Highways with unusual milepost schemes

[Pink Jazz]

I would like to know, could anyone tell me of any highways using unusual milepost schemes?  Such highways could include mileposts increasing backwards (north to south or east to west, rather than south to north or west to east), or some other weird scheme.

I know that SR 85 in Arizona has an unusual milepost scheme, with the zero point at Gila Bend where mileposts increase going both north and south from there.

[Alex]

Texas SH 130 sees mileposts in the 400s east of Austin.

[Pink Jazz]

Also, on PR-22 in Puerto Rico, the kilometer posts increase east to west.  I presume the reason for this is for DTOP won't have to re-post everything if the extension to Aguadilla is ever built.

[hbelkins]

I would like to know, could anyone tell me of any highways using unusual milepost schemes?  Such highways could include mileposts increasing backwards (north to south or east to west, rather than south to north or west to east), or some other weird scheme.

I know that SR 85 in Arizona has an unusual milepost scheme, with the zero point at Gila Bend where mileposts increase going both north and south from there.

I-795 in North Carolina. Starts at 0 on the north end and increases as you go south.

[oscar]

Alaska is full of milepost weirdness.  Many highways have mile zeros on other highways, most notably the Alaska Highway (part of AK 2) whose mile zero is in northern British Columbia, and the highest conventional milemarker is 1421 (plus an elaborate mile 1422 monument at highway's end in Delta Junction).  The Parks Highway (AK 3) starts in Palmer AK, but its mile zero is about 35 miles south in downtown Anchorage. 

AK 1's mile zero is at the south end of AK 9 in Seward.  That applies to both the Seward Highway segment east of Tern Lake Junction, and the Sterling Highway segment west of Tern Lake (Alaska mileposts are keyed to named rather than numbered highways).  So travelers on AK 1 see mileposts descending to around 38 as they approach the junction (whether coming on the Sterling Highway from Homer, or the Seward Highway from Anchorage), then the mileposts start increasing again on the other side of the junction.

Keying mileposts to named rather than numbered highways also results in numbered highway mileposts resetting as they switch from one named highway to another.  For example, AK 2 is milemarked east to west on its Alaska Highway segment, then the mileposts reset in Delta Junction as AK 2 switches to the Richardson Highway, then they reset again in Fairbanks when AK 2 switches to the Steese Highway, with Steese mileposting continuing in Fox on both AK 2's continuation on the Elliott Highway and AK 6's continuation of the Steese Highway.

The milemarkers on the Richardson Highway (all of AK 4 plus part of AK 2) start at its old south end at the old Valdez townsite, four miles east of the highway's current south end.  When Valdez was moved after it got trashed in a 1964 tsunami, the highway was extended, but there are no mileposts on the extension and all the other mileposts were left as is.  Since people use mileposts as street addresses, Alaska DOT&PF is especially reluctant to recalibrate mileposts.

[pianocello]

IIRC, US 75 in Texas has numbers that start at 200 at the Red River and increase going southward.

I seem to recall reading somewhere that all non-Interstate mile markers in Texas are unconventional in some way, I just don't quite remember the method.

[NE2]

Arizona usually keeps mileposts going on branches, with some rather odd results (most obviously I-17). The 77/79 and 77/177 splits show the general idea well. And SR 85 north of Gila Bend is numbered as former US 80 (unless it's changed recently).
http://www.azdot.gov/docs/business/state-milepost-map.pdf

Texas does the weird thing on all non-Interstates. Someone else can explain it.

[NE2]

I-795 in North Carolina. Starts at 0 on the north end and increases as you go south.

That's not unusual. Spur Interstates are supposed to start at the parent.

[1995hoo]

A portion of VA-294, the Prince William Parkway, has kilometre posts running from east to west. The markers are also erroneous because they incorrectly abbreviate it as "KM" instead of the proper "km."

[J N Winkler]

Canned explanation of Texas non-Interstate milepointing:

*  The basis of the Texas uniform reference marker system is two lines.  One is east-west and parallels the top edge of the Texas Panhandle ten miles to the north, while the other is north-south and passes through a point ten miles due west of the far western tip of the state near El Paso.

*  Each segment of state highway has an origin point whose value is equal to that point's distance in miles from the east-west line (if the route runs north-south) or from the north-south line (if the route runs east-west).  All reference markers on the state highway segment count up from this origin point according to miles along the route centerline.  East-west routes count up going east and north-south routes count up going south.

Routes like SH 130 that have mileposts as their reference markers are actually fairly rare, and this usually occurs in coordination with exit numbering.  The typical reference marker on a Texas state highway is three digits (white on green, Series D, zero fills used as necessary) mounted just below a route marker on the same post.

The ten-mile offset built into the reference marker system ensures that only Interstates can have mileposts below 10 in Texas.

[jeffandnicole]

In NJ (Both East to West):
   I-76
   AC Expressway

[NE2]

Canned explanation of Texas non-Interstate milepointing:

One thing I haven't been able to determine: do these reset at county lines?

