dictated by how much you earn. You're probably further out than you would like since the real estate in the center is ridiculous. So, Leander, Hutto, Garfield, or Dripping Springs.

Now, plot the flows on a map. Note the massive pileup in the center as people attempt to change sectors across the city. Most modern cities are like this. There are not two or three obvious points that would get a big chunk of the traffic.

I chose Austin, but San Diego, Pittsburgh, Nashville, etc. all look similar.

And, the news is worse than that. The "white collar" jobs that could be concentrated have also been the most displaced by surburban office parks or by WFH. The types of jobs that most people are employed in (service and warehousing--Walmart/Amazon/FedEx/UPS/etc.) are generally specifically positioned where real estate is cheaper which is almost by definition off of any mass transit connector.

These are people coming from several dozen square miles and dispersing into several dozen square miles, flowing through these chokepoints. They're commuting 40, 50, or more miles. You'd need these people to change several bus lines and several train lines. Those busses would be snaking through messy suburbs for them to be actually useful. Those neighborhoods to pick them up are sprawling with poor walkability and a 15 minute+ walk just to the major road bus stop for a lot of those people. And when they get off the train, they're probably going to need a bus to navigate the sparse fields of giant empty parking lots to their actual workplaces.

In fact, for a certain group of people this path practically already has a train service. US-75 runs parallel to the Red/Orange line. Assuming the person starts from Plano to go to their office job in Las Colinas/Irving (a massive assumption here, but practically best case), they won't even need to change trains. Let's ignore how they get to Parker Road station for now (Park and Ride? 15 minute walk through their neighborhood + same time as the park and ride?) and start the clock from there. They're going to commute to the Microsoft offices. They need to get in by 9:00 AM. So they instantly warp to the Parker Road Station, hop on the orange line at 6:46 AM. They get to North Lake College station at 8:07 AM. They then wait for the 229 bus at 8:21, take that 8 stops, hop off, walk a half a mile (~11min), and get there at 8:40. Nearly two hours and they only had to change once. And once again, that's instantly warping to Parker Road Station. Add another several minutes of drive or even more for a bus to connect to the train.

Next, they're going to take the highways, including toll ways. They leave from Parker Road Station, head down US-75, hop on Bush Turnpike, and it is an ~30min trip on average.

Or they want to avoid tolls. They take US-75 to 635 (taking this High Five talked about here), and it takes 30 minutes to an hour.

Now, theoretically this should get a little bit better. There will be a new train line with more of an East-West path that would be useful here. But it'll be a train change at Bush Turnpike Station, and service on this new Silver Line won't be great at first. Potentially a 30 minute wait for the next train at peak times. And it'll still take 50ish minutes after that transfer. So you'd look at maybe a 5 minute train ride from Parker Road to Bush Turnpike, wait 15 minutes for the next train, then still 50 minutes. Plus the 30 minutes for the bus ride and walk. Over an hour and a half, magically warping to Parker Road immediately when the train was about to leave and hopefully only waiting 15 minutes for the Silver Line train. We'll see that it is really like when it opens in late 2025 (hopefully). I'm still excited for it, I'm looking forward to not needing to drive and park all the way out to the airport for travel and having another way to Addison Circle for things like Oktoberfest and other events there will be great.

But this is also kind of a best case of someone taking the train versus driving. This is assuming someone lives close to the train lines in Plano. Their office is one bus line away from the station. Tons of people live even further out from there. Lots of people work at places which would require multiple bus transfers. The train doesn't even go halfway out into the sprawl, and the surrounding cities don't want to join DART.

But honestly, it makes little sense to me to have people living in Fairview, McKinney, Prosper, and Melissa trying to commute to an office job in Dallas/Irving/Grapevine/Westlake/etc. It's insane to me, but people choose it.

But a lot of people here just have a big aversion to taking transit. You're seen as an oddball to many for acknowledging it exists. We live right next to a bus stop that goes to a nice train station. My wife was looking at taking a job that would be immediately outside a train station. She looked at me like I was insane for suggesting she think about taking the train. I usually take it when I go into Dallas and people think it is incredible I survive. I take the bus to the city park with my kids and people wonder what happened to my car.

there is also the weave required between exiting off a loop and other traffic entering onto it. you could theoretically get rid of it by bridging one loop over the other but then you may as well just bridge more of it without the 270 degree turn at that point.

and there are also compromise designs like the cloverstack where only two opposite sides are the 270 loops to avoid the weaving problem.

That wasn't true in the past. My father used to carpool with other teachers for 20+ years. I remember a lot of carpooling through the 1980s and into the mid 1990s where it fizzled out.

I would suggest, however, that self-driving cars would be fine for carpooling. Most people would have no problem carpooling with a self-driving car if it always picked up the same bunch of people every day. If I can commute in 35 minutes or take a self-driving carpool for 45 because it stopped to pick up 3 other people from my department along the way, that would be fine.