Why are Texas interchanges so tall? • Hacker News Board

Re: Why are Texas interchanges so tall?

#21

Post by lagniappe » Tue, Aug 20, 2024, 7:59 PM UTC

tines wrote: post #14

I heard somewhere that the reason is to provide trucks the ability to shed speed to make these sharp turns, and then give them back their speed when coming back down so they don’t have to brake hard or spend a lot of gas to do it. Basically lets trucks use gravity to store their energy when making sharp turns and then get it back for nearly free.

Now THAT is some cool engineering I can get behind.

Re: Why are Texas interchanges so tall?

#22

Post by function_seven » Tue, Aug 20, 2024, 8:05 PM UTC

jessriedel wrote: post #13

Does anyone know why the flyover ramps in a stacked interchange usually have such large vertical clearances? The vertical separation seems much larger than necessary to accommodate the tallest trucks allowed on the highway. EDIT: There are some more theories here: https://engineering.stackexchange.com/questions/4238/why-are... Obviously there may be many contributing factors, but I expect one is dominant. This one is…

This comment was posted the same time you asked your question, but I think it's one decent reason:

https://news.ycombinator.com/item?id=41303428

Re: Why are Texas interchanges so tall?

#23

Post by bob1029 » Tue, Aug 20, 2024, 8:06 PM UTC

jessriedel wrote: post #13

Does anyone know why the flyover ramps in a stacked interchange usually have such large vertical clearances? The vertical separation seems much larger than necessary to accommodate the tallest trucks allowed on the highway. EDIT: There are some more theories here: https://engineering.stackexchange.com/questions/4238/why-are... Obviously there may be many contributing factors, but I expect one is dominant. This one is…

Part of it may be due to constraints of the construction process itself (I.e. not disrupting existing traffic)

I've never seen one built all at once. It's always a bunch of incremental changes over time.

Re: Why are Texas interchanges so tall?

#24

Post by mikepurvis » Tue, Aug 20, 2024, 8:06 PM UTC

missingcolours wrote: post #7

Groxx wrote: post #3

So... > Q: Why are Texas interchanges so tall? > > Frontage roads need grade separation, adding a layer. > Texas has lots of frontage roads. OK, I buy that Texas has more grade-separated interchanges on average... but I'm not seeing how that turns into "Texas has more higher interchanges" which is what it feels like that question is asking. Frontage roads don't generally exist where 3+ highways intersect, so that sho…

There are frontage roads at most freeway interchanges in Texas. Typical stack interchange elsewhere has 4 levels: freeway 1, freeway 2, freeway 1 left turns, freeway 2 left turns. Frontage roads add 5th and occasionally 6th levels. The other thing not mentioned is the proliferation of separate express lanes, which often have dedicated flyover ramps as well.

From a quick skim of Wikipedia, it looks like the main benefit to the stack style over a conventional 2-level cloverleaf is capacity (the left-turn leaves are generally at most one lane) and speed (left turns are direct rather than 270deg, so you don't have to slow down as much when taking them).

Still, to this layman it seems like a colossal engineering cost to build all those bridges and ramps for relatively modest gain.

Re: Why are Texas interchanges so tall?

#25

Post by gosub100 » Tue, Aug 20, 2024, 8:09 PM UTC

jessriedel wrote: post #13

Does anyone know why the flyover ramps in a stacked interchange usually have such large vertical clearances? The vertical separation seems much larger than necessary to accommodate the tallest trucks allowed on the highway. EDIT: There are some more theories here: https://engineering.stackexchange.com/questions/4238/why-are... Obviously there may be many contributing factors, but I expect one is dominant. This one is…

I'm completely guessing, but maybe to prevent harmful oscillations from wind currents blowing between the roads?

Re: Why are Texas interchanges so tall?

#26

Post by nabla9 » Tue, Aug 20, 2024, 8:09 PM UTC

First time I visited the US it was Houston, Texas. My host was perplexed because I laughed out loud while we were driving. Experiencing highways and those massive gas stations felt like someone had made a state that mocked America by taking prejudices we have about the American traffic and making them 200% larger in real life. My mind was blown.

Re: Why are Texas interchanges so tall?

#27

Post by gosub100 » Tue, Aug 20, 2024, 8:11 PM UTC

I remember reading a long form article on HN about 2 years ago that was about Texas roads and highways. It wasn't from practical engineering though. Maybe it was the website of the guy who Grady briefly mentions in his video?

Re: Why are Texas interchanges so tall?

#28

Post by 0cf8612b2e1e » Tue, Aug 20, 2024, 8:11 PM UTC

lagniappe wrote: post #21

tines wrote: post #14

I heard somewhere that the reason is to provide trucks the ability to shed speed to make these sharp turns, and then give them back their speed when coming back down so they don’t have to brake hard or spend a lot of gas to do it. Basically lets trucks use gravity to store their energy when making sharp turns and then get it back for nearly free.

Now THAT is some cool engineering I can get behind.

That sounds like tens of millions in extra infrastructure costs so as to save trucks a tiny amount of gas.

It is Texas, so anything is possible.

Re: Why are Texas interchanges so tall?

#29

Post by nonameiguess » Tue, Aug 20, 2024, 8:12 PM UTC

0cf8612b2e1e wrote: post #2

I only had a quick skim so far, but I did not feel like the article gave a firm enough rationale? I have seen scenarios in Texas when there is seemingly no other infrastructure or geography that could explain the over building.

It's addressed. The frontage roads by any reasonable measure seem like an objectively stupid idea that the state largely regrets building so much of, but they did it because it made it easier to acquire the adjacent land to build the highways. There may not be any infrastructure or geography that would have made building differently any harder, but the land had owners and Texas is probably abnormally deferential to private landowners as US states go.

Re: Why are Texas interchanges so tall?

#30

Post by Zircom » Tue, Aug 20, 2024, 8:13 PM UTC

nonameiguess wrote: post #16

Groxx wrote: post #3

So... > Q: Why are Texas interchanges so tall? > > Frontage roads need grade separation, adding a layer. > Texas has lots of frontage roads. OK, I buy that Texas has more grade-separated interchanges on average... but I'm not seeing how that turns into "Texas has more higher interchanges" which is what it feels like that question is asking. Frontage roads don't generally exist where 3+ highways intersect, so that sho…

The article kind of addresses this. Texas really does have more frontage roads than other US states. In the specific example cited in the article's headline photo, both the US 75 and I-635 have frontage roads that cross each other. They stay at ground level whereas the highways elevate. Part of the reason this specific interchange has so many levels, though, which is not addressed for some reason, is that both highwa…

The 75 lane your talking about is actually an HOV lane, not an express one. And I appreciate that it exits off the highway for exactly one reason and it's that it makes it so incredibly easy for the cops to setup their checkpoints to catch single people using the HOV lane without screwing over the rest of the traffic on the highway. Love to see it.