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Why are Texas interchanges so tall?

practical.engineering

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Re: Why are Texas interchanges so tall?

#31

Post by jessriedel »

function_seven wrote: post #22
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

It's an interesting theory, but ChatGPT was skeptical, and I can't find anything on the internet that supports it.

Thanks for the pointer though.

Re: Why are Texas interchanges so tall?

#32

Post by criddell »

mschuster91 wrote: post #18
lwansbrough wrote: post #4

> 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 Truly pitiful numbers for a project of this scale lol.

And now imagine that these 500k people would be served by a decent network of trains or even buses. That would immediately reduce the demand for infrastructure, simply because how much more efficient even a single decker bus is in passengers/m² of occupied road space.

> more efficient

Depends on what you are optimizing for.

I can drive to work in 15 minutes, door-to-door. I used to ride the bus and sometimes I would be waiting that long just for the bus to appear.

The other factor is weather. Not many of the bus shelters around me (I’m in Austin) are air conditioned. Right now it’s 103F / 39C with a feels-like temperature of 107 / 42. Depending on your health, just walking around can be dangerous.

Re: Why are Texas interchanges so tall?

#33

Post by jessriedel »

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.

Does anyone have a link to something that supports this theory?

Re: Why are Texas interchanges so tall?

#34

Post by kuschku »

lwansbrough wrote: post #4

> 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 Truly pitiful numbers for a project of this scale lol.

Assuming the 500k people all commute between 6am-9am and 3pm-8pm, that means this interchange moves 160k people per hour.

That's 1½-2½ commuter rail lines or about 3-4 subway lines.

Such interchanges are truly a monument to stupidity.

Re: Why are Texas interchanges so tall?

#35

Post by jeffbee »

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…

It's because they let the same company design and build the interchange and own the concrete plant. "Design-build" project delivery. It is the same reason why the new HSR projects in California have ludicrous vertical and side clearance compared to other countries with better/cheaper HSR.

Re: Why are Texas interchanges so tall?

#36

Post by tempoponet »

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.

Another couple aspects of the high-5 not mentioned in the video:

-it doesn't have any lane merges, you can change from one highway to another without merging, your bridge just becomes a new lane on the next road

-these bridges are very long leading up to the turn, so it displaces a lot of traffic and shifts a lot of the lane changing upstream.

Re: Why are Texas interchanges so tall?

#37

Post by seijiotsu »

mikepurvis wrote: post #24
missingcolours wrote: post #7

Earlier quoted context omitted.

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…

Don’t forget ease and safety. With a stack interchange (and many other types), you often have plenty of time beforehand to position yourself into the correct lane. But if you are going to go through a cloverleaf you will often need to make a stressful weave into the leaf, and then back again into the highway.

Re: Why are Texas interchanges so tall?

#38

Post by dylan604 »

mikepurvis wrote: post #24
missingcolours wrote: post #7

Earlier quoted context omitted.

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…

The TFA specifically covers this, so skimming wikipedia should have been totally unnecessary.

In fact, the TFA goes further than your wiki skim to add that the merging from the highway to the clover leaf requires slowing down on the highway while entering the highway from the clover requires accelerating. Both of these things are being done in the same piece of road. It does so with graphic animations. You'd probably enjoy the TFA

Re: Why are Texas interchanges so tall?

#39

Post by pixl97 »

kuschku wrote: post #34
lwansbrough wrote: post #4

> 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 Truly pitiful numbers for a project of this scale lol.

Assuming the 500k people all commute between 6am-9am and 3pm-8pm, that means this interchange moves 160k people per hour. That's 1½-2½ commuter rail lines or about 3-4 subway lines. Such interchanges are truly a monument to stupidity.

You mean hundreds if commuter rail lines because the population isn't centered around dense points that make rail useful.

We're in a catch 22 trap that will cost trillions and likely a century to get out of.

Re: Why are Texas interchanges so tall?

#40

Post by toast0 »

mikepurvis wrote: post #24
missingcolours wrote: post #7

Earlier quoted context omitted.

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…

Cloverleaf intersections have really poor overload behavior. They're fine in low traffic areas, but in places with high traffic, the crossing streams of traffic cause cascading backups.

Stack interchanges increase acheivable throughput because you can often build enough road to allow backups for one direction to not impinge on the other flows.

It's also easier to build effective signage so there's fewer surprise lane changes.

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