There was a lot more of that hazardous carcinogenic stuff 50 years ago though. Baby cribs used to be made from asbestos. Every other person was a smoker. Houses were painted with lead. This trend is very very recent.
there would be a clear geographic distribution of cancers along major highways vs rural areas with little traffic
which may be happening? not sure if anyone actually tried making a survey along those lines yet; I think currently most of the time a geographic distrubution of cancers is assessed to find chemical leaks e.g. superfund sites