It sounds like you were probably at fault. The California vehicle code requires you to make sure it is safe before you make a lane change. It does not matter how long you had your turn signal on. What matters is whether it was safe to move into the lane before doing so.
I agree with the first answer. Based on your description of the damage, it appears that you pulled into the left lane just as the other driver - whom you apparently did not notice - was passing. When you merge to the left, it is important to look quickly to your immediate left in addition to checking the left-side mirror.
Disclaimer: This post does not constitute legal advice and does not establish an attorney-client relationship.
You can't assume anyone is aware of you just because you're signaling. Since you were the one making the lane change, this accident will be considered mostly, if not all, your fault.
If the other driver was speeding, it wasn't by much, and the speeding didn't cause the accident, your lane change did. As noted by my colleague, you have a duty to not only check your review mirror, but also to look over your shoulder before you make a lane change like this.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
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