I get asked that quite frequently from some of my technically astute customers. Personally I think its a fail on the OEMs for designing a bad icon. An icon of a side view of a flat tire/wheel would have been way more understandable by a larger percentage of the motoring public. Its easy to blame the end user (the consumer) but if you are not getting the message accross in regards to a safety issue, then its shame on the OEMs.