The Arakawa cycling road runs along the river from north Tokyo to Tokyo Bay, and is a favourite training route for road cyclists. This route starts at the Iwabuchi water gate near Akabane and follows the river through Senju and Horikiri to Kasai Rinkai Park.
It's almost flat with hardly a traffic light, but there's little shade and the wind along the river can be strong. Plenty of people walk and run here too, so keep your speed down.
Turn-by-turn cycling navigation for iPhone & Apple Watch
