For the best performance in editor play mode, make sure the scene window is behind the game window. If you don't need to view the scene view gizmos, you can disable them in the Preferences window. Edit/Preferences/Simple Traffic System
Editor tools don't seem to work as intended.
Often resetting the unity window layout to default (top right corner of Unity Editor) will fix editor related bugs. Unfortunately this is an unresolved Unity bug that has been around for a long time.
Traffic cars are not spawning as expected.
When cars are spawned, the spawn point's collider bounds are checked to make sure it's empty. Ensure there are not any other objects touching the spawn point colliders.
Collections is not available to import via Package Manager in Unity 2020.1 or 2020.2.