1.7) The two US Highways of interest are 90 and 67. Presidio is a large, spread-out town, and quite busy as it has the only official border crossing from USA into Mexico for 450 miles between El Paso and Del Rio. The main road north, US 67, is a well-traveled 4-lane highway that crosses the edge of the Chinati Mountains which offer some good views over the desert to the east but there is little else of interest along the long journey to the next town, Marfa on US 90. (Texas Travelers Guide: http://www.americansouthwest.net/texas/big_bend_ranch/location.html)
US Highways were the primary travel routes of the USA from their adoption by AASHO on November 11, 1926 until the signing of the bill creating the National System of Interstate and Defense Highways on June 29, 1956. Up until 1927, there were only about 44,000 paved roads, and most other roads were nothing more than dirt roads with ruts which tended to become muddy and impassable when it rained in most areas. (www.us-highways.com/)
In the Rand-McNally road map for 1927, only Highway 90 through Marfa is indicated. By the time the 1938 version of the map was printed, Highway 67 from Marfa to Presidio was included. According to the table at www.us-highways.com, it was a 'North-South' highway that originated in MO and Ar and ended in Dallas Texas in 1927. But then in 1934, a northward extension was added that ended in Presidio and included IA and IL.(www.us-highways.com/) . Year 1928 Map, Year 1938 Map
An old map of the area from 1895 shows the old road that became Highway 90, but no road connecting to Presidio. The population of Presidio County at that time was listed as '316' people. (www.livgenmi.com/presidioTX.htm)
The Lay of the Land:
From the topo map, I marked spots on Highway 67 every 1.6 kilometers from the intersection with Highway 90 in town. I used the topo map to get the elevation of the road, and created the following plot: below. Note the Viewing area is at 4900 feet. There was no data for the roadway for the first 5 kilometers. The roadway does a very gradual dip to 4450 feet by 10 kilometers near the junction of Highway 165. Then after 2 kilometers it begins a gentile rise at 12 kilometers at a rate of 300 feet in 6 kilometers until it reaches a ridge at the end of the valley and rises sharply by 100 feet in 1.6 kilometers. This is the point at which lights from the road may first be seen, and they follow a descending path northbound to Marfa on Highway 67. In the following plot '0.0' is in thetown of Marfa; 20 is out at the ridge.
I have also used the same points and measured the angle of each point as seen from the Viewing Area. Each 1.6 km on the road takes 1.01 minutes driving at 60 mph...the likely road speed. Note at 20 km, near the descending pass, the car will seem to stand still for 3-4 minutes! It will probably be hard to see it shift from 38 degrees to 35 degrees over a time of 5 minutes. From the time a car is spotted, it takes nearly 20 minutes for it to travel the 35 degrees from there to the due-west direction of Marfa. In terms of a clock face with Noon at south and West at 3:00, cars would be spotted at about 2:00.
It is commonly said that cars can be seen as bright lights on Highway 67, even though the road is no closer than the 8.8 miles to Marfa, and on the line of sight to the ridge at the 24 km mark, it is 36.8 km or 23 miles distant. I am amazed that anyone can see ordinary car headlights at 23 miles!
I have calculated how far apart a pair of headlights would appear at various distances, based on a 6-foot separation and viewed at the angles defined by the road and the line-of-sight. Even a full-on view at 1 mile provides a separation of just under 4 arcminutes. I have seen cars on Maryland Highway 95 at one mile distance and the lights are just at the limit of being easily separable. At 20 miles, the separation is 12 arcseconds and unresolvable to the naked eye. So, any car at the ridge would not be resolvable as a pair of headlights. Two cars at that distance, separated by 30 feet, or 3 car-lengths, would however appear resolvable as two INDIVIDUAL lights. The figure below shows the angular separation of two headlights at the appropriate, instantaneous, viewing angle. I calculated this by drawing a line to the particular marker on Highway 67. I measured the angle between the roadway and the line-of-sight at that point. If the angle is zero, you are viewing the headlights head-on. This happens near the Ridge at 30 kilometers (19 miles) where the road between markers 18, 19 and 20 is along the line-of-sight so you get the beams head-on with their full separation. Because headlights are directional within a cone that is about 45-degrees to the normal, the brightness of the lights should begin to fade noticably once the angle below becomes more than 45 degrees as the car approaches Marfa. At about 1/2 the way down Highway 67 from where the car was first spotted, the lights should begin to fade.
This next plot is the actual headlight separation along Highway 67 from the above data. The human eye cannot see things closer than about 60-120 arcseconds, so this means that only when the cars are close to Marfa and out of the viewing region for the lights, do you actually discern that they are pairs of lights. The rest of the time, they look like unresolved 'stars'.
The following figure shows the angle at the road marker between the road surface, and the line-of-sight from the Viewing Area. Notice that the angle is very shallow! For all but the last 5x1.6 km = 8 kilometers into Marfa, the angle is under 0.5 degrees! No wonder people claim that roadway heating may cause a mirage effect! These conditions, on a daytime-heated road surface radiating after sunset, seem like they are possible at these shallow roadway angles!
Copyright (C) 2001Dr. Sten Odenwald