Great idea! I've been riding my bike on the car-free quay ever since August, when the crowds at the Sziget Festival started obstructing the bike path near Filitorigat. After that, on my evening ride home from Szentendre, I'd get off the circuitous designated bike route at Auchan and get on the quay. With two wide lanes of asphalt and hardly a car in sight, it felt like entering a cycling autobahn. Wonderful. The further south you go, the more other users you see -- not only cyclists, but runners, walkers, parents pushing prams, people walking dogs, even people stretching and doing tai chi. It's all happening in the middle of a road that until this summer had been clogged daily with rush-hour traffic jams. It's easy to imagine how much more it would be used if properly developed with paths, benches, landscaping and the rest (i.e. beer gardens). It'd probably become like an extension of Margit Sziget, with some choice jogging and cycling circuits formed via Margit and Arpad bridges.
An open letter from someone named Aron was posted on the criticalmass.hu site urging Mayor Demszky to make the de-motorisation of the quay his major legacy. You can cut and paste the Hungarian text and send it to email@example.com. Or surprise him with something original in English or Sanskrit.
Another article about the "rakpark," comeplete with an artist's rendering of what it would look like, is here. And then there's a blog posting about how Vienna did something like this with one of its riverside arterials. For that matter, check out what the city of Seattle did recently with its waterfront. Seems like lots of cities are realising that embankments are too precious to waste on roads.