should we go outside?

I would like to take my piggies outside to play in the grass. A couple of threads on the forum mentioned that fleas shouldn't be much of a concern, but we have wild rabbits that travel through our yard. Could this increase the risk of our piggies catching fleas? Also, I had read not to offer forages from areas that are high (automobile) traffic areas due to possible toxins. We live on a major street in our city, and we have a train track that borders our backyard. Would it be dangerous to let them graze the grass in our yard? If so, it doesn't really seem worth taking them outside. Any thoughts would be greatly appreciated.