I'm in favor of saying, "passing on your left" in a strong voice. Also, when someone says it to me, I hold my line and say, "thank you" in strong voice. I don't think there's a good solution to this problem. The people in the real world (non-cyclists) are not aware and familiar with this concept. Also, I ride often in a tourist area and many of the people don't understand english.

I did notice something on the century ride I did this weekend. For about 50 miles of the ride, I was riding with only people doing the century. For the other 50 miles, I was mixed in with metric and century riders. I noticed a big difference in how the metric riders handled this.

It's a big generalization, but it sure seemed to me like the metric riders were less familiar with holding their line -- and with general group riding procedures in general.