Don't know how thick the clouds were, but Iridium flares are pretty bright and last for a bit.https://en.wikipedia.org/wiki/Satellite_flare
I think the clouds were the outflow from Hurricane Sandra - pretty thin by the time they got to Texas, but they would have illuminated if lit from behind. Here's why it wouldn't have been an iridium flare.
* It was too bright (I've seen iridium flares, and while they can be quite bright at the peak, they are never really bright for several minutes.
* The time was about an hour early for an iridium flare. According to an iridium flare forecasting site, there was one visible in the area (assuming no overcast), about an hour later.http://heavens-above.com/flaredetails.a ... t=0&tz=CST
I was recording a gps log at the time. I just opened it up to confirm I didn't remember the time one hour off for some reason. Looking up the log, I realize the highway was heading just east of south at that point (between Old Settlers Boulevard and 79 on I-35 in Round Rock, Texas, not just west of south as I had thought. That doesn't really change things much. Whatever I saw was angled a couple degrees left of my own heading.