How do I stalemate this recalcitrance!?!?

[Pythias900KMB]

Many salutations!!!  Guys, as you can tell from the screen captures, I have a very frustrating engineering situation on my hands.  I have an avenue viaduct that I would like to make into an inside diamond interchange; unfortunately, SC4 is refusing to cooperate saying that it is an unsuitable grade for construction.  I have revised the grading under the northbound carriageway as much as I am able; still, the game will not allow me to build the eastern half of the interchange after building the western half (and vice versa!!!)  ARGH!!!

[APSMS]

The obvious suggestion is that you take out the hammer that is god-mode terrain tools, and properly level the entire corridor.

Alternatively, you could do the same thing in a more precise manner by plopping single road tiles all across the carriageway (you'd have to delete your highway first, but this would avoid potential collateral damage.

After the flatness of the area is confirmed, the obvious solution is that you use the right tools for the job. You should be using the draggable versions of Avenue and OWR/MIS. In fact, I would recommend that you use the draggable MIS ramps instead of the OWR, as the OWR will take longer due to the transition from MIS to OWR that has to occur (and IIRC, the slope of the RHW ramp is longer, but I could be wrong).

Using the appropriate draggable pieces, everything should work out find with a bit of clicking. The proximity is a small issue, and in general I would recommend you space out your intersections more to give the game more space to work out the network RULs, but theoretically it shouldn't be a problem.

Also, technically PPs are deprecated, and no longer supported, so their presence is mostly for legacy support.