I think arrivals are not the problem, as they are already spread out over more days than exodus. No matter how many people you have attending, they are arriving over the course of more than 5 days and leave over the course of 2. Also, the number of early arrivals is already pretty big. Bigger than the number of late departures.
You could try to encourage spreading out exodus more, but people are already doing that. I know lots of people that have left right after the man burn, or even before, in the last couple of years, to avoid traffic. Either way you slice that, the bulk of the people is going to try to leave in a 48 hour or so window.
Now expanding the road *would* help, but I have no idea how realistic that is. And with USG closed, there is probably even less arguments to do that. So unless we get to pour some stimulus money into 447, I am not holding my breath.