As Roley Poley said, York & the Vale of York are flat, by comparison most of West Yorkshire is very lumpy, which can quite easily be perceived as being just too hard to commute, do a days work and then commute home again, similarly Hull is quite flat, so quite a few regularly commute by bike there too, i think geography can also be off putting for new cycle commuters, in fact when on holiday last year in the Poitou-Charentes region of France,which is also quite flat, my average speed went up by 1.5 mph despite the head winds the area is famed for.