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 there's no single answer. I've spoken to several engineers about this and they give a couple of reasons.

First, many interchanges in Texas are built after development has already hemmed-in the right-of-way. This can be either older interchanges that are being replaced, or an interchange is being built to connect to a new freeway. In either case, because the adjacent property is already developed, it would be costly and disruptive to acquire that additional land. So, when you can't build out, you have to build up in order to maintain the necessary radii for the connectors to facilitate safe and fast traffic flow.

The constrained footprint also requires the flyovers to be concentrated in the center of the crossing, which means they have to stack directly on top of each other. With a typical minimum clearance of 16 feet, plus 5 feet or so for the bridge beams, that results in 21 feet or so for each level, which adds up to about 85 feet minimum for a basic stack interchange.

The second reason is Texas' ubiquitous frontage roads. In many cases, accommodating frontage roads requires adding another level to an interchange, which can add 20-25 feet 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 equally tall interchanges now in other states.

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 rules:

• 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
	EB 1604 to WB 10 Near center of interchange to lower level access roads (Under construction)	125'
	WB 1604 to EB 10 Near center of interchange to lower level access roads (Under construction)	125'
East	WB to SB Over EB 10 mainlanes (Under construction)	120'
	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 access road	85'
	WB 410 to EB 10 Over EB access road	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 available, so I don't have height information on them yet.