According to the Sustainable Cities website: http://sustainablecities.dk/en/city-projects/cases/budapest-cyclist-numbers-double numbers of cyclists in the Budapest in just two years. This story was published in 2010 so I would suspect that numbers have continued to rise still further.

(source: cafebabel.com)
There are number of reason for this: As mentioned at least half the city, pest, is super flat (important to mention that along the river on the Buda side is extremely good also and you have an opportunity to get the best views of landmarks such as the parliament building). The climate from April onwards is very pleasent with little and ride and importantly for me, coming from Britain as I do, minimal wind! Thirdly i've no doubt that finances have contributed to change, with fuel prices continuing to rise while wages remain stagnant.
Cycling in the city has been championed by what may have previously been two opposing sides: Activists from the vibrant local cycling subculture and the City council. I want to cover cycling culture in the city in a seperate post, but in order to quickly illustrate its strength in Budapest, the city hosted its first Critical Mass (C.M.) ride (where cyclists take over the local road network to protest against primacy of cars.) September 2004. By 2008 the C.M. spring ride had attracted a record 80,000 participant! http://criticalmass.hu/english.

During this time the city council has also been busy both accomodating, encouraging cycling and cyclists. As previously mentioned a large network of cycle paths have rapidly taken shape (though as yet, far from perfect, there are still places where the paths simply end with the unlucky cyclist having the option of either cycling on the pavement or one the BUSY city centre roads). Other enlighted changes include allowing cyclists to travel the opposite along one way roads and also onto suburban trains. These have all encouraged locals to get on their bikes!