What time limit is needed for Apple maps to redirect my route based on traffic

Typically Apple Maps on my Iphone 10 has been good at redirecting around traffic. Since I can't display Waze on my car via Car Play, I plug my Iphone in and use the Apple maps app.

This morning on my drive to work, I noticed it was routing me in a way which was through some seriously heavy traffic. Typically it would not. I restarted and selected "more options" but the app insisted this was the best route.

It said 56 minutes for a 23 mile drive. I decided to get off the road and drive a route I knew would be faster.

As such, I arrived fifteen minutes earlier than the time the map would have sent me.

Here it is shortly after I took a detour, showing an 8:38 arrival vs. the original 8:50 arrival. I ended up arriving at 8:28.

Is there a threshold over which maps will re-route me? I thought it would find the best route by default?