US Interstate mile markers, how are they determined?

[somepumpkin]

TLDR: how can they assign (ending) mile markers when parts of the route have not been planned?

Around the turn of the century, we started to build a large highway (I-540) to loop within most of Wake County. As you can imagine, funding (as well as politics and lawsuits) have caused the completion to move at a snail’s pace. The first ~26 miles (Miles 1-26) or so were payed for by all of our tax dollars (thank you). The last ~20 miles (miles 53-70) have been completed, paid for with a bond and the road is managed as a toll way. There are still about 30 miles in the center that have yet to be completed. A good chunk of this missing highway is still being debated/planned including which route to take and who’s land to acquire. The point is they still don’t know exactly which route the road will follow.

As mentioned above, the completed section of the I-540 highway and its exits are signed with mile markers from 53-70 and then it starts over at mile 1, ending at mile 26. The miles from 1 to 26 raise no issue with me. However, how can they be so bold to declare miles 53-70 while some of the route is still under discussion?

[PastTense]

They just use their best guess. And if they are wrong they can either ignore it or change the signs (an extremely cheap thing to do).

[chacoguy]

Colorado had to change its 420 marker.

[DSYoungEsq]

Although the exact route won’t be known yet, the likelihood that it will substantially alter the total milage by any significant amount is very small. If it does, the mile markers can be re-sited.

It should be noted that mile-markers for I-74 in the south part of the state also exist, and have replaced the old US 74 markers, despite the fact that the exact route of that interstate across North Carolina is quite undetermined.

[Chronos]

I don’t know what US Route I-74 would be replacing, but it wouldn’t be 74. The Interstate system was deliberately set up such that they would never be close to US Routes with the same number, to avoid confusion.

[Ranger_Jeff]

Interstate mile markers and exit numbers start from the western state border on even numbered interstates and from the southern state border for odd numbered interstates.

[Tom_Tildrum]

@Chronos

I don’t know what US Route I-74 would be replacing, but it wouldn’t be 74. The Interstate system was deliberately set up such that they would never be close to US Routes with the same number, to avoid confusion.

There are exceptions. From Wikipedia:

Crossing the Pee Dee River and into the Sandhills region, US 74 meets up with Future Interstates 73/74, in Rockingham. After a future interchange near NC 38 that will end its overlap with Interstate 73, US 74/Future I-74 continues southeast, bypassing Laurinburg and Maxton. East of Maxton and through Lumberton, the highway is officially US 74/Interstate 74, before dropping back to Future I-74 west of Boardman; the concurrency with Future I-74 ends at Bolton, where a future interchange will split from US 74 to continue south towards South Carolina. This is one of only two instances (along with I-41 in Wisconsin) of similarly-numbered U.S. and Interstate routes being designated on the same road.

[N9IWP]

Drat, I was going to bring up I41. Though that was an interstate in all but name that got converted.

Brian

[Toledo_Jim]

Interestingly, I-69 in Indiana has a bit of a story: From when it was built and opened, it ran from Indianapolis to Detroit. The mileposts in Indiana started in Indy, and went to the Michigan line, at about milepost 157.

Then somebody decided to run an interstate from Indy SW to Evansville, in the lower corner of the state. Somebody else said it should be I-69. So milepost 1 would be just north of the Ohio river at Evansville. And also just outside of Indy, on the I-470 loop around Indy. Can’t have that now, can we? So all the mileposts (and exit signs, etc) north of Indy got 200 added to them. So exit 102 (near Fort Wayne) became 302. Exit 64 (Marion) is now 264. Etc. The actual addition is not 200 miles exactly, but some creative counting around the belt way was used to make it work. The beltway exits are numbered from I-70. The I-69 numbers run up from SW to the beltway (exit 188 I think) and pick up again at milepost 201 where mile post 1 used to be NE of Indy.

Actually there is still a gap of non-interstate from Bloomington to Indy, but the route is surveyed and distances are known.
Kind of odd.

[Mr_Downtown]

State DOTs sometimes fudge mile markers a bit. Several of the original two-lane tunnels along the Pennsylvania Turnpike were later replaced by roadway sections that wound around a bit as they went over rather than through the mountains, and thus lengthened the route a bit. Rather than renumber the entire turnpike, they moved a few mile markers to be 1.something miles apart.

[ftg]

The current starting-with-borders convention only goes back to the 70s or so.

There were various systems in places and the feds mandated a consistent one everywhere. So Interstate mile markers, exit numbers, etc. all got changed.

It happened again here and there when new sections were added.

Changing the mile markers/exit numbers isn’t a death sentence.

[DSYoungEsq]

@Chronos

I don’t know what US Route I-74 would be replacing, but it wouldn’t be 74. The Interstate system was deliberately set up such that they would never be close to US Routes with the same number, to avoid confusion.

You apparently have not had the joy of driving the combined I-74/US 74 in Southern North Carolina.

[somepumpkin]

Thanks, Guys. I could see folks driving past mile 39 to the next one labeled 43 and not notice.

The highway dept did something similar many years ago when they put a new I-40 around Winston-Salem. The miles before and after didn’t change but the route differs by a mile or so. I’ve not taken the time to watch each mile marker fly by in an effort to find the missing link or anything, though.

@chacoguy

Colorado had to change its 420 marker.

Ha! Well, it’s not that funny, really.

This quote from Snopes (concerning the 420 sign) was interesting.

“Obviously people steal these signs,” [CDOT spokeswoman Amy] Ford said. “In the past, if a sign was stolen too much, we wouldn’t replace it. This is sort of an innovative way for us to keep the sign there.”

“So this is our way to test it out. So far it’s working, said Ford. “It’s a traffic safety thing. It’s a helpful thing to have these signs on the road. But people kept ripping them off.”

Few highways run long enough in Colorado to merit such a sign. Two that do, U.S. 40 near Hugo and U.S. 50 near McClave, have no signs marking their 420th miles, according to Google Street View images.

[Lord_Feldon]

@ftg

There were various systems in places and the feds mandated a consistent one everywhere. So Interstate mile markers, exit numbers, etc. all got changed.

Several northeastern states still use the old exit numbers. They’re supposed to change soon IIRC, but I don’t think there’s any actual plan to force it.

[DesertDog]

@chacoguy

Colorado had to change its 420 marker.

Similarly, a whole highway, US 666 had its number changed to US 491 in part because of sign theft, in part due to objections of “The Devil’s Highway.”

@wiki

Although sign theft has always been a problem along this highway, thefts reached epidemic proportions when the pending number change was announced. Within days of the announcement, virtually every US 666 sign had been stolen, some for sale on eBay. Officials in Utah reported that five entire sign assemblies had been cut down with a chainsaw and stolen, while New Mexico officials reported that even signs welded to metal posts, as a theft deterrent, had been stolen. Officials speculated from one scene that someone had intentionally crashed a car into the sign post to break the welds.

More to the OP, around 1980 my brother and I were headed south on I-81 to see our parents in Abingdon, Va. A few miles out we passed mile marker 8, then a half mile later, mile marker 9. “Faster, brother; it’s gaining on us!”