This is a complicated answer, and yes, it is very dependent on country and even the type of accident.
First, be aware that there can also be a big difference in "incident" vs "accident", and they may be reported differently.
Next, each country has its own laws and regulations about the dissemination of materials generated by government entities. I don't know French law or it's rules for releasing records, so I would have to defer to somebody familiar with it. In the United States we have the Freedom of Information Act which regulates how documents are released, exemptions from release, and a request process.
Again, I'm not sure about the French, but in the U.S. we have something called Procedural Due Process which allows both sides of the proceedings equal access to evidence. This comes from Due Process and by extension, the Magna Carta. Really now we are getting into the rights and obligations under law and is a bit outside the scope of this site. Point is, at least in the U.S. a person in a civil or criminal lawsuit has the right to examine the evidence as equally as the prosecuting party.
Now, as far as current access to records... This is a digital age, everything is done on computers, fed to servers, and can easily be made available to the general public, usually if it is in the public interest. 1989 was a different story. The first "laptop's" weren't introduced until the 80's and ones that would run word processing software weren't readily available (or very affordable). Most investigators at that time would take notes on paper then go back to an office and type them up (on a typewriter mind you). These reports would then be given to others in the organization and filed.
The U.S. has done a pretty damn good job of digitizing old reports and it may be that the French have not have bothered to put the resources into digitizing these reports. If your search for information is purely "on the web", you may need to actually visit a library (national) or make a request to the investigating agency.