To get my leaflets better seen by strollers, I always put them under the windscreen wipe which is closer to the sidewalk (don't care whether it's the wipe in front of the driver or the passenger's one).
If both wipes are equal to the sidewalk (as with cars parked nose to the kerb) I choose passenger's wipe as I prefer to place flyers out of reach from the driver seat (I don't want my micro-billboards quickly removed).
I see many of you are trying this with 50, 100, 200 leaflets... It would be long and kinda offtopic to explain the statistical formula involved but, to keep it easy, whatever conclusion you get to after so few flyers distributed, don't trust it AT ALL (you have more chance to be right just by flipping a coin).
If your pocket forces you to cut expenses some way, it's more advisable to test water with a cheaper version of your promotional material (so you know that your results can only improve from those obtained in test). If you can't afford to print 5,000 color flyers then print them Black-on-White, but don't waste your time (and ink) in so-called "tests" that are nothing but a joke.