You've never heard anyone choose to take side streets instead of the highway because of traffic jams? No one ever goes out of their way to avoid heavily trafficed areas?

I don't understand what the point is you're trying to make. When people at t0 take detours because of traffic jams on the direct route, and then at t1, there are less traffic jam on the direct route due to additional lanes, so they decide to take the direct route, then total traffic is down, because they no longer take a detour. Even if they are still part of a newly induced traffic jam.