I never used a mannequin, but when I did live in Utah after graduation, I would dress up the passenger seat to look like
someone was in it. I went to work in the morning when the light was not too good. I raised the headrest, kept a jacket over it, and put a hat on top of it (even in the summer). I used the HOV lane at various times practically every morning on my commute (only when the other lanes began to slow down). 3 years before I moved away and I was never pulled over for speeding or carpool violations.