I've called companies about poor driving on the part of their employees. I'm almost always satisfied with the response. I recently had an electrical company call me back twice to get further details because management wanted to be sure they clobbered the right guy. If one is polite and gives the facts without vitriol or threats, the person at the other end usually wants to make sure there's not a repeat performance. I've also sent emails when I couldn't find the proper phone number and those appear to have worked as well.
This next part is painful to say, because I'm politically a big-government fan. However, the only fleets that appear to be immune to these calls are public fleets (city, municipal power, school districts, county). Maybe I just don't speak their language, but they are highly resistant to improving their driving skillset.