[lucumo]

It's not really accurate to call them the "European system". These signs are used in many countries. Which also immediately tells you why icons are more useful than text: language barriers.

Where the US is one big area with a single language, that's not the case in the rest of the world. Processing words in a foreign language takes longer than just seeing the same traffic signs in a slight variation.

As for intuitiveness. I don't know, I kind of subscribe to the notion that the only intuitive interface is the nipple. Everything else is learned. What people call intuitive is just familiarity. The traffic signs are easy to get familiar with if you grow up with them. A systems-minded (or traffic-interested) kid can easily learn the meaning of most traffic signs long before they can read fast enough.