Did you miss out on the black borders for your Scalextric Classic or Sport track that had the flat kerbs (curbs)? Were you not able to procure enough of them for your layout? Or, did you need the R1 outer as a flat kerb, which Scalextric never even produced? If you still have the bump kerbs with the red and white stripes, and are tired of your performance cars getting upset by the transition over that bump, then fret no more! Just print these up and put them in place of the bump curbs (kerbs).
Currently, they do not have any clipping mechanism, so you will likely need to use some kind of adhesive to keep them in place. I've heard hot glue works well, but have not tested it. I have printed and test fitted all these pieces, and they fit well, but I already have flat kerbs where I need them, so I have not installed them for long term testing.
Hopefully I will find the inspiration to try and add the clips soon, but would be just as happy if someone else did. I've even uploaded the step file for easy remixing. Try not to laugh at my poor CAD work too much. ;-)