The section of Interstate 40 between Tucumcari and Glenrio was one of the last to be built. Several people told me that the towns along that stretch (including San Jon, Endee, etc.) were hesitant to see the freeway get built in that it might hurt businesses in those towns. It was a long time before construction began on that stretch of Interstate 40. Here's some information from Jim Williams about that section of Interstate 40 and about an abandoned US 66 bridge you can see near Glenrio:

"The most recent US 66 (pre-Interstate) alignment closely followed the current Interstate alignment. I think that the north frontage road is the old two-lane highway but I'm not sure. Just before entering Texas, the old alignment veered to the south to enter Glenrio directly. You can still see the old bridge from the Interstate (near the New Mexico welcome center/rest area). I'm guessing that this is the bridge you were talking about. I don't think that you can drive to it, but I image you can walk to it from Glenrio.

The earlier alignment, which was replaced back in the 1950s (or maybe 1940s), is now drivable. From San Jon, take the south frontage road east and just before the Bard exit from the freeway, the frontage road turns due east (leaving the Interstate), crosses NM 93 and goes through Glenrio. It is unpaved but in good condition (I drove it back in May on a trip to Albuquerque).

As you leave Glenrio going to the west, the paved highway starts to turn to the north (the alignment abandoned in the 1970s when the Interstate was built), but it is fenced off. (It is from here that I suppose you could walk to the old bridge mentioned above.) At this point, you can turn to the south and follow the even older alignment."

I had also asked Jim about the New Mexico pratice of building freeways with crossovers and 1/2 freeways where only one section of main lanes went in and then the rest was completed several years later. Here's what Jim told me:

"When the Interstate standards were first drawn up, 2 lane sections as the ultimate design were permitted. The FHWA (actually BPR back then) published a map of the ultimate Interstate system (to be finished by 1969!) that showed a large number of two lane sections in the west. Legislation in the mid-1960s changed that to require minimum four-lane divided cross sections. Many sections of Interstate in the west were built in stages. For example, on the section of I-25 south of San Antonio in New Mexico, the southbound lanes were built first, and the northbound lanes built several years later. The southbound lanes carried two-way traffic until the northbound lanes were completed. In this case, the old highway was on an alignment too far away to be used as one of the freeway roadways, even temporarily.

However, I-40 was built in stages west of Albuquerque, out just past Correo. US 66 was first expanded to a four-lane divided highway by using the original two lane highway for west bound traffic, and the new parallel roadway used for eastbound traffic. When this section was upgraded to Interstate status, new eastbound lanes were built, the old eastbound lanes were upgraded and converted to westbound lanes, and the old westbound lanes (which was the original two-lane highway) was used as the frontage road. At the Rio Puerco crossing (midway between Albuquerque and Correo), you can easily see this by the old bridge on the frontage road.

To my knowledge, there is only one 2-lane section that is considered complete in the country. It is on I-93 in New Hampshire where it is going through an environmentally sensitive area.

Talking about interstates with crossovers:

Regarding the at-grade crossings you noticed on I-40 just on the Texas side: Interstate standards allow at-grade crossings as long as the traffic volume on the side street is very, very low. These ranch entrances certainly fill that bill. The other at-grade crossings that I knew of have all been eliminated. There was one on I-10 east of El Paso that was essentially signed as an exit with the big green signs. There was the standard advance guide sign (town name, exit 1 mile), but instead of an exit direction sign with the upward slanting arrow, the sign was at the cross over with an arrow pointing to the left. This was replaced with a diamond interchange in the late 70s or early 80s."

Jim and others also told me not to rely on the earliest maps to show "controlled access" interstate highways in New Mexico. For example, the section of Interstate 40 near Clines Corner was not always controlled access, but maps showed it as interstate.

From Jim: "The piece of I-40 from about 10 miles west of Clines Corners to a few miles west of Santa Rosa was marked as I-40 (and US 66)for many years, but had many crossovers, even where state highways crossed (NM 3, as I recall). The highway department did an upgrade in the 1970s on that piece of freeway. Prior to the upgrade, eastbound drivers just crossed the median (and the westbound lanes) to get to Clines Corners. After the upgrade, these drivers have to get off at US 285, cross over I-40 and backtrack a bit on the frontage road."