Complex sentence: On motorways and high quality dual carriageways they appear after every junction (except on the M50, on which they are not typically used), while on other national roads they appear when leaving built-up areas.
Split into simpler sentences: On motorways and high quality dual carriageways, they appear after every junction (except on the M50, on which they are not typically used). On other national, roads they appear when leaving built-up areas.