Use of triangle in yield sign?

When American road signs were being conceived, a general concept was devised that the more sides a sign has, the more important its implication (circular signs, having "infinite" sides, are considered the most urgent). A triangle is used with the YIELD sign because it's meant primarily as a notice. It is the driver's responsibility to observe traffic and only proceed when it's clear, but the driver will not get in trouble for simply driving past a YIELD sign--only for causing trouble once past it, which is covered under other traffic laws.