[dcotter]

Not silly. Here's why:

1. Passing on the right, i.e. passenger side, means being in the driver's blind spot longer.

2. On- and off-ramps are almost always on the right, as are police making traffic stops, ambulances, cars pulled over on the shoulder, sidewalks, pedestrians, cross streets, etc.

3. Slower traffic is supposed to be in the right lane (because of #2), so a car accelerating to pass the car in front of it, suddenly in a lane of slow traffic, is a safety hazard to the slower drivers, regardless of #2.

I'm writing from the US, so for me left lane = driver's side.