Legal Question in Traffic Law in California

You might send the driver and vehicle's owner a certified, return receipt letter demanding what you are stating. Keep a copy of the letter. No doubt, they will reject your request. Then you will have to duke out whether you were negligent to let the dog run free, whether the driver was negligent, etc. If you were to sue in small claims court, your claims (i.e. causes of action) would probably be negligence and trespass to chattel (pets are considered objects rather than members of a person's family).