請教加州關於dash cam使用的規定

水中撈月給了這個鏈接。看完後還是有些地方不明白。

http://www.ehow.com/info_12333580_legal-use-car-dash-camera-california.html

這裏說：

A crash or "unusual motion” of the vehicle can automatically trigger the camera’s recording function, but the driver can also manually operate it for the purpose of monitoring his driving performance. The recorder may not store more than 30 seconds of data before and after the event that triggers its recording function.

這個30s before the event到底什麽意思？如果recording function還沒有被trigger，哪裏來的data？

我在考慮買一款開車時就自動啟動的dash cam，能記錄24小時。到底合法嗎？