This post from BBC talked about the thousands of limos and other vehicles used to chauffeur stars and other "important" folks around Los Angeles on that all "important" night for Hollywood. As someone who has lived in LA and taken these things for granted, I found the post interesting.
The closest Hollywood related events I've been to are premieres, especially those in Westwood by UCLA. Streets would be cordoned off for blocks depending on how bit the movie is and who will be there.
Of course, that was nothing like the award shows like the Oscar. I could not imagine what it would be like to be a part of something like that, either sitting in traffic because of that or someone like a driver who is an integral part of that.
Already, LA traffic is pretty bad. Those of us who have lived here most of our lives know to avoid those areas. What I found interesting is that the Hollywood Bowl (wiki) is essentially turned into a limo parking lot.
I imagine dispatchers at these limo companies use technologies akin to those used by air traffic controllers to keep track of drivers, routes, arrival times, etc.