How much CO2 emissions is caused by traffic?

Traffic can include a lot of things. I think you need to be more specific about what you are referring to. If you mean cars jammed up in traffic jams, it can cause quite a waste in gas and therefore emissions. It might be up to 20% of the cost of driving in busy places and that might be 10 percent of the 22 percent that S suggested since the 22 percent in his link included all transportation.

worldwide, 22%

more than 20 per cent