The Japanese word for 'bicycle' is jitensha, written 自転車 in Chinese characters. This mostly how it is written, as can be seen in the signs below.
There is one place, however, where the characters are not used -- in road signs painted directly onto the road surface.
This appears to be a case of simplification for safety and technical reasons. Kanji would require a larger number of strokes than is suitable for the medium of paint-on-bitumen. To ensure clarity, hiragana is thus used.
A Google search in August 2003 revealed the following distribution: