The only traffic allowed _____ buses and bicycles.
'Are' is plural, 'is' is singular. Next!
traffic is a singular word, therefore "is" is correct here
Traffic, like family, everyone, etc is singular and therefore "is" would be the correct answer.
this is the easiest one yet! some people do use is, but they are incorrect!!
Juliana. Don't understand your comment. As others have said, traffic is a single noun, so IS is correct. Simple.