I-10 in Texas... some wierd nagging questions.

I just moved to San Angelo, Texas, from Monterey California. I notices as soon as I passed the TX border, I began to see signs on the side of the road saying “HC” in a circle. What is that?

Also, along I 10 between El Paso and Fort Stockton, we kept seeing sections of 2 laned, two way highway that would randomly start and stop alongside the interstate. There were points where you could access the roads at the exits, and they were inside of the fence that runs alongside all interstate highways. At first I thought perhaps they’d just build the interstate alongside old 2 lane highways, and never bothered removing them, but they don’t seem decrepid, and they have fairly new roadsigns and lane markings.

What’s up with these?