There's this thing they call "clip level", the way vector scenery is designed everything clips either on QMID 11 boundaries or in the case of traffic lines QMID 15 boundaries. Technically you should always exclude and replace an entire QMID 15 cell when you're doing traffic lines (and you should always exclude and replace an entire QMID 11 cell when you're doing anything else), that way you know there's no way something underneath it in the scenery library can interfere, whether it be on your own system, your buddies system, your customers system, or whoever regardless of whether they have FTX Vector, UTX, or default CVXs running underneath your scenery.
In Sbuilder you can turn on QMID 15 grids (View > QMID level > 15, and then with the line tool active you can right click your line and choose "slice to QMID 15" which does exactly that, cuts the line wherever it crosses a QMID 15 boundary. If you plan for it you can start your initial line just outside a QMID 15 boundary and when you get to this point you're at you can just slice that little part off, which is what I did in that .spb I posted. Then you throw down a 3-cornered polygon, tag it "exclude all freeway traffic", right click and "fill to QMID 15". You now have a perfect traffic replacement that can't be affected by anything below it in the scenery library.
It doesn't always work that way though so here's another trick. Save your project, select the traffic line, then do Select > Invert selection. Hit the delete key leaving nothing but your traffic line. Whatever you do don't save your project in this state! Now do File > export > SbuilderX SBX and export your line, you'll "append this back into your project again later. After that, right click your line and do "manual check", you'll get a little dialog with some instructions, basically you cycle through all the points using your up/down arrow keys and you can hit delete to remove one or hold down the delete key to delete a bunch of them in rapid succession. Delete all the points down one lane of your traffic line including the U-turn at the end. Right click the line and set width to .5m or so, then right click again and do set transparency, turn it bright pink or something, you do this so you can tell the lines apart when you import your recently saved SBX back into the project. Now do File > Append > SBuilderX SBX and import the line you just exported. On the imported line do "manual check" again, this time deleting all the points for the opposite lane plus the U-turn. When you're finished you'll have the two individual lines side by side without the U-turns on either end. Export them to SBX again, both together this time and close your project without saving. Open the project back up, delete the existing traffic lines, and append the SBX you just exported. Now save and compile and you should be good to go.
EDIT: You can see how you might click out a road, asphalt, 2 lanes, undivided median for example, and then use the export SBX trick to bring it back into your project again, this time turning it into a traffic line with the convert to poly/make line/delete poly process. Saves a lot of clicking, and your road traffic will follow your vector road perfectly.
It's worth noting that you can also export your line as an ESRI shape or a Surfer BLN as well, either way works, the SBX and BLN options are basically just text files, BLN being the simplest. Sometimes it comes in handy being able to edit either the SBX or BLN in a text editor like Notepad++. The .shp is binary and can't be edited, but it can be used with GIS applications, for example you mentioned earlier blend masking out your PR so the vector road could show through, you could conceivably export your road as a .shp and use gdal_rasterize to burn it onto a .tif you could import into your blend mask. That's providing you know the exact corner coordinates and pixel dimensions of your photoreal source. Just an FYI, in practice it's probably easier to do it manually in PhotoShop, lol.
