There are some major differences about guide sign signage for both eastbound 270 and westbound 70 at mile 15 just north of Troy. I got photos of the signage from eastbound 270 but none are on here because the photo scanner made them look extremely distorted. However, I do got ones from westbound 70 at the same junction and it does not take a dummy to figure out the differences.

This is the first such photo on westbound 70, found at mile marker 17. I-70 in Illinois east of Troy is marked with 270 mileage, so right now I'm ignoring the signage on I-55 and I-55/70. Note that the guide sign appears to have both of them multiplexed and the next one (below) is the same way:

The only major difference is that this one is on a gantry and note the fact the first two photos have the exit just being "15B". If there was an Exit 15A, the tab would be on the left guide sign on this gantry. Note you got a left exit coming up!

Exhibit 3 is about 3/4 mile from the I-55 North exit. Note that I-70 is leaving this highway right now. That exit would had an "Exit 15A" tab in reality but IDOT is not fond of having interstates "exit themselves".

About 3/4 mile later, the exit for northbound 55 finally happens. Disregard the two down arrows on the left guide sign -- you're already on 270!!

Now think the signage would be on eastbound 270 at the same interchange. You'll notice a big difference. Exit 15A does exist -- but it does not recognize the I-55/70 multiplexion. (There is no I-70 shield.) Exit 15B is the same as it is on westbound 70. The control city guide sign says "I-70 Indianapolis" -- you first start noticing these right before the IL 159 exit.

The left photo is the one mile advance guide sign on eastbound 270. There is no further warning (except for a guide sign to Troy) until you have reach I-55. Note what is missing on the right sign in the right photo (badly distorted). Note that there is a "Exit 15A" here. And the left guide sign has no down arrows at all.

Illinois is usually good about signing the ends of highways, but not here. There is no "END I-270" signs in sight. Worst yet, the next mile marker on the highway once it becomes I-70 reads "16"!

Due to that, locals occasionally believe that I-70 doesn't exist even though it does. I have caught my mom telling people to "Take route 4 to 270 west". I have seen newspapers call the Troy to Highland stretch "270" as well.

Here is why I-70 mileage is off:

I-70 enters Illinois multiplexed with I-55. The reason why the mile markers on I-70 east of Troy are wrong because of these signs. I added the mile marker to your right because Exit 20 A-B is really at the 19 mile marker. Other than that, I-55 is pretty consistent here.