US 281/Loop 1604 interchange

(Photo source: Google Maps)

One question I get with some regularity regards the height of local interchange flyovers. Texas is notable in that the state builds spectacularly tall interchange flyovers, and their height is often a topic of curiosity. San Antonio was a bit of a latecomer to the high-flying interchange party, with the first modern "Texas-style" stack interchanges starting to rise from the ground here in the first decade of this century. Prior to that, the tallest interchanges here topped out at about 65 feet. But there are now five interchanges that have at least one flyover that's 80 feet tall or higher, and two more under construction will have flyovers at least 120 feet tall, which will be the tallest in the area and among the tallest in the state.

Given that, I thought it was time to compile a list of the tallest interchange flyovers in San Antonio.

Why are Texas flyovers so high?

Before delving into the height of local interchanges, I have to try to answer the question that inevitably comes-up: why are Texas interchange flyovers so tall?

If you search the Interwebs, you'll find that, like many things, there's not a single answer, but rather a combination of factors.

First, this type of interchange, known as a "stack" interchange, is widely used for urban freeway junctions. San Antonio has three older generation stacks built in the 1960s: two on I‑10 south of downtown, and one at I‑37 and Loop 410. But these only top-out at about 65 feet each, and there are a couple of reasons these are not nearly as tall as newer stack interchanges. First, they were generally only built with the minimum clearance between levels (15 or 16 feet), whereas with modern interchanges, engineers typically provide an additional few of feet of clearance between levels for added safety and as a requirement of the state's designated freight network. Second, those older interchanges were built with shorter bridge spans using smaller girders, typically less than five feet in height. Today's interchanges use fewer columns and longer spans. Those longer spans require heftier girders, oftentimes eight or nine feet high. These larger girders, plus the additional clearance, can add 10-15 additional feet per level and therefore are a big reason for the additional height in contemporary stack interchanges.

Another reason is that modern interchanges are designed to allow for higher speeds over their older counterparts. This requires additional banking of the ramps, which can add a few more feet of height. Also, it's desirable to provide a consistent grade across the entire span so that it can clear the the other levels without excessive undulation. This means sometimes it must go a little higher than would otherwise be needed in order to maintain that consistent grade. And, accommodating the terrain in and around the interchange can add a few feet.

But perhaps the most significant reason interchanges in Texas soar so high are Texas' ubiquitous frontage roads. Accommodating continuous frontage roads through an interchange in most cases requires adding another level to the interchange, resulting in five levels and adding 25 feet or more of additional height, pushing the height of such interchanges to over 100 feet.

It's worth mentioning that while tall interchanges are widely seen as a Texas phenomenon, you'll find similarly tall interchanges now in other states such as California and Florida.

I‑10/Loop 1604 interchange

(Photo by Marvin Pfeiffer/San Antonio Express-News)

Methodology

Determining the height of a particular flyover is not as easy as it may seem. Both the structure itself, as well as the ground below it, are not level, with elevations varying continuously across the length. On curves, banking will cause a significant variation in elevation laterally across the bridge deck. And on the ground, besides the varying terrain, one big question arises: should the extra height that a depressed roadway adds be included in the calculation?

So for my calculations, I used the following methodology:

• The elevation of the flyover was determined along the centerline of the roadway on the top surface of the bridge deck.
• The flyover height at that spot was calculated by subtracting that elevation from the elevation of the ground directly below, whether that's the natural ground level, on a man-made embankment, or in a depressed location.
• For flyovers that pass over roadways depressed below grade, I show two top heights for that flyover: one for the tallest point over the depressed roadway, and one for the tallest point over a location that's at the normal ground level. You can decide for yourself which counts. 😉
• I didn't calculate the elevations at specific intervals along every flyover. Instead, I field surveyed each interchange and determined anecdotally where the tallest spots were, then performed the measurements in that area using trial-and-error until I found the highest point.
• The elevations for completed interchanges were taken from Google Earth. To help ensure accuracy, I took multiple measurements from different angles. If there were discrepancies, I used my best judgment to reconcile them.
• For interchanges under construction, the heights come from bridge profile elevations in the construction plans.

Tallest interchange flyovers in San Antonio

The following table lists all flyovers in San Antonio over 80 feet tall, with the height at the highest point on the flyover shown. The table is sorted to keep all the flyovers in each interchange together with the interchanges ranked by their tallest flyover.

As mentioned above, if a flyover crosses over a depressed section of roadway, one height will be shown for the highest point over the depressed road, and one for the highest point over normal ground level along an adjacent roadway. Locations over a depressed roadway are marked with an asterisk (*).

INTERCHANGE	CONNECTOR	HEIGHT
	WB 1604 to EB 10 Over existing lower level Over future NB lower level access road (Under construction)	126' 128'
	EB 1604 to WB 10 Near center of interchange over lower level Park & Ride (Opened December 3, 2024)	123'
East	WB to SB Over EB 10 collector/distributor (Under construction)	122'
	WB to SB Over NB 281 mainlanes* Over eastern turnaround	109' 87'
	EB to NB Over SB 281 mainlanes* Over SB access road	100' 84'
	EB to NB Over SB access road* Over SB 410 mainlanes	93' 87'
	EB to NB Over WB access road	88'
	EB to SB North of Halm Blvd.	85'
	WB to SB Over EB 410 mainlanes.	81'
	SB to WB Over NW cloverleaf	88'
Northwest	EB 10 to EB 410 Over WB 410 mainlanes and collector/distributor	85'
	WB 410 to EB 10 Over EB collector/distributor	82'

Here are a few other interchanges that may be of interest:

• Loop 1604 at US 90: 66 feet over the westbound US 90 mainlanes
• Loop 410 at Bandera Rd: The westbound flyover to northbound Bandera is 55 feet over the western turnaround at Evers Rd.
• I‑37 at Loop 410: 65 feet at the center of the interchange
• I‑10 at I‑37: 65 feet at the center of the interchange
• I‑10/I‑35/US 90: 64 feet at the center of the interchange

The I‑35 NEX project will also have some tall flyovers; however, as a design-build project, the plans were not readily available, so I don't have height information on them yet.