Install and use: get the updated track from the link inside the zip, test it and verify it's working. Then drop all items from the 7z into "..\content\tracks\californiacoast\" and confirm the prompt. This is traffic for up to 17 AI cars but if using CSP and AI flood you may get better experience with ~10 cars (CSP v0.1.75 or newer is recommended). Or if you have nice classic weathers (and ppfilters) you may deactivate CSP and see improvement in performance (and graphics). The AI pits are scattered along the course and when not using CSP flood they mostly keep nice distance.
The active line is restricted so no overtaking happens and no head-on occurs. Included is the normal line where overtaking is allowed and for that you need to unpack it from "NORMAL fast_lane.7z" and replace the "fast_lane.ai". After that you need to remove both "side**.csv" files from "..\content\tracks\californiacoast\2way\data" and all "**_payloads.bin" files from "..\content\tracks\californiacoast\2way\ai" or the restriction will still apply.
Another options are interchangeable road models. Track now has 3 kinds of surface to meet different needs - cruising or drifting. Try "patches_phy.kn5", "bumpy_phy.kn5" or "smooth_phy.kn5" by editing "models***.ini"
[MODEL_3] FILE=patches_phy.kn5 POSITION=0,0,0 ROTATION=0,0,0