From Practical Engineering, transcribed from a YouTube Video:
Why Are Texas Interchanges Texas So Tall?QuoteThis is the Dallas High Five, one of the tallest highway interchanges in the world. It gets its name from the fact that there are five different levels of roadways crossing each other in this one spot. In some ways, it's kind of atrocious, right? It's this enormous area of land dedicated to a complex spaghetti of concrete and steel; like the worst symbol of our car-obsessed culture. But in another way, it really is an impressive feat of engineering. 37 bridges and more than 700 columns are crammed into this one spot to keep the roughly half a million vehicles flowing in every direction each day.
FULL ARTICLE HERE (https://markholtz.info/2vi)
This is right close by to me, and covers I-635 running East-West and US-75 running North-South. While it is an impressive structure, there are still bottlenecks. As an example, and this was on a weekend, the traffic comes to an almost standstill when going from eastbound I-635 to US-75 until you get beyond the North-South split. There is no advantage to using the I-635 Express Lanes as the exit to US-75 puts you at the back of the line. You need to pay attention to the road and not your phone there.
I can understand why they they use ramps and "bridges". I'm reminded of the cloverleaf at I-580/I-680 in Pleasanton, CA where going to from Westbound I-580 to Southbound I-680 felt like taking your life in your hands at the speeds dropped tremendously and you had to deal with the semis who are going from Northbound I-680 to Westbound I-580, and CA-84 was a recommended bypass route.
To answer the question in the subject line, obviously Texas interchanges are often so tall because they have to accommodate all those levels of intersecting flyover bridges, freeway main lanes and continuous frontage roads and/or surface streets.
In some cases there are flyover ramps that seem needlessly high. One that stands out to me is the EB Kell Freeway ramp to the Broad Street overpass and start of I-44 in Wichita Falls. It's a single lane ramp that feels like it goes up pretty high. I have to focus on the road and not look off to the left or right otherwise I'll start getting queasy. I'm guessing the flyover ramps on that interchange are built high enough to (maybe?) fit another level of thru lanes if Kell Freeway was ever extended farther East into the old downtown area of Wichita Falls.
The High Five interchange in Dallas is really impressive. But I think they should have built wider flyover ramps. Most of the flyovers and bridges leading to them are just 1 or 2 lanes wide. The interchange was completed almost 20 years ago. In retrospect some of those flyovers and all of the approach bridges should have been at least 3 lanes wide. Then again, there's only so much you can do. Katy Freeway in Houston is wide as hell, but gets backed up. In that case it's largely due to the horrible surface street grid.
A couple of things I've noticed as an outsider;
* Texas highways have frontage roads. This adds a fifth level to what would be a four-level interchange in most other places
* many large interchanges with sweeping flyovers only have a single lane. So its capacity is not that great.
He mentioned in the beginning that these interchanges take up a lot of land. This is actually not true. The High-Five interchange is very large, but most stack interchanges in Texas actually have a pretty small footprint.
Here's an example of Loop 375 / I-10 east of El Paso. This shows that a large stack interchange is not larger than a cloverleaf (which has a far lower capacity).
(https://i.imgur.com/y9HfSVl.jpeg)
There are a few examples in Europe of stack interchanges having a substantially smaller footprint than a well-designed cloverleaf (those with collector/distributor lanes and loop ramps designed for 40 mph).
Quote from: Chris on August 21, 2024, 05:54:29 AMHere's an example of Loop 375 / I-10 east of El Paso. This shows that a large stack interchange is not larger than a cloverleaf (which has a far lower capacity).
Slight problem with that image. At the bottom level is a cloverleaf for the frontage roads, The High 5 has an actual signalized intersection with the East-West frontage road going over US-75. Also, the traffic volume is probably much less than the High 5.
Why are Texas interchanges so tall? I guess they build everything bigger in Texas than in other states.
Quote from: The Ghostbuster on August 21, 2024, 12:01:47 PMWhy are Texas interchanges so tall? I guess they build everything bigger in Texas than in other states.
