Just ride with and without. Personally I think I get another couple of feet passing distance with the vest. However, it's hard for me to judge these days because I also run a Dinotte, which also DEFINITELY generates a lot more passing room - between the vest and the Dinotte, people not only pull over into the far side of the far lane to pass, they actually often slow down (I ride 60 MPH roads).
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Work: the 8 hours that separates bike rides.