Also, it would be possible to have a number less than 10 if a route is extended west or south from its original endpoint, and the extension is rather indirect. Unless they always renumber after extending.

[lordsutch]

One thing I haven't been able to determine: do these reset at county lines?

Nope, Texas mileposts are continuous across county lines. Typically you get a reassurance marker along with the milepost every 2 centerline miles.

[dfwmapper]

In Texas, US 75's exit numbering starts with 1A (the exit to Woodall Rodgers/Spur 366) and climbs to 75 (the welcome center just over the river from Oklahoma). I've never paid attention to the markers.

[J N Winkler]

One thing I haven't been able to determine: do these reset at county lines?

As Lordsutch says--No.  The numbering is supposed to be continuous and statewide.  However, TxDOT's documentation is unclear on whether a given route can have multiple origin points.  I suspect this is the case because it offers a convenient way of handling milepointing for routes that exist in multiple pieces in Texas (e.g. US 54, US 62, US 180), but it does create at least a theoretical possibility that a route of sufficiently circuitous alignment could have duplicate markers measured from different origin points.

Also, it would be possible to have a number less than 10 if a route is extended west or south from its original endpoint, and the extension is rather indirect. Unless they always renumber after extending.

This is a nitpick, but in the case of north-south routes, the situation you describe would need a northward extension to arise.

I don't know how TxDOT handles such awkward extensions in its scheme.  I do know, however, that reference marker values less than 10 are explicitly forbidden.  In principle they could handle an extension by defining a new origin point specifically for it, but this could easily result in duplicate marker values and I don't know how those are handled.  For that matter, I don't know how they deal with alignment changes that affect mile progression.  The documentation just talks about programming in new reference markers, but says nothing about ahead/back mile equations (a very common technique for other state DOTs) or postmile prefixes (the Caltrans solution).

It has been reported in MTR that zero reference markers exist and can be found deep in the interior of Texas.  This is clearly impossible with the current system.  However, the former system was based on control sections (milepointing reset at each control section terminus), and the current one was devised in 1990 by remapping the control-section mileposts then existing for each route onto the route length across the entire state.  Since I don't think route-based control sections straddle county lines, presumably it would have been normal to see zero markers at county lines.  Stragglers that somehow survived in the field after the 1990 conversion may have been the basis of the MTR report.

[KG909]

Interstate 19 has its markers in kilometers

[SignGeek101]

Interstate 19 has its markers in kilometers

It has mileposts installed as well.

Doesn't Delaware SR 1 use km based exits? I believe it uses standard mile posts though. I guess that can be considered unusual.

[KG909]

Interstate 19 has its markers in kilometers

It has mileposts installed as well.

Doesn't Delaware SR 1 use km based exits? I believe it uses standard mile posts though. I guess that can be considered unusual.

Oh I haven't been on it since like 2007, before mileposts. Also I'm not sure about Delaware route 1, I still don't know that much about other roads.

[Alex]

Interstate 19 has its markers in kilometers

It has mileposts installed as well.

Doesn't Delaware SR 1 use km based exits? I believe it uses standard mile posts though. I guess that can be considered unusual.

Oh I haven't been on it since like 2007, before mileposts. Also I'm not sure about Delaware route 1, I still don't know that much about other roads.

Yes, exit numbers along the SR 1 Turnpike are in kilometers. Nothing else remains metric along the turnpike, though it was fully signed with those units when the Dover to Smyrna portion opened to traffic in December 1993.

[Henry]

The mileposts and exit numbers on the I-90 part of the NY State Thruway increase to the west instead of to the east, due to their continuing from the I-87 part that starts in Yonkers, north of NYC.

[PHLBOS]

When the earlier portions of DE 1 first opened, the exit numbers were originally mile-marker based but then changed to kM-based exit numbers shortly thereafter.  IIRC, the highest exit number (near or at I-95) used to be just over 100; then it jumped to the 160s.

[vdeane]

What is the rationale for Texas banning mileage below 10 on non-interstates?

The mileposts and exit numbers on the I-90 part of the NY State Thruway increase to the west instead of to the east, due to their continuing from the I-87 part that starts in Yonkers, north of NYC.

Technically that's just the Thruway mileage, so another example of mileposts following a named road rather than the interstate number.  They're all over.

More interesting: the Garden State Parkway's mileposts in NY are backwards because it's considered part of the Thruway.

[wxfree]

Canned explanation of Texas non-Interstate milepointing:

One thing I haven't been able to determine: do these reset at county lines?

