Are dashboard video cameras legal??

It's not illegal to record anything that you can see from a public place or your private property. (People who don't put up curtains or who leave them open are considered to have given up their right to a reasonable expectation of privacy with respect to what can be seen from a public location through that window.)

Posting those recordings online is also legal, but might not be wise. Even lawsuits without a chance of winning can cost the defendant time and money.