[Note that this article is a transcript of the video embedded above.]

This 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.

They say everything’s bigger in Texas, but that’s not always true when it comes to engineering projects in the US. The tallest concrete dam is split between Arizona and Nevada. The longest bridge span is in New York. The longest road tunnel is in Alaska, and the longest water tunnel, not only in the US but the whole world, is the Delaware Aqueduct in New York. The largest hydroelectric plant is the Grand Coulee Dam in Washington, while the largest nuclear plant is in Georgia.

But one thing that Texas really does do bigger is highway interchanges. If you’ve driven from one major Texan highway onto or over another, you may have been astonished to find yourself and your vehicle well over a hundred feet or 30 meters above the ground. There’s no clearinghouse of data for flyover ramp heights, as far as I can find. Plus there’s the complexity of what a true height really means since many interchanges use excavation below grade for the lower level. Still, even the most conservative estimate puts the High Five taller than the Statue of Liberty from her feet to the top of her head. And if you do a little digging, you’ll find that many, if not most, of the tallest highway interchanges in the world are right here in the Lone Star State. Let’s talk about why. I’m Grady, and this is Practical Engineering.

The idea of a freeway really started in the 1920s with what’s now the Autostrada A8 in Italy: an automobile-only road with controlled access. Freeways are separated from local roads with limited ways to get on and off. And if you’ve driven a vehicle in the past century, the idea of a controlled-access freeway is pretty much taken for granted. Smooth curves and limited chances to enter or exit mean more speed and more capacity. But eventually, those big roads intersect other roads (sometimes other big roads) and that creates an obvious challenge.

Unlike most roads that cross at the same level on the ground, or as engineers say, “at grade,” freeways use grade separation at intersections. Roads go over or under one another. No traffic signals, stopping, or interruptions. Again, this is nothing groundbreaking. But what if you want to turn from one road onto the other? Just like that, we’ve gone from an intersection to an interchange. And this is where things get a lot more complicated. But we have to build up to it.

The diamond interchange is probably the simplest way to get grade separation because it kind of half doesn’t. Through traffic on the freeway flows right by, in most cases without any need to slow down. But that’s not true at the crossroad. Ramps enter and leave the highway at gentle angles and meet the crossroad nearly at right angles. Viewed from above, the ramps form a rough diamond shape, giving the interchange its name. The intersections of the ramps and the crossroad are just that: intersections. They are usually controlled by stop signs or traffic signals. Diamond interchanges can often get away with having just one bridge, a relatively small one carrying the crossroad over the highway. So, this can be the cheapest and easiest to build type of interchange to build. But, those intersections create limitations on how much traffic it can handle, so it’s really only used when the cross road is a minor one.

This kind of interchange is sometimes called a service interchange, in contrast to a system interchange, where two controlled access highways cross. As traffic increases, the only way to increase capacity is to eliminate at-grade intersections. So, the largest interchanges implement grade separation for every lane. The classic system interchange is the cloverleaf. Four ramps form a diamond, usually for the right-hand turns. These are directional ramps, that is, they curve toward the ultimate direction a traveler is trying to go. You exit right and end up driving to the right. The OTHER four ramps give the cloverleaf its name. The loop ramps, usually used for left-hand turns, curve around while ascending or descending so they can cross over themselves. So, you can get traffic flowing in any direction with no at-grade intersections and just one bridge.

The loop ramps make the whole thing look like a four-leafed clover, but finding yourself on this type of interchange doesn’t usually feel very lucky. For one, the loops are often pretty tight, requiring motorists to slow way down. And for two, there’s the weave. Consider traffic entering the highway from one of the loops. In the same place vehicles are trying to get back up to speed and merge left onto the freeway, drivers trying to exit the highway are slowing down and moving right. This inevitably creates traffic as people struggle to merge and cross paths with one another. Along with suboptimal traffic conditions, cloverleaf interchanges eat up a lot of of land. When cloverleaf interchanges were at their height of popularity in the mid-20th century, land was plentiful, and there were fewer cars, but as the volume of traffic increased AND the cost of land went up, engineers had to come up with new solutions to build better grade-separated highway crossings. And so they did.

Now, there’s such a huge variety of freeway interchange designs that it would be impossible to cover them all. The turbine, the windmill, the braided interchange, the ITL, mixes of various designs, and more. Each of these balances the constraints of a project like this in a different way: land requirements, cost, capacity, safety, et cetera. And the design that generally provides the most capacity, on the smallest footprint, (often for the highest cost), is the stack.

