Professor M.J. Crawley FRS, at Imperial College, has studied the population dynamics of Ragwort, He writes this in his book Flora of Berkshire.

Research has shown that most of the commonly-adopted means of ragwort control
have exactly the opposite effect to that intended. For example, pulling up
ragwort by the roots leaves behind a ring of 4 or 5 broken root fragments,
each of which is capable of producing a new rosette in the following year.
So, instead of reducing the pest, hand-pulling increases ragwort numbers
4- or 5-fold. Another method beloved of farmers (I suspect because it has
such immediate and impressive visual impact) is to mow down the ragwort when
it is in flower, in order to prevent it from setting seed. A worthy aim, you
might think. But what actually kills ragwort is setting seed, not mowing.
If you prevent the plant from filling its seeds, then it retains the
reserves in its root stock and, instead of dying, the plant survives to
grow and flower again another year. Herbicides are effective, but they
kill all the other grassland herbs as well (the herbicides are "selective"
only in the sense that they do not kill pasture grasses); legumes, orchids and
other attractive pasture species are lost under this regime. The Silwood research
demonstrates that the best way to control ragwort is to fence against rabbits and
then let the plants go to seed. Going to seed will kill the established ragwort
plants, and in the absence of soil disturbance and gap-creation by rabbits, the
seeds will not produce ragwort plants. The strategy works, because in a well-managed
grassland, recruitment by ragwort is not seed-limited. Recruitment from seed requires
competition-free microsites of the kind that are produced by cultivation or by
heavy grazing from rabbits (or by over-grazing with domestic stock like sheep
or horses)."

What is clear from this reseach is that Ragwort seeding from roadsides and waste land is not a significant problem and ragwort control is a matter of management of the site on which it occurs.

Site Author's note:-It has come to my attention that some people on the internet are misunderstanding this article as being about Oxford Ragwort which is quite a different plant. It is NOT about Oxford Ragwort BUT about the ordinary common Ragwort which is being discussed on the rest of this site. The webmaster of the Warmwell site which is promoting this false idea using the material on this page has been contacted but, as is so often the case with ragwort propaganda, has failed to correct matters.