This happens at the onramps. One could be happily tooling along on the Interstate at 60MPH, when suddenly the cars in front of you step on their brakes. They're doing this to avoid hitting the cars coming onto the I-5, even though the onramps are all governed by "Yield" signs.

In one instance I remember being forced over from the right lane to the left on the I-5 because an SUV shoved its way over from the onramp. The driver saw me clearly, and looked pretty annoyed that I hadn't cleared the lane for him earlier, despite him having a "Yield" sign.

On the other side of the coin, I approached the I-5 from an onramp once and heeded the "Yield" sign. I was there for all of five seconds when the car behind me leaned on his horn -- despite there being a steady stream of traffic on the Interstate. I waited until it was clear (another ten seconds or so) then away I went, all the while tailgated by the car behind me, horn still blaring away until he passed me. I think he might've given me the finger.

Which brings me to my question: with the understanding that it's always better to drive defensively and to not get into a fuss on the roads (hurtling metal being what it is), at what point is it better to ignore the law and just go with what everyone else is doing?