Like the cloverleaf, a stack has the four directional ramps, usually for the right-hand turns. But we move the exit for the left-hand turn off the main highway to avoid the weaving problem, and fly them over the middle of the intersection where they meet up with the opposite directional ramp. These ramps are often called flyovers, and it’s easy to see why. The gentle curves and elevation changes of the stack mean that drivers can safely maintain speed whether they’re going straight through the interchange or changing direction. The curved ramps often bank to the inside of the curve, called superelevation, making it even easier to maintain speed through the turn. This conventional configuration is called a four-level stack. There’s one level for the freeway, another for the crossing freeway to pass over, and two levels for the flyovers. It’s bridges on bridges, each one providing enough clearance underneath for large trucks. So these upper ramps end up pretty high off the ground. Four-level stacks are actually fairly ubiquitous in the US these days. These are impressive structures in their own right, but this is where Texas takes it to another level, literally. And it mostly has to do with feeder or frontage roads.

Lots of highways use frontage roads running parallel to connect areas alongside that would otherwise be cut off from the roadway network. They allow businesses to develop right up to and facing the freeway with easy access to those coming on and off it, basically keeping areas attached to the roadway network. Texas took the idea and ran with it. Apparently, they started as a way to reduce the cost of acquiring land for road projects. If you could promise the landowner access to a new highway along a frontage road, you're making their property more valuable, so they’re willing to sell a portion for the highway at a much lower cost. Now, Texas has over 6,400 miles (or 10,300 kilometers) of frontage roads. That’s almost the circumference of the moon, and as far as I can tell, way more than any other state in the US. I won’t go into the pros and cons of this approach here. Some research has shown pretty conclusively that the money saved on acquisition costs doesn’t make up for their many disadvantages. And Texas has since changed its policy to only include frontage roads on new freeways where necessary and justified. Although, from what I can tell seeing new construction these days, there don’t seem to be many projects where they’ve been left out. And one major effect of putting frontage roads alongside every highway happens at interchanges. Because these are more roads that need grade separation from all the others. So, at stack interchanges around the state, there aren’t just four levels but five.

In fact, this kind of interchange is often referred to as the Texas stack because it's so popular here. In a typical configuration, one freeway goes below grade at the bottom level. The frontage roads sit at grade. The crossing freeway is elevated. Then there are the two layers of flyovers. With a minimum vertical clearance of 16 feet or about 5 meters, plus the thickness of each bridge, vehicles on the highest flyovers are often more than a hundred feet or 30 meters above the ground. It’s a nice way to get a good look at the city, even if you only get to enjoy the view for a moment.

The Dallas High Five is probably the most famous interchange in Texas with its cool nickname, but it doesn’t stand alone. There are quite a few five-level stacks around the state and even a couple that qualify as six-level stacks with flyovers connecting to other highways. My friend Brian, better known as the Texas Highway Man, documents a lot of new construction in Texas, including this replacement of an old cloverleaf crossing with a five-level stack in San Antonio. These flyovers will be higher than a twelve-story building when they’re done. The frontage roads for this new interchange use a pretty innovative concept. Four partial roundabouts morph into one funny-shaped roundabout that’s been lovingly nicknamed the “fidget spinner.”

Of course, Texas stacks don’t exist only in the Lone Star State. The Big I is another famous interchange in Albuquerque decorated with a tumbleweed snowman each winter. The Judge Harry Pregerson Interchange in Los Angeles gets its fifth level not for frontage roads but the high occupancy lane. Plus, it has a railroad at the lowest level, which I always appreciate. Not just because I like trains, but also because it’s a reminder that these artfully sculpted ribbons of concrete carefully woven together represent a tremendous investment of public money, our money, into a way of getting people from A to B that has a lot of downsides. Everyone has different thoughts about what a city should look like, but there’s a growing recognition that the way we prioritize motor vehicle traffic in the US may not have been the best path forward. And so, I admit that my ideal city has a lot fewer of these towering interchanges that kind of stand as a testament to a transportation network that doesn’t necessarily reflect our highest values and aspirations. But, I still find them pretty impressive in their own right, and whenever I’m in a new city, I try to plan my driving to hit those tallest ramps at the top of the stack to get a bigger, if momentary, perspective on the built environment. It’s always a nice reminder of our capacity for grand designs and ambitious projects, even if they might not always be the best solutions.