highest milepost in Texas: US-67 in Presidio has, IIRC, 987.
It runs less than 700 miles through the state, so how did they get a milepost number of 987?
a similar mileposting system to Arizona. US-89 goes up to something like 604 in Arizona.
I've looked at this for two minutes and still don't understand it. Seems needlessly confusing to me.
It is somewhat complicated. They're called reference markers, not mile markers, because they don't represent actual mileage (except when within a single county when there has been no rerouting). For US 67, measure the distance from the given latitude (the latitude of the given point) directly south to the northernmost point of the highway (where it enters the state). Remember that the beginning point is 10, not 0(a highways starting at the state line at the north end of the Panhandle has an initial number of 10). A marker is placed every two miles, with the number (almost always even) increasing by two. They are generally placed on alternating sides of the highway, with one on each side every four miles (the number is displayed on both sides of the sign post and can be seen from both directions during daylight).
An additional complication is that the measurement starts again at each county line (unless the marker falls close to that line, in which case the marker will be set accurately according to mileage but not directly on the county line). If the last marker from the north is 486, and it's one-half mile to the county line, the next marker, 488, will be directly on the county line. This results in numbers that grow faster than mileage.
Also, once the marker is placed, it is not moved. If a bypass is built, the original markers downstream from it are not moved, and no longer represent accurate mileage from the markers upstream from the bypass. The markers along the bypass are not placed every two miles, but are spaced evenly to fill the gap in the numbers. Markers along the bypass that are not spaced at 2 mile intervals have an A used as a suffix (such as 486A while the marker along the business route retains the number 486).
To see the placements and numbers of markers, go to the Statewide Planning Map, and select Markers near the bottom of the list. Note that Interstate highways have actual mile markers accurately placed every mile, but not all of them are shown on the map. Also, markers are not shown on the map along business routes, although in my experience they are left in place.
http://www.txdot.gov/apps/statewide_mapping/StatewidePlanningMap.html
