IVS ONVIF triggering is problematic for LPR duty, especially at night with the black image.
Since you have a great camera overview already, I would suggest setting up a clone of that camera to trigger the LPR camera.
You can also set it up as motion in BI.
I found this to be the setup that worked best for me. The one zone approach at this location allowed me to catch the front plate of the cars going right to left and the back plate of the cars going left to right. Occasionally I would get a large truck or a very fast vehicle that was missed, but I also run this continuous motion in the event that happens or it missed the motion. And then at night I simply make the object size box a lot smaller like the size of the plate.
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The other way is since this thread originated, BI now has DeepStack integrated with it. It has added the ability to only send when AI detects a vehicle. You can assign a leading edge trigger now as well with DeepStack, But some fields of view are still problematic and you could do as another member did here and create a custom model based on your field of view that will only take a snapshot when a plate is in view. Member said it took about an hour or so to train the custom model.