Zero, because it would require the USA to "trust" that Canada had already done the screening for every visitor in the continent. For example, if someone entered at YVR from country X, the USA had to be 100% comfortable that that person had been screened. They never will be.

However, one thing I don't understand is why Canadian and American citizens can't move freely back and forth across the border. I don't understand why, as a Canadian, I have to deal with all this "why are you working and what are you doing business" and ditto I dont' understand why US citizens have the same rigamorole coming here. We should just 100% prove we're a citizen of each other's nation and that should be enough.

Cheers,
Geoff Glave
Vancouver, Canada