Wikipedia, note that not all years are represented). The degree of improvement in a short time period is evocative of East German swimmers of the 1970s.
It is routinely and correctly stated that performance improvements are not sufficient evidence of doping violations, but the Alpe d'Huez performance shown above sure should have raised some eyebrows. I have long wondered why the Tour did not publish a readily accessible time series of rider times. Perhaps this is why.
Thanks to @ReinerGrundmann for the inspiration and link to data for this post!