Part of it is that there are not a lot of days where ice on the bridges is relevant. SO icing is not an issue.
https://en.wikipedia.org/wiki/High_Five_Interchange
https://en.wikipedia.org/wiki/Judge_Harry_Pregerson_Interchange
Interestingly the designs for the High Five interchange has some of its origins in Los Angeles.
(https://upload.wikimedia.org/wikipedia/commons/e/e6/Los_Angeles_-_Echangeur_autoroute_110_105.JPG)
For the record, the fidget spinner roundabout will be here (https://www.google.com/maps/@29.5909986,-98.5986632,848m/data=!3m1!1e3?entry=ttu) if I'm not mistaken.
Also, the video mentions that there are quite a few disadvantages to having frontage roads, like Texas so often does. I did not know that.
Texas
On Central southbound there is a big bottleneck south of the High Five where you have no less than three entrance ramps merging within about a quarter-mile, one of them being traffic off of LBJ. A C-D lane there would help immensely.
Because they drank milk as children.
Quote from: bwana39 on August 22, 2024, 09:24:55 AMQuote from: The Ghostbuster on August 21, 2024, 12:01:47 PMWhy are Texas interchanges so tall? I guess they build everything bigger in Texas than in other states.
Part of it is that there are not a lot of days where ice on the bridges is relevant. SO icing is not an issue.
But on the days where they are, HOO boy ....
Also Texas
Quote from: Chris on August 21, 2024, 05:54:29 AMA couple of things I've noticed as an outsider;
* Texas highways have frontage roads. This adds a fifth level to what would be a four-level interchange in most other places
* many large interchanges with sweeping flyovers only have a single lane. So its capacity is not that great.
He mentioned in the beginning that these interchanges take up a lot of land. This is actually not true. The High-Five interchange is very large, but most stack interchanges in Texas actually have a pretty small footprint.
Here's an example of Loop 375 / I-10 east of El Paso. This shows that a large stack interchange is not larger than a cloverleaf (which has a far lower capacity).
(https://i.imgur.com/y9HfSVl.jpeg)
There are a few examples in Europe of stack interchanges having a substantially smaller footprint than a well-designed cloverleaf (those with collector/distributor lanes and loop ramps designed for 40 mph).
What interests me with that particular interchange is how they reworked the frontage/access roads into the old cloverleaf design while also incorporating the direct flyover connectors for the mainlaines. More capacity, but still 4-level. Slick.
That interchange in El Paso is indeed pretty neat. I like the frontage roads having their own cloverleaf interchange with a freeway to freeway directional stack interchange floated over it. There should be more of those built.
I forgot to mention: Texas
Quote from: LilianaUwU on November 12, 2024, 05:08:45 AMQuote from: NE2 on November 12, 2024, 01:41:26 AMI forgot to mention: Texas
Don't forget Texas.
I came here to say, Texas. And I won't mess with it.
But I also see that many ramps will stay parallel to the road they diverge from as they rise in elevation before the split to go either direction on the intersecting road, which reduces the amount of real estate needed, but as a result makes taller bridges a necessity. The photos posted above show good examples of this.
Quote from: wriddle082 on November 12, 2024, 12:13:21 PMQuote from: LilianaUwU on November 12, 2024, 05:08:45 AMQuote from: NE2 on November 12, 2024, 01:41:26 AMI forgot to mention: Texas
Don't forget Texas.
I came here to say, Texas. And I won't mess with it.
But I also see that many ramps will stay parallel to the road they diverge from as they rise in elevation before the split to go either direction on the intersecting road, which reduces the amount of real estate needed, but as a result makes taller bridges a necessity. The photos posted above show good examples of this.
I'm from Arkansas. Texas messes with us plenty and I'd rather they keep it there.
The IH 410/ US 281 interchange north of SA is high due to the tight space around the freeways. Remember this exchange got built after the development around the area happened as both freeways had no direct ramps between them for many decades.
NC when connecting I-42 to I-95 in Selma should use this intersection as a blueprint as both sides of I-95 there are as clustered as I-410 is in SA. Tall flyovers there would fit the ramps without taking too much ROW along frontage businesses.