Guitarmans AIW/CAM editor is the tool for you. It's not the most simple software to use but it'll get the job done if you only want a few corridors changed. But like i said, after that alteration, the track is not online compatible: the server and client versions need to be identical (well, some files need to be identical and AIW is one of them.)
Here's the tool:
http://www.rfactorcentral.com/detail.cfm?ID=AIW CAM Editor
Just a little sidenote, AIW works like this: It has a centerline, made from individual waypoints. Attached to that waypoint we have left and right corridors, marked by their distance from the waypoint.
This is what a line in the AIW file says:
wp_pos=(-418.3721,6.0256,-6.0161)
That's the waypoint location. Then you got:
wp_width=(3.975,4.050,8.975,9.050) // road left, road right, far left, far right
wp_dwidth=(20.786,30.000,0.000,0.000) // collision left, collision right, cut left, cut right
The first parameters on wp_width marks the legal road width, calculated by difference to the waypoint (centerline, usually). The last two parameters on wp_dwidth define the difference to the road width, ie if the cut is wider (positive value) than the road or narrower (negative value) You can either edit that value in guitarmans editor or alternatively locate the waypoint you want to edit and do it manually. So don't move the waypoint as it moves everything around it. You may have to edit AI fastline too to get the right results as you can draw a "illegal" fastline, one that crosses the cut corridor and AI either breaks your corridor rules or brakes and swerves.
Road far left marks the full driveable width of the track, including runoffs and is marked as difference to road width. Collision is the full driveable road before any collision objects are calculated (again difference to road width) but this affects AI only! You can put a concrete barrier on track, AI drives thru it (as it is inside the collision corridors) and you'll crash.
I hope you got enough info to convince that it's a lot of work for very little change and you can't compare your results or drive online competitions on that track anymore. EDIT: oh, and you can only modify community created addon tracks and you can not release the modified track without the authors permission (from experience, this can be impossible). I'm not going to teach you how alter default game content as that is like opening a can of worms...
