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Why are Texas interchanges so tall?

practical.engineering

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Re: Why are Texas interchanges so tall?

#51

Post by estebank »

criddell wrote: post #32
mschuster91 wrote: post #18

Earlier quoted context omitted.

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…

You need to compare not against what you currently have, but rather what you'd have if there was political will and budgets comparable to what was spent on the existing car infrastructure. Otherwise is saying that no-one wants a bridge to cross a river because no-one swims across.

Re: Why are Texas interchanges so tall?

#52

Post by gnatolf »

reaperducer wrote: post #8
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…

Frontage roads don't generally exist where 3+ highways intersect… Or do they, in Texas? I've lived in several states where frontage roads are common (under various names), and Texas was the place where they were most plentiful. Older interchanges tended to run the frontage roads below grade to make them cheaper. But more recent ones tend to keep everything above grade due to flooding. Or is it just that Texas has one…

Maybe it's just me but I expected these links to open in the recently established apple maps web page. However, on an Android phone, Google Maps opens.

Re: Why are Texas interchanges so tall?

#53

Post by mikepurvis »

dylan604 wrote: post #38
mikepurvis wrote: post #24

Earlier quoted context omitted.

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…

To be honest I did read the first few paragraphs and then went looking for info about the Dallas High Five as I don't think I've even been on anything taller than ~three levels— the major highway near me is the 401 and I believe it is mostly cloverleafs other than the junction with 410 and 403, so I'm much more used to that arrangement.

Re: Why are Texas interchanges so tall?

#54

Post by aidenn0 »

gullywhumper wrote: post #49

I lived in Dallas for a couple years, and while driving the Hive Five interchange (mentioned in the article) always seemed a little crazy, what really blew my mind was that a specific interchange could warrant its own Wikipedia page. For some reason that more than anything else really underscored its scale. https://en.wikipedia.org/wiki/High_Five_Interchange

One close to where I grew up with its own page is the Springfield, VA interchange[1][2]. 24 lanes wide at its widest point.

1: https://en.wikipedia.org/wiki/Springfield_Interchange

2: https://www.openstreetmap.org/#map=17/38.791557/-77.175887

Re: Why are Texas interchanges so tall?

#55

Post by Animats »

Frontage roads force a design decision. Texas likes to run the frontage roads through the interchange ramps. California generally displaces the freeway about 200m from major surface roads to avoid running the surface street through the ramps. CA-92 at US-101 does that.

When too many layers are needed, CALTRANS often goes for a tunnel underneath. See CA-92 at I-280, and SF's 19th Avenue at I-280.

At US-101 and I-280 in San Francisco, a frontage road does go through the ramps. The interchange was spread out horizontally to avoid piling up all the levels.

Re: Why are Texas interchanges so tall?

#56

Post by tines »

dylan604 wrote: post #44
0cf8612b2e1e wrote: post #28

Earlier quoted context omitted.

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.

I've never heard of the proposed gravity assist theory in any discussion of these interchanges. Does it happen? Sure. Is it one of the reasons behind the design or just something that happens to also be true is the actual question. Pontificating on the interwebs is fun

At https://www.fhwa.dot.gov/modiv/programs/intersta/docs/interc... I see the short comment

> At service interchanges it is desirable to design the interchanges with the crossroad above the freeway due to:

> - The crossroad above the freeway results in longer sight distances to the exit ramp and gore area.

> - The crossroad above the freeway allows gravity to assist the operation of both accelerating vehicles (the on-ramp has a down-grade) and decelerating vehicles (the off-ramp has an up-grade). In addition, the resulting grades generally provide longer sight distances.

But that's the main thing I can find right now.

Re: Why are Texas interchanges so tall?

#57

Post by WheatMillington »

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.

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

Assuming every single commuter is coming from and going to the same place, sure.

Re: Why are Texas interchanges so tall?

#58

Post by dylan604 »

mikepurvis wrote: post #53
dylan604 wrote: post #38

Earlier quoted context omitted.

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…

To be honest I did read the first few paragraphs and then went looking for info about the Dallas High Five as I don't think I've even been on anything taller than ~three levels— the major highway near me is the 401 and I believe it is mostly cloverleafs other than the junction with 410 and 403, so I'm much more used to that arrangement.

You must have skimmed over the "graphic animations" too which implies the source video from which the text was just transcribed

Re: Why are Texas interchanges so tall?

#59

Post by criddell »

estebank wrote: post #51
criddell wrote: post #32

Earlier quoted context omitted.

> 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…

You need to compare not against what you currently have, but rather what you'd have if there was political will and budgets comparable to what was spent on the existing car infrastructure. Otherwise is saying that no-one wants a bridge to cross a river because no-one swims across.

Well that's a problem. I'm not willing to pay anywhere near as much for public transportation as I am for a private vehicle and everything that goes with that.

But I do think you are on the right track. I think public transportation in the form of buses doesn't make a lot of sense. But if Musk had been able to deliver on his promise of self-driving cars, then a giant city operated (or at least funded) fleet of small autonomous vehicles offering door-to-door service and route flexibility could change everything.

Re: Why are Texas interchanges so tall?

#60

Post by TeMPOraL »

0cf8612b2e1e wrote: post #28
lagniappe wrote: post #21

Earlier quoted context omitted.

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.

Tens of millions in extra infrastructure costs to save a lot of trucks tiny amount of gas per ride, over many rides per year, over decades? That's literally what infrastructure is meant for.
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