Law enforcement sources tell TMZ, Javaris was pulled over for going over 60 in a 45 -- and when the cop wrote up the ticket ... Javaris refused to sign it, something you have to do to acknowledge receipt of the citation.

As a result, we're told the officer asked Javaris to step out of the vehicle -- and Javaris refused again.

We're told words were exchanged -- and after the officer reasoned with Javaris, the former NBA player agreed to step out of the car ... at which point he was placed under arrest for speeding and obstruction of an officer.

He was subsequently transported to Clayton County jail, where he is currently being processed.