Well if the officer did not see your signal, perhaps other divers would have missed it too. Police only stop drivers to ensure the roads are safe. If you feel strongly and have been written up you can always take it to a judge (you could lose). If you have just been warned then take it on the chin and try and use the signal light rather than you hands to signal.
They don't need 'probable cause', just a 'reasonable suspicion'.
The police officer has to have probable cause to believe a particular person commited the crime in question. In court, probable cause is NOT enough to convict you of the same crime.
If probable cause is given then yes, the officer can.
Under no circumstances can a police officer stop you or search you without probable cause. Probable cause is one of the ways that an ordinary citizen's right to privacy is protected from unlawful search and seizure.
Probable cause.
A computer check is an investigatory tool, no probable cause or reasonable suspicion is necessary.
Probable cause and reasonable suspicion are legal actions that can be enforced by a law officer. These two actions are similar in that they give the police officer the ability to gain access when investigating a criminal action.
Difficult and expensive to prove lack of probable cause. Probable cause can be anything from suspicious activity in the vehicle to weaving while driving or failure to signal. I am sure you could find a lawyer who would try but I am also sure the judge would eventually find for probable cause in the end.
You have the right to refuse a search of your car unless the officer has a valid search warrant or probable cause to believe there is evidence of a crime in your vehicle. However, it's important to comply with the officer's instructions to avoid escalating the situation.
The most probable cause is that the brake light switch is out of adjustment.
Probable cause is anything an officer wants it to be....whether it would hold up in court? Probably, if they find something else.
In most jurisdictions the search is valid if there is probable cause. Probable cause is what the officer thinks and can substantiate. Don't forget those dash cams.