I have to walk up a busy main road EVERYDAY and I constantly smell car fumes. I am worried sick that I will get Lung Cancer from this. Will I? If I wear a scarf around my nose and mouth will it benefit in any way? Please give me some advice because I'm worried sick:( Thanks.

Won't help in the slightest.
If you are that concerned then you need to get a proper respiratory system (gas mask) as car fumes are incredibly small particulates.