No Two-Person Riding on Large and Standard Motorcycles
大型自動二輪車及び普通自動二輪車二人乗り通行禁止(おおがたじどうにりんしゃおよびふつうじどうにりんしゃふたりのりつうこうきんし)
Meaning
A sign prohibiting motorcycles from traveling with two people on board (carrying a passenger). It is mainly placed on certain sections such as the Metropolitan Expressway (首都高速).
Common mistake on the test
It does not prohibit motorcycles themselves, only riding with two people.