In the direction of increasing numbers (east or south) the next even number will be posted at a county line, regardless of the distance from the previous marker.  They make exceptions when the normal placement of a marker is close to the county line (I've seen them a few hundred feet off, but don't know what the limit is).  These county line reference markers are on the right side of the road when viewed traveling in the direction of increasing numbers.  From there, they alternate between left and right, except on some divided roads that have markers on both sides every two miles.  This means that sometimes there are two consecutive markers on the south or west side of the road, and that the numbers add up faster than mileage does.  I can't find the procedure for placing markers on county lines in the manual, but they clearly do.  You can see marker placement on this map http://www.txdot.gov/apps/statewide_mapping/StatewidePlanningMap.html

Also, it would be possible to have a number less than 10 if a route is extended west or south from its original endpoint, and the extension is rather indirect. Unless they always renumber after extending.

If you open the map and see the first marker on US 83, it's 10 on the Oklahoma state line.  In order for the number to be below 10, the highway would have to be "extended" into Oklahoma.  In the west, the map shows a mile 10 on Loop 375 east of the state line.  I don't know what they'd do if that highway were extended westward.  One of the rules is that markers are generally not to be moved, even if route changes make their numbers inaccurate in terms of mileage.  They're "reference markers," not "mile markers" (they're routinely inaccurate across county lines, anyway).  Having read the manual, my guess is that they'd either make an exception and change the 10 to 10A and move it to the west end and have a more than two mile gap between 10A and 12, or they'd move mile 10 to the new west end and change the existing 10 to 11 (odd numbers are used to fill gaps in some situations).

A useful map showing how the strange grid system works is here http://onlinemanuals.txdot.gov/txdotmanuals/trm/reference_markers_coordinates.htm 

This all leads to the question of "why?".  I have no idea where the idea for the grid system comes from, but the ritualistic placement seems to be based on a wish never to move the markers after they're placed (presumably for purposes of consistency in identifying any particular point on a road).  That intent is evident multiple times, such as when two routes are combined and the markers along the cancelled designation are left in place regardless of inaccurate mileage and renumbered according to the new designation.  http://onlinemanuals.txdot.gov/txdotmanuals/trm/reference_marker_installation_procedures.htm#i1009720

[J N Winkler]

What is the rationale for Texas banning mileage below 10 on non-interstates?

The documentation doesn't explain, but since reference marker values are posted vertically with zero fills, I suspect the motivation is to build in a reality check in the event someone reads a reference marker from bottom to top instead of top to bottom.  If marker values in the zero-to-ten range were permitted, you could have marker values in the northwestern corner of the Panhandle where "002" was intended (which is plausible for a north-south route) but "200" was read instead (which is plausible for an east-west route).

Bit of trivia:  in its documentation, TxDOT uses US 82 across Texas as an example of the reference marker system in operation.  Lowest marker value is 220 at the Yoakum County line (western edge of the Texas Panhandle) and highest marker value is 798 in Texarkana.  This is a difference of 578 miles.  However, Google Maps gives the length of US 82 across Texas as 565 miles.  Where did the extra 13 miles go?

(Edit--Wxfree's post, which came through as I was composing this one, offers a possible explanation for the 13-mile discrepancy.)

[J N Winkler]

Also, it would be possible to have a number less than 10 if a route is extended west or south from its original endpoint, and the extension is rather indirect. Unless they always renumber after extending.

If you open the map and see the first marker on US 83, it's 10 on the Oklahoma state line.  In order for the number to be below 10, the highway would have to be "extended" into Oklahoma.

The difficulty NE2 describes would never arise in the case of a route that starts at the state line.  Rather, it would arise in the case of a route that starts X distance from the state line at a point where the latter is fairly near one of the axes of the marker system--say, the western tip near El Paso, or the northern edge of the Panhandle.  If the length of the extension is greater than X, then you have a situation where a marker value less than 10 (which is not permissible) must be used if marker values on the extension are generated by counting back from the origin point.

It is quite easy to produce an extension with this characteristic.  All you have to do is to follow a circuitous path back to the state line, e.g. by winding back and forth across multiple switchbacks, or going straight back to within a mile or two of the state line and then turning at a right angle to follow it for an extended distance.

This all leads to the question of "why?".  I have no idea where the idea for the grid system comes from, but the ritualistic placement seems to be based on a wish never to move the markers after they're placed (presumably for purposes of consistency in identifying any particular point on a road).  That intent is evident multiple times, such as when two routes are combined and the markers along the cancelled designation are left in place regardless of inaccurate mileage and renumbered according to the new designation.  http://onlinemanuals.txdot.gov/txdotmanuals/trm/reference_marker_installation_procedures.htm#i1009720

Yes, the purpose of the marker system is to allow feature descriptions and highway performance data (such as crash reports) to be bound to a route, marker value, and offset from marker.  Where a route is renumbered but no alignment change (due to relocation, etc.) is involved, it is convenient for the only transformation applied to a marker to be change of the route and marker value (i.e., no translation along route centerline), as this allows feature and performance data pre-dating the renumbering to be directly compared to that following the renumbering.

Relocations typically involve not just changes in alignment but also upgrades to features that affect the safety performance of the road, so before/after performance data cannot be compared on the same basis.  In such cases there is provision for resurveying the markers and communicating their new locations to TxDOT TPP for inclusion on the statewide marker map.