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Why are Texas interchanges so tall?

practical.engineering

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Re: Why are Texas interchanges so tall?

#11

Post by seijiotsu »

missingcolours wrote: post #6
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.

The tl;dr to the article is "Texas freeways have frontage roads, which add at least one additional level to a traditional stack interchange, and that makes them taller than interchanges in e.g. California. The broader question I've always had, that's not addressed in the article, is "so why do California and Texas build so many stack interchanges while the rest of the country mostly does not?"

From the article:

> And the design that generally provides the most capacity, on the smallest footprint, (often for the highest cost), is the stack.

This would explain why there are plenty in coastal California, where there are tons of people to move around and where space is very much at a premium.

Re: Why are Texas interchanges so tall?

#12

Post by 486sx33 »

Perhaps Texas is uniquely specialized in building “up” and in a compact space due to all the oil and petrochemical industries and specialized knowledge ? This offers financial efficiency in building these stacked interchanges that might not be so cost effective elsewhere ?

I mean the less you move the cranes around the cheaper I’m sure

Re: Why are Texas interchanges so tall?

#13

Post by jessriedel »

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 most convincing to me: The flyover ramp generally needs to go over some things and under other things. Additionally, for both safety and rider comfort, the ramp needs to be vertically smooth, not having fast drops or rapid climbs. That means ramp A might pass over ramp B by a lot more than the minimum amount because ramp A also needs to pass over ramp C just a bit later, where it does so by a much lower amount. With many overlapping constraints like this, most ramp intersections are much higher than the minimum clearance even though none can be lowered without either (1) making one of the clearances below minimum or (2) forcing a ramp to rapidly climb/descent.

Re: Why are Texas interchanges so tall?

#14

Post by tines »

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.

Re: Why are Texas interchanges so tall?

#16

Post by nonameiguess »

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 highways have high-occupancy express lanes that are separated from other traffic and have separate interchanges with the other highway as well as separate on-ramps and off-ramps to the frontage roads. This one is particularly bad because it's where the express lane for the US 75 starts and stops. Normally, the express lanes don't have their own dedicated exits and you have to first exit to the normal highway, but US 75 has an express lane north of I-635 but not south, so there are separate entrances and exits for it here.

Thankfully, I've worked from home for years now, but back in the day, this used to feature heavily in my daily commute. My wife and I worked at the same place, so typically used the US 75 express lane where it existed, and it was a bitch. To go south, you have to get off the highway onto the frontage road and then back onto the highway.

Re: Why are Texas interchanges so tall?

#17

Post by stetrain »

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 video shows multiple 5-level interchanges in Texas where two highways intersect along with their frontage roads, turning what would otherwise be a 4-stack into a 5-stack or 6-stack interchange.

Re: Why are Texas interchanges so tall?

#18

Post by mschuster91 »

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.

Re: Why are Texas interchanges so tall?

#19

Post by stevenwoo »

robotnikman wrote: post #5

As cool as the engineering is, I avoid such interchanges at all costs since the being so high up gives me panic attacks when driving...

Normally you are either going so fast you are looking far down the road and not over the side or really slowly, trying to not rear end the car in front of you, the height is not that noticeable unless one is a passenger. One time I was on a freeway in Houston at night and suddenly sparks started flying down on me while I was going at normal speed - it was a truck that had flipped on its side in the merge lane that was above me and to the left and was grinding against the road and the guardrail at about highway speed (for a little bit), that was a new fear unlocked.

Re: Why are Texas interchanges so tall?

#20

Post by stetrain »

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 in the video the quote about keeping traffic flowing is laid over footage where the traffic is nearly stopped and creeping through the interchange.
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