1) Accident witness, often 2) Footer 3) Just one of the crowd 4) Passer 5) Passer-by 6) Pedestrian 7) Random witness 8) Sidewalk user 9) Walker 10) Window shopper 11) Witness 12) Witness by chance 
[n] - a person who passes by casually or by chance
passer-by noun a person who passes by casually or by chance